Ins. Soc. 42: 17-29 (1995) 1015-1621/95/010017-13 $1.50 + 0.2010 © 1995 Birkhiiuser Verlag, Basel

Division of labor in post-emergence colonies of the primitively eusocial instabilis de Saussure (: )

S.O'Donnell Department of Entomology, University of California, Davis, CA 95616, USA

Key words: Foraging, polyethism, specialization, task partitioning.

Abstract

Polyethism was quantified in post-emergence colonies of the primitively eusocial wasp, , and compared to polyethism in a sympatric advanced eusocial wasp, occidenta­ lis. Like P. occidentalis, P. instabilis foragers collected food ( and prey) and materials (wood pulp and water). P. instabilis foragers showed some evidence of specialization with respect to which materials they gathered, but most foragers, divided their effort among food and nest materials, a pattern that is rarely seen in P. occidentalis. In colonies of both species, more foragers collected nectar than any other material; in contrast, most water foraging was performed by one or two workers. Upon returning to the nest, P. instabilis foragers gave up part or all of most nectar, prey, and pulp loads to nestmates, while water was rarely partitioned. Prey loads were most likely to be given up entirely. P. instabilis workