Who Are the "Lazy" Ants? The Function of Inactivity in Social Insects and a Possible Role of Constraint: Inactive Ants Are Corpulent and May Be Young and/or Selfish

Item Type Article

Authors Charbonneau, Daniel; Poff, Corey; Nguyen, Hoan; Shin, Min C; Kierstead, Karen; Dornhaus, Anna

Citation Daniel Charbonneau, Corey Poff, Hoan Nguyen, Min C. Shin, Karen Kierstead, Anna Dornhaus, Who Are the “Lazy” Ants? The Function of Inactivity in Social Insects and a Possible Role of Constraint: Inactive Ants Are Corpulent and May Be Young and/ or Selfish, Integrative and Comparative Biology, Volume 57, Issue 3, September 2017, Pages 649–667, https://doi.org/10.1093/icb/ icx029

DOI 10.1093/icb/icx029

Publisher OXFORD UNIV PRESS INC

Journal INTEGRATIVE AND COMPARATIVE BIOLOGY

Rights Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology 2017. This work is written by US Government employees and is in the public domain in the US.

Download date 27/09/2021 05:55:48

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Link to Item http://hdl.handle.net/10150/632218 Integrative and Comparative Biology Integrative and Comparative Biology, volume 57, number 3, pp. 649–667 doi:10.1093/icb/icx029 Society for Integrative and Comparative Biology

SYMPOSIUM Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019 Who Are the “Lazy” Ants? The Function of Inactivity in Social Insects and a Possible Role of Constraint: Inactive Ants Are Corpulent and May Be Young and/or Selfish Daniel Charbonneau,1,* Corey Poff,† Hoan Nguyen,‡ Min C. Shin,‡ Karen Kierstead§ and Anna Dornhaus§ *Graduate Interdisciplinary Program in Entomology and Insect Science, University of Arizona, Biological Sciences West, 1041 East Lowell, Room 235, Tucson, AZ 85721, USA; †Mathematics and Computer Science Department, Davidson College, 405 N. Main Street, Davidson, NC 28036, USA; ‡Department of Computer Sciences, College of Computing and Informatics, University of North Carolina Charlotte, 9201 University City Blvd, Charlotte, NC 28223, USA; §Department of Ecology and Evolutionary Biology, University of Arizona, 1041 E Lowell Street, Tucson, AZ 85721, USA From the symposium “The Development and Mechanisms Underlying Inter-individual Variation in Pro-social Behavior” presented at the annual meeting of the Society for Integrative and Comparative Biology, January 4–8, 2017 at New Orleans, Louisiana.

1E-mail: [email protected]

Synopsis Social insect colonies are commonly thought of as highly organized and efficient complex systems, yet high levels of worker inactivity are common. Although consistently inactive workers have been documented across many species, very little is known about the potential function or costs associated with this behavior. Here we ask what distinguishes these “lazy” individuals from their nestmates. We obtained a large set of behavioral and morphological data about individuals, and tested for consistency with the following evolutionary hypotheses: that inactivity results from constraint caused by worker (a) immaturity or (b) senescence; that (c) inactive workers are reproducing; that inactive workers perform a cryptic task such as (d) acting as communication hubs or (e) food stores; and that (f) inactive workers represent the “slow-paced” end of inter-worker variation in “pace-of-life.” We show that inactive workers walk more slowly, have small spatial fidelity zones near the nest center, are more corpulent, are isolated in colony interaction networks, have the smallest behavioral repertoires, and are more likely to have oocytes than other workers. These results are consistent with the hypotheses that inactive workers are immature and/or storing food for the colony; they suggest that workers are not inactive as a conse- quence of senescence, and that they are not acting as communication hubs. The hypotheses listed above are not mutually exclusive, and likely form a “syndrome” of behaviors common to inactive social insect workers. Their simultaneous contribution to inactivity may explain the difficulty in finding a simple answer to this deceptively simple question.

Introduction species (e.g., anolis lizards 4–28%, seaside sparrows Most animals spend the better part of their days 4%, chimpanzees 23% [Herbers 1981]) and within doing what appears to be nothing in particular. species (wild woolly monkeys 0–65% [Defler 1995], Non-sleep resting time consistently takes up >50% e.g., Indian wasps 0–70% [Gadagkar and Joshi 1984], of animal time budgets, even for animals with vastly and great gerbils 6.6–16.6% [Tchabovsky et al. different life histories and ecologies (e.g., birds: hum- 2001]). However, we know very little about the evo- mingbirds 57–86%, blackbirds 60%, mammals: fish- lutionary reasons either for inactivity or for this var- ers [Martes] 68%, short-tailed shrews 68%, walruses iation within and across species. 67%, lions 75%, howler monkeys 70% [Herbers High levels of inactivity are also common in social 1981]). Although high levels of inactivity are preva- insect colonies. This is perhaps surprising because lent, there can still be enormous variation among social insects are one of the most ecologically

Advance Access publication July 25, 2017 Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology 2017. This work is written by US Government employees and is in the public domain in the US. 650 D. Charbonneau et al. successful animal groups. They dominate most ter- organization did not include inactive workers restrial habitats and constitute up to 80% of all in- (Lenoir and Mardon 1978; Mirenda and Vinson sect biomass (Holden 1989; Wilson 1991; Samways 1981; Herbers 1983; Mersch et al. 2013; Pamminger 1993). Data from the literature show that in most et al. 2014), but when included, they may make up

colonies of social insects, upward of 50% of workers 40% of the colony (compared with 20% for Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019 appear to be inactive at any one time. This is true extra-nidal workers and 40% for brood carers across most species of social insects, including honey [Lindauer 1952; Mirenda and Vinson 1981; Herbers bees (Lindauer 1952; Moore et al. 1998; Moore 1983; Cole 1986; Corbara et al. 1989; Retana and 2001), bumble bees (Jandt et al. 2012), wasps Cerda 1990, 1991; Rosengaus and Traniello 1991; (Gadagkar and Joshi 1984), termites (Maistrello Hasegawa 1993; Dornhaus et al. 2008; Ishii and and Sbrenna 1999), and ants (Herbers 1983; Hasegawa 2013; Charbonneau and Dornhaus 2015a, Herbers and Cunningham 1983; Cole 1986; Retana 2015b; Hasegawa et al. 2016]). and Cerda 1990; Dornhaus 2008; Charbonneau et al. An additional difficulty when comparing studies 2014). However, beyond the prevalence of inactivity, on insect worker activity or lack thereof is that dif- we know very little about this behavior. There is ferent operational definitions are used when identi- little detailed individual-level data on inactivity fying “inactive” workers. For example, some studies (although see [Herbers and Cunningham 1983; (Ishii and Hasegawa 2013) include behaviors such as Fresneau 1984; Cole 1986; Corbara et al. 1989; moving around the nest or self-grooming, while we Retana and Cerda 1990, 1991; Ishii and Hasegawa (Charbonneau et al. submitted for publication, 2014; 2013]), and few studies have attempted to explicitly Charbonneau and Dornhaus 2015b) did not. test potential functions or costs associated with inac- Workers moving around the nest (wandering) form tivity (reviewed in [Charbonneau and Dornhaus a group that is behaviorally (Charbonneau and 2015a], and below). Dornhaus 2015b) and, as we show in the present manuscript, possibly physiologically distinct from “inactive” (immobile) workers. This behavior may What do we know about inactivity? be a functional task linked to sharing local informa- Most reports of high levels of inactivity in social tion with the rest of the colony (Johnson 2008). If insects come from observations of workers in the more than half of all workers in many social insect laboratory (e.g., Gadagkar and Joshi 1984; Cole colonies are consistently inactive (immobile and not 1985; Moore 2001; Charbonneau and Dornhaus performing visible tasks), then this raises the obvious 2015b). Thus, the simplest explanation for high lev- question of why the colony would produce more els of inactivity is that inactivity is the result of col- workers than appear to be necessary, only to have onies being insufficiently challenged in simplified thembestandidlybyastheirnestmatesdoallof laboratory environments. However, we have previ- the work. Do inactive workers serve a specific un- ously shown that colony-level inactivity as well as known function, or is inactivity the result of con- time allocated to each specific task did not vary be- straints on age or physiology? tween laboratory and field observations in the ant Temnothorax rugatulus, suggesting that at least in this species, inactivity is not simply a laboratory ar- Previously tested hypotheses for the function of tifact (Charbonneau et al. 2014). inactive workers Not only is inactivity common, but specific work- The response threshold model may provide a prox- ers are consistently inactive (Fresneau 1984; Corbara imate mechanism for inactivity (Robinson 1992; et al. 1989; Retana and Cerda 1990; Ishii and though see Jeanson and Weidenmu¨ller 2014), but Hasegawa 2013; Charbonneau and Dornhaus functional explanations remain unclear. The most 2015b). That is, workers have consistently different commonly invoked explanation for inactivity is the activity levels at least over medium term periods “reserve worker” hypothesis: inactive workers consti- (3weeks in Temnothorax [Charbonneau and tute a reserve labor force that becomes active when Dornhaus 2015b]). Inactive workers form a group needed. Although it is a commonly proposed hy- that is distinct in their task profile from such pothesis, it is often not well defined. For instance, groups as “extra-nidal” workers (e.g., performing it could mean that inactive workers replace lost foraging or building) or “intra-nidal” workers per- workers in the case of a major catastrophe that forming brood care or grooming (Charbonneau eliminates or fatigues all other workers (Hasegawa and Dornhaus 2015b). Many detailed studies that et al. 2016). It could also mean that, at shorter time- have previously identified broad patterns of colony scales or less extreme fluctuations in either worker Lazy ants and their role 651 availability (e.g., loss of foragers to predation) or reproduction (c. Reproduction hypothesis) workload (seasonal, daily, or stochastic), the colony (Beekman et al. 2000; Jandt and Dornhaus 2011). has a pool of workers available that help mitigate Alternatively, inactive workers may in fact be per- these fluctuations by picking up the slack when forming a task or function that simply appears as

needed (e.g., providing additional defense in case inactivity, such as acting as communication hubs Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019 of a predator attack, [Jandt et al. 2012] or as fanners (d. Communication hypothesis) (O’Donnell and helping cool honey bee hives [Johnson 2002]). Or it Bulova 2007), or food stores (e. Repletism hypothesis) could mean that the colony maintains a pool of rest- (Blanchard et al. 2000; Robinson et al. 2009). Finally, ing workers that regularly replace active workers as we consider the novel hypothesis that individual work- they become tired (i.e., “shift-work” hypothesis; ers may differ in their “pace-of-life” (Pearl 1928; Re´ale Charbonneau and Dornhaus 2015b). These different et al. 2010),whereinactiveworkersareattheslowend interpretations of what it means to be a “reserve” of the spectrum (f. Pace-of-life hypothesis). result in different conclusions about the role of in- active workers in a group (see Charbonneau and Dornhaus [2015a] for a short discussion on this a. Immaturity hypothesis and b. Senescence hypothesis topic). Age could be linked to worker activity in two ways. There is little evidence to support this broad class Young workers may be less active due to a still de- of hypotheses. Inactive workers are common even in veloping physiology (physiological and neural devel- emergency/high workload conditions (Dornhaus opment), inexperience (social role development and et al. 2008; Pinter-Wollman et al. 2011), and at- experience-related improvements in task perfor- tempts to induce inactive workers to undertake tasks mance), or being more vulnerable (e.g., not fully by either removing active workers or increasing col- sclerotized cuticles) (Farris et al. 2001; Seid et al. ony workload have shown that already active work- 2005; Giraldo and Traniello 2014). ers, rather than inactive ones, tend to increase their On the other hand, older, senescing workers may activity in response (Fewell and Winston 1992; be less active. Senescence is defined as a decline in O’Donnell 1998; Johnson 2002; Gardner et al. physiological functioning with age accompanied by a 2007; Jandt et al. 2012). Indeed, the only study to decrease in reproductive performance and an in- effectively activate inactive workers by manipulating crease in mortality rate (Rose 1991). Older workers workload or workforce removed all workers except may be less active due to degraded physiology the inactive workers (Ishii and Hasegawa 2013). Thus, (Porter and Jorgensen 1981; Cartar 1992; Schofield while many authors failed to find empirical support et al. 2010), decreased locomotion (Ridgel and for the reserve worker hypothesis in the context of Ritzmann 2005), or perhaps even decreased immune fluctuations in workload or worker availability, the function (Doums et al. 2002; Amdam et al. 2005; possibility remains that they represent an emergency Schmid-Hempel 2005; Armitage and Boomsma reserve force to prevent interruptions in colony activ- 2010). Locomotor function (e.g., movement speed) ity in such extreme cases (Hasegawa et al. 2016). also tends to decrease with age in many animals, including insects (e.g., mice [Forster et al. 1996], shrews [Punzo and Chavez 2003], fruit flies, cock- Hypotheses for the function of inactive workers roaches, and locusts [Ridgel and Ritzmann 2005]). tested here However, in some cases, honey bees show no decline Here, we simultaneously test predictions of five hy- (Rueppell et al. 2007) or even an increase (Schippers potheses (Table 1). All of these hypotheses are “func- et al. 2006) in performance with age, and have been tional” (i.e., “ultimate” [Tinbergen 1963]), providing shown to increase cognitive and learning abilities as potential answers to the question of why inactive they age (Farris et al. 2001). Thus, overall, it is far workers are evolutionarily maintained in social in- from obvious whether immaturity or senescence or sects. Specifically, we evaluate the previously pro- both contribute to inactivity in ant workers. posed hypotheses that inactive workers are either Worker age can be approximated by using differ- immature (a. Immaturity hypothesis) or senescent ent measures, which can be helpful in long-lived ant individuals (b. Senescence hypothesis) (both pro- species (such as T. rugatulus), where actually moni- posed by Fresneau [1984]andCorbara et al. toring workers over their whole lifetime is difficult [1989]) who are constrained in their ability to (workers may live >2 years). In many ant species, work. We also consider the hypothesis that inac- young workers tend to have more developed ovaries tive workers are avoiding risky and energetically (e.g., ovariole length and oocyte presence and size) costly tasks and instead investing in their own because they have yet to be fully suppressed by the 652 D. Charbonneau et al.

Table 1 Predicted (P) and observed (O) outcomes of behavioral, physiological, and morphological measures of inactive workers compared with their nestmates

Effect on inactive workers Hypothesis (a) Immaturity (b) Senescence (c) Reproduction (d) Repletism (e) Communication (f) Pace-of-life Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019 Predicted/Found POPOP O POP O PO Walking speed Fast Slow Slow Slow Slow Slow Slow Slow Dist. from center Near Near Far Near Far Near Spatial fidelity zone Small Small Body size Large NS Large NS Corpulence High High Low Low High High High High Network centrality Low Low High Low Behavioral repertoire Small Small Large Small Small Small Ovariole length Long NS Short NS Long NS Notes: The most promising hypotheses are the pace-of-life, worker age, worker reproduction, and workers acting as repletes. Predictions that were supported are highlighted in dark grey and those that were rejected in light grey, while inconclusive results (neither supported nor rejected) are highlighted in medium grey. queen and/or have not yet atrophied with age (Otto producing their own worker-laid male eggs 1958; Weir 1958a, 1958b; Ceusters et al. 1981; Billen (Beekman et al. 2000; Jandt and Dornhaus 2011). 1982; Fresneau 1984). Furthermore, both the age/ However, other studies show no relationship be- temporal polyethism (Seeley 1982; Gordon 1996; tween worker reproduction and inactivity (Cole Tschinkel 1999) and the foraging-for-work (Franks 1981, 1986; Ishii and Hasegawa 2013). Worker re- and Tofts 1994) task allocation models predict that production may also be linked to spatial fidelity pat- younger workers should tend to hold spatial posi- terns within the nest (Jandt and Dornhaus 2011), tions closer to the nest center and brood pile, nearer which can be driven by workers seeking to decrease to where they first emerge as adults, and engage their exposure to the queen’s reproductive suppres- preferentially in safer, more centrally located tasks sion pheromones (Keller and Nonacs 1993; Brunner (e.g., brood care). As workers age, they frequently et al. 2011), policing from nestmates (Stroeymeyt progress to less central and potentially more risky et al. 2007) or worker age (Fresneau 1984). These locations/tasks (e.g., nest maintenance and foraging). studies show a relationship between activity level Such relationships between worker age, ovarian de- and ovary development, however they do not di- velopment and task allocation (Kuehbandner et al. rectly observe inactive workers laying eggs or test 2014), and task allocation and spatial organization the viability of worker-laid eggs; instead, they use have been shown in other species in the genus measures of reproductive potential (e.g., ovary or Temnothorax (Sendova-Franks and Franks 1995). In oocyte development). Hence, the possibility that some cases, as worker progress from more central to these workers were laying trophic eggs (unfertilized less central positions and tasks, they gain new tasks eggs that serve as food) cannot be excluded. In this rather than switch specializations, effectively increas- case, egg-laying workers would not be acting self- ing their task repertoires with age (Seid and ishly, but rather be serving a role by providing Traniello 2006). Thus, if inactivity is the result of food stores for the colony (Ho¨lldobler and Wilson age (either immaturity or senescence), then ovary 1990; Perry and Roitberg 2006). development, spatial position, and behavioral reper- toires are predicted to correlate with worker inactiv- d. Communication hypothesis ity in specific ways (Table 1). Another possibility is that inactive workers are per- forming a function for the colony that is not easily c. Worker reproduction visually recognized. For example, inactive workers There is already some support for the hypothesis that may be facilitating communication by acting as an inactivity results from a worker-colony conflict in information relay (Bonabeau et al. 2000; O’Donnell which inactive workers may be selfishly resting and and Bulova 2007), for example, facilitating task allo- avoiding work, particularly risky tasks such as forag- cation. There is evidence that a subset of workers ing, to conserve energy which is then shunted into may specialize on sampling global information by Lazy ants and their role 653 patrolling the nest (Johnson 2008) and that these larger, have reduced behavioral repertoires and spa- highly interactive individuals may expedite informa- tial fidelity zones, and lower mobility. tion spread, for example about a food source, throughout the colony (Pinter-Wollman et al. f. Pace-of-life hypothesis

2011). If inactive workers are acting as information Inactivity may also result from “pace-of-life” varia- Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019 relays, we should expect them to hold highly central tion between workers. In the personality and behav- positions in colony interaction networks. ioral syndrome literature, inter-individual variation within a species has often been suggested to be linked to metabolic scaling and/or life-history strat- e. Repletism hypothesis egies, such that individuals within a population may Inactive workers may also be acting as food stores vary along a fast to slow paced continuum (Stamps (Blanchard et al. 2000), a behavior that allows colo- 2007; Wolf et al. 2007; Re´ale et al. 2010); see also nies to mitigate the effects of stochastic or predict- “tempo” literature in social insects (Oster and able (e.g., seasonal) variation in food availability by Wilson 1978; Cole 1992; Burkhardt 1998; Franks having energetic reserves on hand (Stumper 1961; et al. 1999; Mason et al. 2015). This is sometimes Tschinkel 1987; Yang 2006). Social insects have the called the pace-of-life hypothesis (Pearl 1928), and ability to store fat in specialized organs called the fat supposes a “syndrome” of linked morphological, bodies (Tschinkel 1987) and liquid food (typically physiological, and behavioral traits. It predicts that carbohydrates) in their “social stomach,” the work- larger body size, slower mass-specific metabolic rates, ers’ crops (Børgesen 2000). This results in increased and decreased activity should be associated with slow corpulence (i.e., larger, more distended gasters). paced strategies (Ricklefs and Wikelski 2002; Re´ale However, corpulence can also result from reproduc- et al. 2010). For example, rapid growth and high tive development/presence of oocytes in the ovaries fecundity (best suited to conditions of high external (Heinze 1996), and the presence of parasites and/or mortality) should require a more active lifestyle, and disease (Carney 1969). Workers specialized on food does not allow for reaching very large body sizes. storage, known as “repletes,” have been found in Empirical evidence of such relationships within spe- most ant genera (e.g., Carebara, Prenolepis, and cies has been found in mammals (Montiglio et al. Proformica [Ho¨lldobler and Wilson 1990], 2014; Careau et al. 2015), birds (Patrick and Leptothorax, Myrmica, and Lasius [Børgesen 2000]). Weimerskirch 2015), fish (Fu¨rtbauer et al. 2015; Honey pot ants (Myrmecocystus sp.), an extreme ex- Rosenfeld et al. 2015), amphibians (Urszan et al. ample of repletism, have highly specialized workers 2015), and arthropods (crabs and aphids) (Bridger whose sole purpose (and activity) is to store food for et al. 2015; Schuett et al. 2015). the colony; but food-storing workers need not have In social insects, workers largely do not reproduce these extreme traits (Ho¨lldobler and Wilson 1990). (though see above), so one may think that adaptive Repletes have been shown to typically be larger than life-history strategies are not relevant. However, the their nestmates (Hasegawa 1993; Ho¨lldobler and main interest of this hypothesis lies in the fact that it Wilson 1990; Tsuji 1990) (but see [Børgesen motivates a relationship between a series of behav- 2000]), develop and remain inside the nest ioral and physiological traits because these traits (Børgesen 2000), typically have reduced mobility complement each other. Thus, it is also possible (Sempo et al. 2006), and do not engage in other that to increase worker lifetime work output (rather colony tasks (e.g., foraging [Kondoh 1968; Porter than lifetime reproductive output) these same traits and Jorgensen 1981] and defensive tasks [Lachaud need to be linked. Lifetime work output of workers et al. 1992]). In the ant Temnothorax albipennis, is the trait that is presumably under colony-level workers have been shown to vary in corpulence (typ- selection in social insect colonies. Thus, variation ically measured as gaster distension or dry weight, in worker inactivity could be a result of intra- sometimes relative to body size) with season and colony variation in worker “pace-of-life,” where in- with age (Blanchard et al. 2000; Robinson et al. active workers represent the slow paced end of the 2009). Leaner workers have also been shown to re- spectrum. Colonies may benefit both from such a spond more quickly to increases in foraging demand pace of life syndrome (i.e., from a linkage among (Toth and Robinson 2005; Toth et al. 2005; traits within workers) and from intra-colony diver- Robinson et al. 2009) as well as be generally be sity (i.e., from producing both slow and fast work- more active (Blanchard et al. 2000). Thus, if inactive ers). If inactive workers evolved through selection on workers act as repletes, they should be more corpu- the classic pace-of-life syndrome, we should expect lent than their nestmates; they may also be generally inactive workers to be larger in size than their 654 D. Charbonneau et al. nestmates and have slower walking speeds (as a cavities where the ants could nest (either a small proxy for metabolic rate [Nespolo and Franco central cavity with a small entrance, or a back wall 2007]). made of cardboard around which the ants could Here we use an extensive dataset including spatial build a stone wall from small ceramic pellets).

tracking data, morphological measurements (includ- Such nests have been used in previous studies to Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019 ing ovary dissection data), and behavioral data for emulate the small rock crevices they inhabit in the workers in the ant T. rugatulus to dissect the traits of field (Charbonneau et al. 2014). The nests were kept “lazy” ants. We use these data to identify possible in small plastic containers that had their walls functional explanations for ant worker inactivity, and painted with fluon (BioQuip “Insect-a-slip”) to pre- to quantitatively assess the potential relative contri- vent the colony from escaping. Colonies were given butions of each of these hypotheses. We also use our water and food ad libitum. Water-filled plastic test dataset to determine which traits form a “syndrome” tubes, stoppered with cotton balls, were provided with inactivity. Relative contributions of different semiweekly, and 2 mL Eppendorf tubes of honey wa- proposed, not mutually exclusive explanations for ter and 10 frozen adult Drosophila flies were pro- inactivity can only be compared when multiple traits vided weekly. They were kept on a regular 12 h are measured simultaneously on the same individ- light cycle (8 a.m.–8 p.m.), and at consistent tem- uals. Specifically, we sought to determine whether peratures (21–24 C) and humidity (20–25% rel- highly inactive workers are younger (a. Immaturity ative humidity). Workers in colonies were typically hypothesis) or older (b. Senescence hypothesis) marked (see below) within 1 week of collection, and workers by using data for reproductive potential as filmed within 2 weeks after marking. proxies for age. We also test whether inactive work- ers have smaller behavioral repertoires, higher cor- pulence, and slower walking speeds as predicted for Behavioral data younger workers (and the opposite predictions for For each colony, we marked each worker with a older workers). If inactives are acting as communi- unique combination of four paint spots (one on cation hubs (c. Communication hypothesis) they the head, one on the thorax, and two on the abdo- should hold central positions in colony interaction men), so that they could be individually identified networks, while if inactives act as repletes (d. and tracked. Videos (5 min long) of normal colony Repletism hypothesis), we expect them to be more activity were taken once each afternoon (between corpulent, slower, have larger body sizes, smaller 12 p.m. and 5 p.m.) with an HD camera equipped behavioral repertoires, and minimal spatial fidelity with a macro lens. Videos were analyzed by tracking zones. Finally, if inactive workers represent the the task each worker performed for each second of “slow” end of within-colony variation in “pace-of- the video (see Table 2 for a complete list of tasks and life” (e. Pace-of-life hypothesis), they should have definitions). Each colony was filmed 3 or 4 times larger body sizes and slower walking speeds (proxy (see Supplementary Table S1) with an interval of for metabolic rate) (detailed hypotheses and their 0–7 days between videos. Our previous work on T. predictions are summarized in Table 1). rugatulus using identical methods to observe individ- ual workers over 24 h periods and multiple days has shown that 3 5 min observations is sufficient to Methods obtain data representative of individual time budgets We collected 33 colonies of T. rugatulus ants during and that individual behavior is consistent over at summers between 2012 and 2015 from the Santa least 2–3 weeks (Charbonneau and Dornhaus Catalina Mountains near Tucson, Arizona, USA 2015b). That work also showed that data from after- (mean colony size ¼ 57.99 workers, which is toward noon observations can predict overall daily behav- the lower end of the typical range for this species ioral patterns, since there are no cohorts of ants [Bengston and Dornhaus 2013]; see Supplementary consistently active at different times of the day Table S1 for detailed demographic data for each col- (Charbonneau and Dornhaus 2015b). A total of ony, as well as collection dates and data types avail- 105 five-min videos were analyzed by 20 different able). Colonies were typically allowed to emigrate to observers in total; data from each video were spot artificial nests in the laboratory within 1–3 days of checked by a single person to ensure uniformity of collection. Artificial laboratory nests were composed behavioral observations. of two glass slides (76.2 50.8 mm) separated by a Tasks were broadly classified as either “active” 2-mm thick piece of cardboard (Charbonneau and (e.g., brood care, see Table 2), “undifferentiated” Dornhaus 2015b). The cardboard had pre-formed (walking inside the nest with no clear task), or Lazy ants and their role 655

Table 2 List of possible behaviors observed during video analysis, second. Distances and speeds were scaled using the their broad class of activity, codes, and detailed descriptions dimensions of known objects in the video (e.g., the Class Task Definition width of the glass slides used to build nests). Temnothorax workers tend to have spatially lim- Active Foraging Located outside of the nest

(foraging, carrying food, or ited movement zones that are thought to be linked Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019 stones), or manipulating a to worker age and task (Sendova-Franks and Franks stone in any way (moving, 1995). We established spatial fidelity zones by build- pushing, pulling), or return- ing a convex hull around all observed worker posi- ing to the colony from for- aging and performing tasks tions within each video and measuring the area of associated with foraging the convex hull and the distance of the hull’s cen- (e.g., trophallaxis) troid to the colony center defined as the centroid of Brood care Manipulating brood (feeding, all brood in the nest. By limiting spatial fidelity grooming, moving) zones to times where workers were inactive, we ob- Self-grooming Grooming itself tain a measure that is potentially independent of the Grooming other Grooming another ant particular activities chosen by the worker. We also (giver) measured the mean distance of workers from the Grooming other Be groomed by another ant colony center while workers were wandering inside (receiver) to estimate the overall range that workers move Trophallaxis Receive or give liquid food to/ within the nest. from another adult ant Movement speed, a measure of locomotor func- Eating Feeding on drosophila inside tion (and a proxy for metabolic rate [Nespolo and nest (brought back by foragers) Franco 2007]), was calculated as the mean speed of workers during bouts of wandering inside the nest Undifferentiated Wandering Anytime an ant is mobile in- inside nest side the nest wall and not (defined as any time an ant is mobile inside the nest engaged in any “active” task walls and not engaged in any “active” task; Table 2). Inactive Inactive Immobile and not engaged in Because of occasional tracking tool errors (96% ac- any “active” task curacy [Poff et al. 2012]), we removed all movement Note: For every second of analyzed video, each ant has one of these speeds greater than 3.356 mm/s (corresponding to behaviors attributed to it. (Similar to Charbonneau and Dornhaus twice the median worker head to petiole length, 2015a). which was 1.678 mm) which were deemed unlikely (pre- and post-removal of extreme values: walking speed mean ¼ 0.407 mm/s vs. 0.378 mm/s, me- “inactive” (completely immobile), comparable to the dian ¼ 0.192 mm/s vs. 0.190 mm/s, SD ¼ 0.655 vs. broad classification in Cole (1986) and as used by 0.490, respectively). We also excluded all workers Charbonneau et al. (2014) and Charbonneau and who were seen walking for <10 s (number of seconds Dornhaus (2015b). If <10 s separated two events of observed per worker mean ¼ 122.0 s, brood care, feeding, or foraging, or 20 s for building, median ¼ 99.5 s). the task was considered to be uninterrupted. We determined interaction networks based on Behavioral repertoire size (akin to “task repertoire,” proximity (i.e., ants within one median ant head to but includes wandering inside and inactivity as pos- petiole length—1.678 mm—of each other were con- sible behaviors) was defined as the sum of behavioral sidered to be interacting). Proximity is a commonly states that each worker spent any time on. used proxy for interactions in studies of social insect interaction networks (Otterstatter and Thomson Spatial data 2007; Moreau et al. 2011; Jeanson 2012; Mersch We used semi-automated tracking software that et al. 2013), as is using body length to infer interac- tracked the spatial positions of individual workers tions between workers (Otterstatter and Thomson in the same videos we used for behavioral analysis 2007; Moreau et al. 2011; Jeanson 2012). Although to measure worker spatial fidelity, movement speeds, proximity data are likely noisier that data derived and proximity interaction networks (Poff et al. 2012; from physical interaction (though proximity net- Nguyen et al. 2014). The software provides x, y co- works do capture a wider range of contact modali- ordinates for each worker at each frame of the video ties), proximity data have been shown to (at 24 fps) as well as the orientation of the worker. approximate interaction data within reasonable To reduce data redundancy and noise, we used the bounds (Farine 2015; though see Castles et al. median x, y coordinates of the 24 frames within each 2014), and the networks that we generated here 656 D. Charbonneau et al. using proximity data are used only to determine the et al. submitted for publication; Ceusters et al. network centrality of worker task groups. 1981; Fresneau 1984).

Morphological data Worker task groups Worker body sizes and abdomen widths were ob- Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019 Previous work has shown that workers of T. rugatu- tained by measuring the length of the dorsal line lus ants can be grouped into four distinct task from the mouthparts to the petiole (length of the groups, or behavioral castes: inactives, foragers, sclerotized body parts) from screenshots of the videos. nurses, and walkers (Charbonneau and Dornhaus These measurements were validated by comparing them 2015b). These groups specialize (i.e., spend more with head width measures obtained under a dissecting time than their nestmates) on the behaviors of inac- scope (LM: df ¼ 103, F ¼ 87.44, estimate [slope] ¼ tivity, foraging and building, brood care, and wan- 2.31, P < 0.0001, R2 ¼ 0.459; Supplementary Fig. S1). dering inside, respectively (Table 2). Here, in Gaster widths were obtained by measuring worker gas- addition to using the continuous measure of ters at their widest point on a screenshot of the videos. “amount of time spent inactive,” we use these tasks Body size and abdomen widths were measured once on groups to identify the most inactive workers, and a single frame for each of the videos (3–4 per col- compare them to their nestmates in the other three ony; see Supplementary Fig. S1) and averaging val- task groups. ues for the same individual across videos. Worker task groups were established using princi- Corpulence was defined as the ratio of head to pet- pal component and hierarchical cluster analyses iole length to gaster width to account for body size (prcomp and hclust, base “stats” package in R variation between workers (as in Robinson et al. Version 3.1.2; same methods as Charbonneau and [2012] who also show a strong positive correlation Dornhaus [2015b]). The PCA was used to determine between gaster dry mass and gaster width in T. the number of task groups (4), based on the total albipennis; R2 ¼ 0.77, N ¼ 80, P < 0.0001). contribution of each task to the principal compo- nents (tasks with absolute sum >1; Supplementary Ovary dissections Fig. S2) and the hierarchical cluster analysis to sep- Ovaries were extracted and oocytes measured for arate workers along similarities in time spent on workers from 8 of the 33 colonies immediately after tasks (Supplementary Fig. S3; see Supplementary the last video was recorded. Workers were placed in Fig. S5 for the distribution of worker inactivity in a sealed CO2-filled vial for 30 min, then placed into each task group and Supplementary Fig. S6 for the a petri dish containing Tween-20 phosphate buffer. proportion of workers in each task group for each Under a dissecting microscope, Ultrafine forceps colony). Workers were clustered into (1) inactives, were used to grasp and pull the most posterior ster- (2) foragers, (3) nurses, and (4) walkers. To avoid nite free from each ant’s gaster, followed by the the effects of sporadic (low repeatability) tasks, we newly-exposed underlying stinger apparatus while performed the clustering analysis only on highly con- another pair of forceps grasped and held the ant’s sistent (high repeatability) tasks: inactivity, wander- petiole. Ovaries were separated from other tissues ing inside, foraging, and brood care. Principal attached to the stinger mechanism with ultrafine for- component analyses and clustering analyses have ceps (see Dolezal and Brent [2009] for detailed dis- been used to describe colony organization in prior section methods), then photographed through the studies (Lenoir and Mardon 1978; Retana and Cerda microscope at approximately 80 magnification 1991), including in T. rugatulus (Charbonneau and next to a micrometer placed into the petri dish. Dornhaus 2015b). The number of visible, whitened oocytes in all work- Data used for both the PCA and hierarchical clus- ers’ ovaries was counted from the photographs. ter analysis are aggregated individual data (i.e., a Yellow bodies were not seen in any workers, which single value per trait per individual) from workers may indicate that oocytes were being produced as of all 33 colonies (PCAs on individual colonies show trophic eggs rather than for reproduction. similar patterns to the pooled data; see However, we also did not find yellow bodies in Supplementary Fig. S4). Workers that did not appear queen dissections which may indicate issues with in at least two videos were excluded from the identifying or dissecting them. As measures of repro- analyses to ensure sufficient data for representative ductive potential, we counted the number of visible individual time budgets. In order to account for oocytes, measured the diameter of all oocytes, and inter-colony variation in inactivity levels, the data the length of all worker ovarioles (Charbonneau for each colony were centered (task mean subtracted Lazy ants and their role 657 from task values) and scaled (task values divided by group (mean speed 0.205 mm/s/mm), followed by task standard deviation) separately before being nurses (0.218 mm/s/mm), walkers (0.251 mm/s/ pooled and clustered. Rotations were not necessary mm), and foragers (0.334 mm/s/mm). Note that all for the PCA as only seven variables were used and speeds are measured during “wandering” (mobile,

the spread between variable vectors was good. The but not engaged in any other apparent task; Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019 first three components of the PCA were retained Table 2), thus not directly dependent on what tasks (determined using parallel analysis [Franklin et al. are more often performed by workers in each of these 1995]). These explained 64.5% of inter-worker vari- groups. Post hoc analyses (Tukey) showed that inac- ation in time spent on tasks. tives and nurses were significantly slower than for- agers, while walkers were not significantly different Statistical analyses from either inactives or foragers (Fig. 1,right). Data were analyzed using linear mixed-effects models (LME; lme() R function, P-values obtained from Spatial position anova() function in “base” package) for most data The size of worker spatial fidelity zones (convex hull and generalized LME (GLMM; glmer() R function, of all spatial positions observed) was negatively cor- P-values obtained from Anova() function in “car” related with worker inactivity. Overall distance from package) for data following a poisson distribution. nest center (convex hull centroids) and mean dis- Mixed models were used to account for inter-colony tance from nest center when inactive were also neg- variation (i.e., colony as random effect). Specific atively correlated to time spent inactive, but there models for each analysis are indicated in the figure was no relationship between inactivity and mean dis- legends. Analyses were performed on mean individual tance from nest center when wandering (Fig. 2a). values (e.g., mean proportion of time spent inactive Inactives, as defined by the cluster analysis, had across all videos). Boxplots used to represent data the smallest spatial fidelity zones (convex hull built show the lower and upper quartiles (box), median using all observed movement), followed by nurses, (horizontal line within box), and extremes (whiskers). walkers, and foragers (Fig. 2b, upper left). Worker distance from the nest center (measured as either the Results centroid of its convex hull, mean distance from the A total of 1477 individual workers were classified nest center while wandering inside, or mean distance into four distinct task groups: inactives (605 workers, from center while inactive) show nurses as the most 40.8%), nurses (249 workers, 16.8%), walkers (483 central, followed by inactives and walkers in inter- workers, 32.6%), and foragers (145 workers, 9.8%; mediate positions, and foragers as the least central Supplementary Fig. S3). Colonies had an average of (Fig. 2b, upper right and both bottom figures). 19.21 inactive workers (median ¼ 20, SD ¼ 9.77) which corresponded to a mean proportion of work- Body size ers classified as inactive across colonies of 0.32 (me- Workers varied in their body sizes (measured as dian ¼ 0.31, SD ¼ 0.11). head-to-petiole length) by nearly a factor of 2, rang- ing from 1.21 mm to 2.27 mm for head-to-petiole Walking speed length, though most worker body sizes fell within a Workers varied in their relative (body-size-corrected) smaller range (1st quartile 1.58 mm, 3rd quartile movement speed when wandering inside the nest 1.77 mm; Supplementary Fig. S7). No relationship (Supplementary Fig. S6). Walking speed was calcu- was found between body size and inactivity lated relative to body size because we were interested (P ¼ 0.394; Fig. 3, left). Foragers were significantly in walking speed as an indicator of individual “effort” smaller than nurses and walkers who were the largest and as proxy for metabolic rate for the pace-of-life workers; however, inactives were not significantly hypothesis (e.). Time spent inactive was negatively different from either group (Fig. 3, right). correlated to mean relative walking speed across all workers (Fig. 1, left). We found no relationship be- Corpulence tween worker body size and actual movement speed Inactivity increased with worker corpulence (gaster (not relative to body size) while wandering inside the width at the widest point/head-to-petiole length), in- nest (df 138, F ¼ 1.767, P ¼ 0.186). dependently of the presence of oocytes in worker Relative walking speed also significantly differed ovarioles, though workers with oocytes in their ovar- across the worker groups as defined by the cluster ioles were more inactive than their nestmates with- analysis. Inactive workers were the slowest task out oocytes (Fig. 4a). Corpulence was also found to 658 D. Charbonneau et al. Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019

Fig. 1 Walking speed and inactivity. (Left) Inactivity is negatively correlated to mean worker walking speed relative to body size (df ¼ 138, F ¼ 9.952, Marginal R2 ¼ 0.032, Conditional R2 ¼ 0.548). Walking speed is measured as the mean movement of workers observed “wandering inside” divided by their head-to-petiole length. Symbols represent the different task groups to illustrate how these are distributed across inactivity levels and walking speeds (task group is not taken into account in this analysis). *Model: LMM, fixed: Inactivity Mean Speed, random: Colony. (Right) Average walking speed was significantly higher in foragers than in inactive workers and nurses (df ¼ 136, F ¼ 5.620). Walking speed is measured as the mean movement of workers observed “wandering inside” divided by their head-to-petiole length. *Model: LMM, fixed: Mean Speed Worker clusters, random: Colony. be negatively correlated with walking speed across all have workers with oocytes that have significantly workers (Fig. 4b). longer ovarioles (Fig. 7c). Furthermore, inactives and nurses were signifi- Because differences within task groups in other cantly more corpulent than walkers, and foragers measures of reproductive potential such as oocyte were significantly less corpulent than any other task presence may indicate important within group dif- group (Fig. 4c). ferences, such as stratified age classes or subsets of selfishly reproducing workers, we tested for signifi- Proximity networks cant differences between groups with and without oocytes independently. Inactivity was negatively correlated with multiple Workers with oocytes were found in each task measures of network centrality (Fig. 5a). Inactives group, but were in significantly greater number were not the best-connected worker group within than expected in inactives, and significantly lower colony proximity networks, as measured by degree numbers than expected in foragers (Chi-square test: centrality, eigenvalue centrality, betweenness central- v2 ¼ 28.776, P < 0.001; post hoc analysis of absolute ity, or closeness centrality. Foragers were consistently adjusted residuals >2[Agresti 2007], Fig. 7d). the least connected, while walkers were consistently among the most central workers (Fig. 5b). Discussion Behavioral repertoire Analyzing the activity profiles and other traits of over 1400 T. rugatulus ant workers, we find that in- There was a negative relationship between inactivity active workers walk more slowly, have smaller spatial and behavioral repertoire size (total count of tasks fidelity zones located nearer the nest center, are more workers engaged in at least once, including inactivity corpulent, are less well connected in the colony’s and wandering inside; Fig. 6, left). Inactive workers interaction network, have the smallest behavioral had the smallest behavioral repertoire, followed by repertoires, and are more likely to have oocytes walkers, nurses, and foragers (Fig. 6, right). (see also [Charbonneau et al. submitted for pub- lication]) than more active workers. Inactivity does Reproductive potential not relate to worker body size nor ovariole length. Inactivity was not correlated with mean ovariole These results are largely consistent with the hypoth- length (P ¼ 0.290; Fig. 7a) and there were no signif- eses (a.) that inactive ants are immature, (c.) that icant differences in worker ovariole lengths between they function as repletes, and (e.) that workers in a any of the task groups (P ¼ 0.138; Fig. 7b). However, colony vary in their “pace-of-life.” Our results are analyses within each task group comparing ovariole inconsistent with predictions from the hypotheses lengths of workers with oocytes to those of workers that (b.) inactive workers are senescing or that (d.) without oocytes show that both inactives and nurses they function as communication hubs (Table 1). Lazy ants and their role 659 Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019

Fig. 2 Spatial position and inactivity. (a) There was a negative relationship between time spent inactive and spatial fidelity zone size (convex hull built using all observed movement), overall distance from nest center (convex hull centroid) and mean distance from nest center when inactive, but not mean distance from nest center when wandering (Convex hull area: df ¼ 431, F ¼ 154.466, Marginal R2 ¼ 0.206, Conditional R2 ¼ 0.465; Distance of convex hull centroid from center: df ¼ 430, F ¼ 12.815, Marginal R2 ¼ 0.019, Conditional R2 ¼ 0.404; Mean distance when inactive: df ¼ 320, F ¼ 6.279, Marginal R2 ¼ 0.014, Conditional R2 ¼ 0.348; Mean distance when wandering: df ¼ 238, F ¼ 2.451, Marginal R2 ¼ 0.009, Conditional R2 ¼ 0.330). Symbols represent the different task groups to illustrate how these are distributed across inactivity levels and walking speeds (task group is not taken into account in this analysis). *Model: LMM, fixed: Inactivity Spatial Measure, random: Colony. (b) Average spatial fidelity zone size (convex hull built using all observed movement) was smallest in inactives, followed by nurses, walkers, and foragers. Nurses spent most of their time (overall, inactive time only, and wandering inside only) near the nest center, while inactives and walkers held intermediate positions, and foragers were furthest from the nest center (Convex hull area: df ¼ 429, F ¼ 36.813; Distance of convex hull centroid to colony center: df ¼ 429, F ¼ 42.244; Mean distance from colony center when inactive: df ¼ 318, F ¼ 14.695; and when wandering inside: df ¼ 236, F ¼ 13.108). *Model: LMM, fixed: Spatial Measure Worker clusters, random: Colony.

Fig. 3 Body size and inactivity. (Left) There is no significant relationship between worker head-to-petiole length and the proportion of time spent inactive across workers overall (df ¼ 1238, F ¼ 0.727, Marginal R2 < 0.001, Conditional R2 ¼ 0.247). *Model: LMM, fixed: Inactivity HeadToThorax_mm, random: Colony. (Right) Average body size was greatest in walkers (1.697 mm), followed by nurses (1.683 mm), inactives (1.680 mm), and foragers (1.654 mm). Foragers were significantly smaller than walkers and nurses, while inactives were not significantly different from any other task group (df ¼ 1236, F¼ 4.296). *Model: LMM, fixed: HeadToThorax_mm Worker clusters, random: Colony.

Age and inactivity (a. Immaturity hypothesis and smaller behavioral repertoires, and hold positions b. Senescence hypothesis) near the nest center (second only to nurses on the Our results provide some indication that inactive brood pile). However, their ovarioles were not workers may be young and immature. As expected significantly longer than those of their nestmates, under this hypothesis, they are more corpulent, have which would be expected if they are young 660 D. Charbonneau et al. Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019

Fig. 4 Corpulence and inactivity. (a) Corpulence positively correlates with inactivity, independently of the presence of oocytes (Corpulence: df ¼ 92, F ¼ 12.075, P < 0.0001; Oocyte presence: df ¼ 92, F ¼ 12.282, P < 0.0001; Marginal R2 ¼ 0.179, Conditional R2 ¼ 0.307; interaction was non-significant (P ¼ 0.358) and so was not included in the model). *Model: LMM, fixed: Inactivity Corpulence_mm þ OocytePresence, random: Colony. (b) Worker corpulence was negatively correlated with mean worker walking speed (df ¼ 138, F ¼ 5.681, Marginal R2 ¼ 0.054, Conditional R2 ¼ 0.131). *Model: LMM, fixed: MeanSpeed_noBody Corpulence_mm, random: Colony. (c) Average corpulence was greatest in inactives (0.0152), followed by nurses (0.0150), walkers (0.0147), and foragers (0.0141). Inactives and nurses were significantly more corpulent than walkers and foragers (df ¼ 1298, F ¼ 23.667). *Model: LMM, fixed: Corpulence_mm Worker clusters, random: Colony.

(Ceusters et al. 1981; Billen 1982; Fresneau 1984), and nurses (Fig. 7c). An inactive worker group com- and their walking speed was the lowest among the posed of juvenile and senescent workers has been task groups, though increased locomotion might be previously suggested in a study showing identical expected in younger workers (Ridgel and Ritzmann trends in ovary development in the ant Neoponera 2005). Low walking speeds could have indicated that obscuricornis (inactive workers composed of two sub- inactive workers were senescent rather than imma- groups: some with long ovarioles and oocytes vs. ture; however, their small behavioral repertoire and some with short ovarioles and no oocytes high corpulence contradict expectations for the (b.) [Fresneau 1984]). Senescence hypothesis. Older workers may potentially be driving the re- It is possible that inactive worker s, as a group, are lationship between inactivity and slow walking speed composed of both immature and senescent workers. (if senescent workers walk much more slowly than The distribution of inactive worker ovariole lengths younger workers), but another likely scenario is that is not obviously bimodal, but does show a dip near the increased corpulence of young reproductive/ the center of the distribution which may be the re- replete workers may cause reduced mobility (Sempo sult two closely overlapping distributions et al. 2006). Nonetheless, the majority of the inactive (Supplementary Fig. S9). Thus, inactives (and task group appears to be younger workers (31 with nurses) may be composed of two distinct age distri- oocytes vs. 8 without). Thus, they are likely driving butions: younger workers who have longer ovarioles the main effects observed for the inactive task group. and are more likely to have oocytes, and older with significantly shorter ovarioles (Ceusters et al. 1981; Worker reproduction (c. Reproductive hypothesis) Fresneau 1984) and no visible oocytes. Indeed work- Concurrently with age, worker reproduction appears ers with oocytes have longer ovarioles in inactives to play a role in worker inactivity. Indeed, inactive Lazy ants and their role 661 Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019

Fig. 5 Interaction networks and inactivity. (a) Inactivity is negatively correlated to multiple measures of network centrality (Degree centrality: df ¼ 181, F ¼ 11.803, Marginal R2 ¼ 0.087, Conditional R2 ¼ 0.386; Eigenvalue centrality df ¼ 181, F¼ 7.448, Marginal R2 ¼ 0.035, Conditional R2 ¼ 0.285; Betweenness centrality: df ¼ 181, F ¼ 23.347, Marginal R2 ¼ 0.074, Conditional R2 ¼ 0.433; Closeness centrality: df ¼ 181, F¼ 8.377, Marginal R2 ¼ 0.060, Conditional R2 ¼ 0.444). *Model: LMM, fixed: Inactivity Centrality measure, random: Colony. (b) Across multiple measures of network centrality, inactives, and foragers were consistently among the least connected, while walkers were the most connected (Degree centrality: df ¼ 179, F ¼ 7.603; Eigenvalue centrality: df ¼ 179, F ¼ 7.983; Betweenness centrality: df ¼ 179, F ¼ 6.386; Closeness centrality: df ¼ 179, F ¼ 8.766). *Model: LMM, fixed: Centrality measure Worker clusters, random: Colony.

Fig. 6 Behavioral repertoire and inactivity. (Left) There is a negative relationship between behavioral repertoire size (number of tasks observed performing) and time workers spent inactive (df ¼ 1546, v2 ¼ 212.10). Model: GLMM Poisson distribution, fixed: NumBehav Inactivity, random: Colony. (Right) Inactive workers had the smallest behavioral repertoires, followed by walkers, nurses, and foragers (df ¼ 1544, v2 ¼ 145.99). (Task group sample size: inactive ¼ 624, nurse ¼ 259, walker ¼ 513, forager ¼ 153). Model: GLMM Poisson distribution, fixed: NumTasks Worker clusters, random: Colony. workers were more likely to have oocytes in their zones and spent more time near the center of the ovaries than other task groups suggesting that they nest, which is surprising because reproductive work- may be laying eggs in order to produce males and ers would be expected to avoid reproductive sup- increase direct fitness. Furthermore, inactive workers pression by the queen. held less central position in colony interaction net- Inactive workers were also more corpulent, which works which may suggest that they are avoiding po- should be expected as the presence of oocytes in the licing behaviors of their nestmates (either via ovaries will increase the overall volume of worker aggressive behaviors or the removal of their brood). gasters. In addition, even when accounting for cor- However, inactives also had smaller spatial fidelity pulence that may result from storing food for later 662 D. Charbonneau et al. Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019

Fig. 7 Reproductive potential and inactivity. (a) Worker inactivity was not correlated with mean ovariole length (df ¼ 92, F ¼ 1.132, Marginal R2 ¼ 0.010, Conditional R2 ¼ 0.145). *Model: LMM, fixed: Inactivity MeanOvLen_mm, random: Colony. (b) No significant differences in mean ovariole length (mm) were found between any of the task groups (df ¼ 90, F ¼ 1.884; Task group sample size: inactive ¼ 33, nurse ¼ 18, walker ¼ 29, forager ¼ 18). *Model: LMM, fixed: MeanOvLen_mm Worker clusters, random: Colony. (c) Comparisons within task groups of mean ovariole length between workers with oocytes and without oocytes show that both inactives and nurses are composed of workers with oocytes that have significantly longer ovarioles than workers without oocytes (Inactive: df ¼ 28, F ¼ 7.660, P < 0.01; Nurse: df ¼ 14, F ¼ 9.809, P < 0.01; Walker: df ¼ 24, F ¼ 1.237, P ¼ 0.277; Forager: df ¼ 13, F ¼ 0.327, P ¼ 0.577; see (d) for sample sizes of each group). *Model: LMM, fixed: MeanOvLen_mm Worker clusters þ OocytePres, random: Colony. (d) Inactive workers are predominantly reproductive (6 vs. 27). The figure shows the number of workers with and without oocytes in each task group. (Chi-squared test: v2 ¼ 28.776, P < 0.001; post hoc analysis of adjusted residuals [Absolute Adj. Res. >2]); Agresti (2007) shows a significantly greater number of workers with oocytes than expected in inactives, and a significantly lower number of workers with oocytes than expected in foragers. use, workers with oocytes present were more inactive repertoires, as expected for food storage workers than workers in other task groups. (“repletes”). This may result from reduced mobility which may make engaging in strenuous tasks more Inactive workers as repletes (d. Repletism difficult and energetically costly, and from avoidance hypothesis) of riskier external tasks such as foraging, because the Our data show that inactive workers are (with loss of repletes would be costly to the colony (con- nurses) among the most corpulent workers. stituting the loss of not just a worker but also the Inactivity positively correlates with corpulence inde- food reserves she carries). The only prediction that pendently of the presence of oocytes in worker ovar- was not met was large body size, where we found no ioles, which may also increase corpulence. In significant relationship between inactivity and body addition, inactive workers had limited mobility size. Although repletes have been shown to be larger (low walking speed and small spatial fidelity zones). than their nestmates in some ant species (Ho¨lldobler Inactive workers also had limited behavioral and Wilson 1990; Tsuji 1990; Hasegawa 1993) this is Lazy ants and their role 663 not consistent across all species (Børgesen 2000), and Who are the “walkers”? so may not be a universal trait for repletes. Thus, Although our main goal was to describe inactive overall our data are highly consistent with the workers, our results provide additional insight into predictions of the “inactive workers as repletes” another task group whose function is still unclear: hypothesis. the “walkers.” Walkers are the second most inactive Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019 workers in the colony (after inactives), are not oth- Inactive workers as communication hubs erwise specialized on any specific task (based on the (e. Communication hypothesis) hierarchical clustering analysis), have spatial fidelity zones that are comparable to foragers (large), and One of the proposed roles for inactive workers was hold intermediate spatial positions on the outer pe- that they might play a role in colony-wide commu- riphery of the nest. They also have intermediate nication by gathering local information and sharing walking speeds. it globally throughout the nest, essentially acting as a Although “inactive” workers have very low cen- colony-level nervous system (proposed in Bonabeau trality in the interaction network, the opposite was et al. [2000]; O’Donnell and Bulova [2007] and re- true of “walkers.” Thus, walkers may form a group viewed in Charbonneau and Dornhaus [2015a]). of communication specialists that expedite informa- Inactive, and thus primarily immobile, workers tion spread, for example about a food source, may be ideally suited to this role, as in some search throughout the colony, essentially acting as a game scenarios (classic two-person zero-sum game colony-level nervous system (Bonabeau et al. 2000; from game theory), the optimal strategy to be found Johnson 2008; Pinter-Wollman et al. 2011). Indeed, by a searching partner is to be remain immobile “walking” behavior (referred to as patrolling) has (Alpern 1976, 1995). However, inactive workers are also been described in honey bees and been shown among the least central task groups in colony inter- to potentially play a role in gathering global infor- action networks and thus communication is an un- mation and sharing it with workers locally across the likely role for inactives. colony (Johnson 2008).

The (f.) “Pace-of-life” hypothesis Conclusion Inactivity was negatively correlated with walking Here we have shown that inactive workers in T. speed, and inactives were the slowest task group, rugatulus are not likely to be senescing, and are suggesting that they may have low metabolic rates. not acting as communication hubs, but instead are However, there was no relationship between worker likely to be both young and acting as food stores, or body size and activity level. We also show that inac- repletes. Inactive workers often have oocytes in their tive workers are among the least interactive (less ovaries, which may indicate selfish reproduction or central in interaction networks), while slow-paced production of trophic eggs (used as food for other individuals are generally thought to be highly social workers, and thus also an indirect food storage (Cote et al. 2010; Re´ale et al. 2010). However, this mechanism). Given the independent effects of oocyte relationship has mainly been shown non-eusocial an- production and corpulence, it seems likely that imals, such as marmots (Blumstein et al. 2009) and at least some inactives are egg layers (selfish or lizards (Cote and Clobert 2007) and it is unclear trophic), and that some function as repletes. whether and how high sociability may play out in Interestingly, our study also indicates that, while social insects. Thus, other than slow walking speed workers who spend most of their time “wandering” and low activity, there is little evidence that inactive around the colony also do not perform “tasks,” they workers are indeed slow-paced. are a distinct group from “inactive” workers. Indeed, However, there is empirical evidence for a wide “walkers,” unlike “inactives,” may be patrolling or range of traits associated with the pace-of-life syn- serving a communication function. Overall, our drome, and not all traits are necessarily found to be study thus points out that inactivity has physiologi- correlated in all species. Indeed, slow-paced individ- cal underpinnings and is not just a short-term con- uals have been shown to be longer lived, have slower sequence of ant workers not finding work. Since we growth rates, low aggressiveness, low metabolism, did not find body size (which is fixed in adults) to and high immune response (Cote et al. 2010; Re´ale correlate with activity level, it is unclear whether and et al. 2010). These traits can all be measured in social to what extent inactive workers may change into insects with relative ease. Thus, additional work may active workers. Given the physiological differences yet show intra-worker variation in pace-of-life. between highly inactive workers and other workers, 664 D. Charbonneau et al. it is however unlikely that inactive workers can di- National Science Foundation [Grant No. IOS- rectly, quickly, or fully replace workers in other tasks 1045239, IOS-1455983, and DBI-1262292 to A.D.]. in an emergency. This symposium was supported by the US National Our results also indicate that activity level differ- Science Foundation (IOS-1634027), the Division of

ences in ant workers may be part of a syndrome of Animal Behavior, Division of Comparative Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019 traits, where inactivity, corpulence, low walking Endocrinology, Division of Ecology and Evolution, speed, small spatial fidelity zones, and egg produc- and the Division of Neurobiology of the Society tion are associated. These traits may all be caused by for Integrative and Comparative Biology. young age, if young workers still retain many nutri- ents, well-developed ovaries, and perhaps incom- pletely hardened cuticles (reducing mobility), or Supplementary data these traits may be associated as part of a colony- Supplementary data available at ICB online. level strategy to reduce mortality risk (and body wear) of workers to store nutrients (both in their crop and in the form of trophic eggs). Worker inac- tivity could be associated with this set of traits as a References consequence (immature or replete workers are inac- Agresti A. 2007. An introduction to categorical data analysis. tive because they are less able to perform tasks or are 2nd ed. Hoboken (NJ): Wiley-Interscience. avoiding risk), or as a cause (inactive workers may Alpern S. 1976. Hide and seek games. Seminar, Institut Fu¨r use less resources, experience less wear, and thus ac- Ho¨here Studien, Wien, 26 July. cumulate more resources and age more slowly). A Alpern S. 1995. The rendezvous search problem. SIAM J Control Optim 33:673–83. longitudinal study tracking worker age along with oo- Amdam GV, Aase ALTO, Seehuus S-C, Fondrk MK, Norberg cyte production, corpulence, and activity level, is K, Hartfelder K. 2005. Social reversal of immunosenescence needed to disentangle cause and consequence in the in honey bee workers. Exp Gerontol 40:939–47. relationship of worker age and inactivity. In addition, Armitage SAO, Boomsma JJ. 2010. The effects of age and identifying the extent to which inactive workers actu- social interactions on innate immunity in a leaf-cutting ally contribute to adult male production or trophic ant. J Insect Physiol 56:780–7. egg-laying is essential to determine whether egg-laying Beekman M, Calis JNM, Boot WJ. 2000. Insect behaviour: parasitic honeybees get royal treatment. Nature 404:723. might be considered “selfish,” and thus inactivity may Bengston SE, Dornhaus A. 2013. Colony size does not predict be to the detriment of the colony as a whole. foraging distance in the ant Temnothorax rugatulus: a puz- zle for standard scaling models. Insectes Soc 60:93–6. Use of experimental animals and human subjects Billen J. 1982. Ovariole development in workers of Formica No experiments herein were performed on live ver- sanguinea Latr. (Hymenoptera: Formicidae). Insectes Soc 29:86–94. tebrates and/or higher invertebrates and so do not Blanchard GB, Orledge GM, Reynolds SE, Franks NR. 2000. require institutional and/or licensing committee ap- Division of labour and seasonality in the ant Leptothorax proving the experiments. albipennis: worker corpulence and its influence on behav- iour. Anim Behav 59:723–38. Acknowledgments Blumstein DT, Wey TW, Tang K. 2009. A test of the social cohesion hypothesis: interactive female marmots remain at We thank Maxwell Akorli, David Cai, Reena Debray, home. Proc R Soc Lond B Biol Sci 359:873–90. Stephen Dimascio, Alex Down, Abigail Garcia, Galen Bonabeau E, Dorigo M, Theraulaz G. 2000. Inspiration for Gudenkauf, Brittney Hensley, Neil Hillis, Theo Jones, optimization from social insect behaviour. Nature Karen Kierstead, Rouna Mohran, Stefanie Nguyen, 406:39–42. Amy Pierce, Andrew Scott, Bryce Tipton, Israel Børgesen LW. 2000. Nutritional function of replete workers Vallejo, Matt Velazquez, Erik Wankasky, David in the pharaoh’s ant, Monomorium pharaonis (L.). Insectes Woosley, for their help with ant painting and main- Soc 47:141–6. Bridger D, Bonner SJ, Briffa M. 2015. Individual quality and tenance, and data collection. We also thank the en- personality: bolder males are less fecund in the hermit crab tire Dornhaus laboratory, and particularly Nicole Pagurus bernhardus. Proc R Soc Lond B Biol Sci Fischer and Sarah Bengston, for their ongoing feed- 282:20142492. back and support. Brunner E, Kroiss J, Trindl A, Heinze J. 2011. Queen phero- mones in Temnothorax ants: control or honest signal?. Funding BMC Evol Biol 11:55. Burkhardt JF. 1998. Individual flexibility and tempo in the This work was supported by the GIDP-EIS and EEB ant, Pheidole dentata, the influence of group size. J Insect Department at University of Arizona, as well as Behav 11:493–505. Lazy ants and their role 665

Careau V, Montiglio P-O, Garant D, Re´ale DJF. 2015. Energy Dornhaus A, Holley JA, Pook VG, Worswick G, Franks NR. expenditure and personality in wild chipmunks. Behav Ecol 2008. Why do not all workers work? Colony size and work- Sociobiol 69:1–9. load during emigrations in the ant Temnothorax albipennis. Carney WP. 1969. Behavioral and Morphological changes in Behav Ecol Sociobiol 63:43–51. carpenter ants harboring Dicrocoeliid metacercariae.Am Doums C, Moret Y, Benelli E, Schmid-Hempel P. 2002.

Midl Nat 82:605–11. Senescence of immune defence in Bombus workers. Ecol Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019 Cartar RV. 1992. Morphological senescence and longevity: an Entomol 27:138–44. experiment relating wing wear and life span in foraging Farine DR. 2015. Proximity as a proxy for interactions: issues wild bumble bees. J Anim Ecol 61:225–31. of scale in social network analysis. Anim Behav 104:e1–5. Castles M, Heinsohn R, Marshall HH, Carter AJ. 2014. Social Farris SM, Robinson GE, Fahrbach SE. 2001. Experience- and networks created with different techniques are not compa- age-related outgrowth of intrinsic neurons in the mush- rable. Anim Behav 96:59–67. room bodies of the adult worker honeybee. J Neurosci Ceusters R, Bertrands H, Petit H, Van de Peer H. 1981. 21:6395–404. Polye´thisme chez Formica polyctena Fo¨rst, et Myrmica Fewell JH, Winston ML. 1992. Colony state and regulation of rubra L. Stratification des ouvrie`res dans le nid selon leur pollen foraging in the honey bee, Apis mellifera L. Behav e´tat physiologique par rapport a un gradient de tempe´r- Ecol Sociobiol 30:387–93. ature. Bull Sect Fr UIEIS Toulouse 38–44. Forster MJ, Dubey A, Dawson KM, Stutts WA, Lal H, Sohal Charbonneau D, Dornhaus A. 2015a. When doing nothing is RS. 1996. Age-related losses of cognitive function and mo- something. How task allocation strategies compromise be- tor skills in mice are associated with oxidative protein tween flexibility, efficiency, and inactive agents. J damage in the brain. Proc Natl Acad Sci U S A 93:4765–9. Bioeconomics 17:1–26. Franklin SB, Gibson DJ, Robertson PA, Pohlmann JT, Fralish Charbonneau D, Dornhaus A. 2015b. Workers “specialized” JS. 1995. Parallel analysis: a method for determining signif- on inactivity: behavioral consistency of inactive workers icant principal components. J Veg Sci 6:99–106. and their role in task allocation. Behav Ecol Sociobiol Franks N, Tofts C. 1994. Foraging for work—how tasks allo- 69:1459–72. cate workers. Anim Behav 48:470. Charbonneau D, Hillis N, Dornhaus A. Submitted for pub- Franks NR, Sendova-Franks AB, Simmons J, Mogie M. 1999. lication. Are “lazy” ants selfish? Reproductive competition Convergent evolution, superefficient teams and tempo in may bias workers toward self-serving tasks. Anim Behav. old and new world army ants. Proc R Soc Lond B Biol Sci Charbonneau D, Hillis N, Dornhaus A. 2014. “Lazy” in na- 266:1697–701. ture: ant colony time budgets show high “inactivity” in the Fresneau D. 1984. De´veloppement ovarien et statut social field as well as in the lab. Insectes Soc 62:31–5. chez une fourmi primitive Neoponera obscuricornis Cole B. 1985. Size and behavior in ants: constraints on com- Emery (Hym. Formicidae, Ponerinae). Insectes Soc plexity. Proc Natl Acad Sci U S A 82:8548. 31:387–402. Cole BJ. 1981. Dominance hierarchies in leptothorax ants. Fu¨rtbauer I, Pond A, Heistermann M, King AJ. 2015. Science 212:83. Personality, plasticity and predation: linking endocrine Cole BJ. 1986. The social behavior of Leptothorax allardycei and behavioural reaction norms in stickleback fish. Funct (Hymenoptera, Formicidae): time budgets and the evolu- Ecol 29:931–40. tion of worker reproduction. Behav Ecol Sociobiol Gadagkar R, Joshi NV. 1984. Social organisation in the Indian 18:165–73. wasp Ropalidia cyathiformis (Fab.) (Hymenoptera: Cole BJ. 1992. Short-term activity cycles in ants: age-related Vespidae). Z Fu¨r Tierpsychol 64:15–32. changes in tempo and colony synchrony. Behav Ecol Gardner KE, Foster RL, O’Donnell S. 2007. Experimental Sociobiol 31:181–7. analysis of worker division of labor in bumblebee nest ther- Corbara B, Lachaud J-P, Fresneau D. 1989. Individual vari- moregulation (Bombus huntii, Hymenoptera: Apidae). ability, social structure and division of labour in the poner- Behav Ecol Sociobiol 61:783–92. ine ant Ectatomma ruidum Roger (Hymenoptera, Giraldo YM, Traniello JFA. 2014. Worker senescence and the Formicidae). Ethology 82:89–100. sociobiology of aging in ants. Behav Ecol Sociobiol Cote J, Clobert J. 2007. Social personalities influence natal 68:1901–19. dispersal in a lizard. Proc R Soc Lond B Biol Sci Gordon DM. 1996. The organization of work in social insect 274:383–90. colonies. Nature 380:121–4. Cote J, Clobert J, Brodin T, Fogarty S, Sih A. 2010. Hasegawa E. 1993. Caste specialization in food storage in the Personality-dependent dispersal: characterization, ontogeny dimorphic ant Colobopsis nipponicus (Wheeler). Insectes and consequences for spatially structured populations. Soc 40:261–71. Philos Trans R Soc Lond B Biol Sci 365:4065–76. Hasegawa E, Ishii Y, Tada K, Kobayashi K, Yoshimura J. Defler TR. 1995. The time budget of a group of wild woolly 2016. Lazy workers are necessary for long-term sustainabil- monkeys (Lagothrix lagotricha). Int J Primatol 16:107–20. ity in insect societies. Sci Rep 6:Art. No. 20846. Dolezal A, Brent C. 2009. General dissection of female ant Heinze J. 1996. Reproductive hierarchies among workers of reproductive system and brain. Cold Spring Harb Protoc the slave-making ant, Chalepoxenus muellerianus. Ethology 2009:pdb.prot5244. doi: 10.1101/pdb.prot5244. 102:117–27. Dornhaus A. 2008. Specialization does not predict individual Herbers JM. 1981. Time resources and laziness in animals. efficiency in an ant. PLoS Biol 6:e285. Oecologia 49:252–62. 666 D. Charbonneau et al.

Herbers JM. 1983. Social organization in Leptothorax ants: Mirenda JT, Vinson SB. 1981. Division of labour and speci- within- and between-species patterns. Psyche J Entomol fication of castes in the red imported fire ant Solenopsis 90:361–86. invicta Buren. Anim Behav 29:410–20. Herbers JM, Cunningham M. 1983. Social organization in Montiglio P-O, Garant D, Pelletier F, Re´ale D. 2014. Intra- Leptothorax longispinosus Mayr. Anim Behav 31:759–71. individual variability in fecal cortisol metabolites varies

Holden C. 1989. Entomologists wane as insects wax. Science with lifetime exploration and reproductive life history in Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019 246:754–6. eastern chipmunks (Tamias striatus). Behav Ecol Sociobiol Ho¨lldobler B, Wilson EO. 1990. The ants. Cambridge (MA): 69:1–11. Belknap Press of Harvard University Press. Moore D. 2001. Honey bee circadian clocks: behavioral con- Ishii Y, Hasegawa E. 2013. The mechanism underlying the trol from individual workers to whole-colony rhythms. J regulation of work-related behaviors in the monomorphic Insect Physiol 47:843–57. ant, Myrmica kotokui. J Ethol 31:61–9. Moore D, Angel JE, Cheeseman IM, Fahrbach SE, Robinson Jandt J, Robins N, Moore R, Dornhaus A. 2012. Individual GE. 1998. Timekeeping in the honey bee colony: integra- bumblebees vary in response to disturbance: a test of the tion of circadian rhythms and division of labor. Behav Ecol defensive reserve hypothesis. Insectes Soc 59:313–21. Sociobiol 43:147–60. Jandt JM, Dornhaus A. 2011. Competition and cooperation: Moreau M, Arrufat P, Latil G, Jeanson R. 2011. Use of radio- bumblebee spatial organization and division of labor may tagging to map spatial organization and social interactions affect worker reproduction late in life. Behav Ecol Sociobiol in insects. J Exp Biol 214:17–21. 65:2341–9. Nespolo RF, Franco M. 2007. Whole-animal metabolic rate is Jeanson R. 2012. Long-term dynamics in proximity networks a repeatable trait: a meta-analysis. J Exp Biol 210:2000–5. in ants. Anim Behav 83:915–23. Nguyen H, Fasciano T, Charbonneau D, et al. 2014. Data Jeanson R, Weidenmu¨ller A. 2014. Interindividual variability association based ant tracking with interactive error correc- in social insects—proximate causes and ultimate conse- tion. 2014 IEEE Winter Conference on Applications of quences. Biol Rev 89:671–87. Computer Vision (WACV). Steamboat Springs (CO): Johnson BR. 2002. Reallocation of labor in honeybee colonies Published by IEEE - Institute of Electrical and Electronics during heat stress: the relative roles of task switching and the Engineers. p. 941–6. activation of reserve labor. Behav Ecol Sociobiol 51:188–96. O’Donnell S. 1998. Effects of experimental forager removals Johnson BR. 2008. Global information sampling in the honey on division of labour in the primitively eusocial wasp bee. Naturwissenschaften 95:523–30. Polistes instabilis (Hymenoptera: Vespidae). Behaviour Keller L, Nonacs P. 1993. The role of queen pheromones in 135:173–93. social insects: queen control or queen signal? Anim Behav O’Donnell S, Bulova SJ. 2007. Worker connectivity: a review 45:787–94. of the design of worker communication systems and their Kondoh M. 1968. Bioeconomic studies on the colony of an ant effects on task performance in insect societies. Insectes Soc species, Formica japonica Motschulsky. 1. Nest structure and 54:203–10. seasonal change of the colony members. Jpn J Ecol 18:124–33. Oster GF, Wilson EO. 1978. Caste and ecology in the social Kuehbandner S, Modlmeier AP, Foitzik S. 2014. Age and insects. Princeton (NJ): Princeton University Press. ovarian development are related to worker personality Otterstatter MC, Thomson JD. 2007. Contact networks and and task allocation in the ant Leptothorax acervorum. transmission of an intestinal pathogen in bumble bee Curr Zool 60:392–400. (Bombus impatiens) colonies. Oecologia 154:411–21. Lachaud JP, Passera L, Grimal A, Detrain C, Beugnon G. Otto D. 1958. Uber€ Die Arbeitsteilung Im Staate von Formica 1992. Lipid storage by major workers and starvation resis- Rufa Rufo-Pratensis Minor Go¨ssw. Und Ihre tance in the ant Pheidole pallidula (Hymenoptera, Verhaltensphysiologischen Grundlagen: Ein Beitrag Zur Formicidae). In: Billen J (editor). Biology and Evolution Biologie Der Roten Waldameise. Akademie Verlag. of Social Insects. Leuven, Belgium: Leuven University Pamminger T, Foitzik S, Kaufmann KC, Schu¨tzler N, Menzel Press. p. 153–60. F. 2014. Worker personality and its association with spa- Lenoir A, Mardon JC. 1978. Note sur l’application de l’ana- tially structured division of labor. Nascimento FS (ed.). lyse des correspondances a la division du travail chez les PLoS One 9:e79616. fourmis. C R Acad Sci Ser D 287:555–8. Patrick SC, Weimerskirch H. 2015. Senescence rates and late Lindauer M. 1952. Ein beitrag zur frage der arbeitsteilung im adulthood reproductive success are strongly influenced by bienenstaat. J Comp Physiol A Neuroethol Sens Neural personality in a long-lived seabird. Proc R Soc B Biol Sci Behav Physiol 34:299–345. 282:20141649. Maistrello L, Sbrenna G. 1999. Behavioural differences be- Pearl R. 1928. The rate of living. London: University Press. tween male and female replacement reproductives in Perry JC, Roitberg BD. 2006. Trophic egg laying: hypotheses Kalotermes flavicollis (Isoptera, Kalotermitidae). Insectes and tests. Oikos 112:706–14. Soc 46:186–91. Pinter-Wollman N, Wollman R, Guetz A, Holmes S, Gordon Mason KS, Kwapich CL, Tschinkel WR. 2015. Respiration, DM. 2011. The effect of individual variation on the struc- worker body size, tempo and activity in whole colonies ture and function of interaction networks in harvester ants. of ants. Physiol Entomol 40:149–65. J R Soc Interface 8:1562–73. Mersch DP, Crespi A, Keller L. 2013. Tracking individuals Poff C, Nguyen H, Kang T, Shin MC. 2012. Efficient tracking shows spatial fidelity is a key regulator of ant social orga- of ants in long video with GPU and interaction. Colorado: nization. Science 340:1090–3. Breckenridge. Lazy ants and their role 667

Porter SD, Jorgensen CD. 1981. Foragers of the harvester ant, Seid MA, Harris KM, Traniello JFA. 2005. Age-related Pogonomyrmex owyheei: a disposable caste? Behav Ecol changes in the number and structure of synapses in the Sociobiol 9:247–56. lip region of the mushroom bodies in the ant Pheidole Punzo F, Chavez S. 2003. Effect of aging on spatial learning dentata. J Comp Neurol 488:269–77. and running speed in the shrew (Cryptotis parva). J Seid MA, Traniello JF. 2006. Age-related repertoire expansion

Mammal 84:1112–20. and division of labor in Pheidole dentata (Hymenoptera: Downloaded from https://academic.oup.com/icb/article-abstract/57/3/649/4036211 by University of Arizona Health Sciences Library user on 13 May 2019 Re´ale D, Garant D, Humphries MM, Bergeron P, Careau V, Formicidae): a new perspective on temporal polyethism Montiglio P-O. 2010. Personality and the emergence of the and behavioral plasticity in ants. Behav Ecol Sociobiol pace-of-life syndrome concept at the population level. 60:631–44. Philos Trans R Soc B Biol Sci 365:4051–63. Sempo G, Depicke`re S, Detrain C. 2006. Spatial organization Retana J, Cerda X. 1990. Social organization of cataglyphis in a dimorphic ant: caste specificity of clustering patterns cursor ant colonies (Hymenoptera, Formicidae): inter-, and and area marking. Behav Ecol 17:642–50. intraspecific comparisons. Ethology 84:105–22. Sendova-Franks AB, Franks NR. 1995. Spatial relationships Retana J, Cerda X. 1991. Behavioural variability and develop- within nests of the ant Leptothorax unifasciatus (Latr.) ment of Cataglyphis cursor ant workers (Hymenoptera, and their implications for the division of labour. Anim Formicidae). Ethology 89:275–86. Behav 50:121. Ricklefs RE, Wikelski M. 2002. The physiology/life-history Stamps JA. 2007. Growth-mortality tradeoffs and “personality nexus. Trends Ecol Evol 17:462–8. traits” in animals. Ecol Lett 10:355–63. Ridgel AL, Ritzmann RE. 2005. Insights into age-related locomo- Stroeymeyt N, Brunner E, Heinze J. 2007. “Selfish worker tor declines from studies of insects. Ageing Res Rev 4:23–39. policing” controls reproduction in a Temnothorax ant. Robinson EJ, Feinerman O, Franks NR. 2009. Flexible task Behav Ecol Sociobiol 61:1449–57. allocation and the organization of work in ants. Proc R Soc Stumper R. 1961. Radiobiologische Untersuchungen u¨ber den B Biol Sci 276:4373–80. sozialen Nahrungshaushalt der Honigameise Proformica Robinson EJ, Feinerman O, Franks NR. 2012. Experience, nasuta (Nyl). Naturwissenschaften 48:735–6. corpulence and decision making in ant foraging. J Exp Tchabovsky AV, Popov SV, Krasnov BR. 2001. Intra- and Biol 215:2653–9. interspecific variation in vigilance and foraging of two ger- Robinson GE. 1992. Regulation of division of labor in insect billid rodents, Rhombomys opimus and Psammomys obesus: societies. Annu Rev Entomol 37:637–65. the effect of social environment. Anim Behav 62:965–72. Rose MR. 1991. Evolutionary biology of aging. New York: Tinbergen N. 1963. On aims and methods of ethology. Oxford University Press. Ethology 20:410–33. Rosenfeld J, Van Leeuwen T, Richards J, Allen D. 2015. Toth AL, Kantarovich S, Meisel AF, Robinson GE. 2005. Relationship between growth and standard metabolic rate: Nutritional status influences socially regulated foraging on- measurement artefacts and implications for habitat use and togeny in honey bees. J Exp Biol 208:4641–9. life-history adaptation in salmonids. J Anim Ecol 84:4–20. Toth AL, Robinson GE. 2005. Worker nutrition and division Rosengaus RB, Traniello JF. 1991. Biparental care in incipient of labour in honeybees. Anim Behav 69:427–35. colonies of the dampwood termite Zootermopsis angusticol- Tschinkel WR. 1987. Seasonal life history and nest architec- lis Hagen (Isoptera: Termopsidae). J Insect Behav 4:633–47. ture of a winter-active ant, Prenolepis imparis. Insectes Soc Rueppell O, Christine S, Mulcrone C, Groves L. 2007. Aging 34:143–64. without functional senescence in honey bee workers. Curr Tschinkel WR. 1999. Sociometry and sociogenesis of colonies Biol 17:R274. of the harvester ant, Pogonomyrmex badius: distribution of Samways MJ. 1993. Insects in biodiversity conservation: some workers, brood and seeds within the nest in relation to perspectives and directives. Biodivers Conserv 2:258–82. colony size and season. Ecol Entomol 24:222–37. Schippers M-P, Dukas R, Smith RW, Wang J, Smolen K, Tsuji K. 1990. Nutrient storage in the major workers of McClelland GB. 2006. Lifetime performance in foraging Pheidole ryukyuensis (Hymenoptera: Formicidae). Appl honeybees: behaviour and physiology. J Exp Biol Entomol Zool 25:283–7. 209:3828–36. Urszan TJ, To¨ro¨k J, Hettyey A, Garamszegi LZ, Herczeg G. Schmid-Hempel P. 2005. Evolutionary ecology of insect im- 2015. Behavioural consistency and life history of Rana dal- mune defenses. Annu Rev Entomol 50:529–51. matina tadpoles. Oecologia 178:1–12. Schofield RMS, Emmett KD, Niedbala JC, Nesson MH. 2010. Weir JS. 1958a. Polyethism in workers of the ant Myrmica. Leaf-cutter ants with worn mandibles cut half as fast, spend Insectes Soc 5:97–128. twice the energy, and tend to carry instead of cut. Behav Weir JS. 1958b. Polyethism in workers of the ant Myrmica Ecol Sociobiol 65:969–82. Part II. Insectes Soc 5:315–39. Schuett W, Dall SR, Kloesener MH, Baeumer J, Beinlich F, Wilson EO. 1991. Ants. Bull Am Acad Arts Sci 45:13–23. Eggers T. 2015. Life-history trade-offs mediate “personal- Wolf M, Van Doorn GS, Leimar O, Weissing FJ. 2007. Life- ity” variation in two colour morphs of the pea aphid, history trade-offs favour the evolution of animal personal- Acyrthosiphon pisum. J Anim Ecol 84:90–101. ities. Nature 447:581–4. Seeley TD. 1982. Adaptive significance of the age polyethism Yang AS. 2006. Seasonality, division of labor, and dynamics schedule in honeybee colonies. Behav Ecol Sociobiol of colony-level nutrient storage in the ant Pheidole morrisi. 11:287–93. Insectes Soc 53:456–62.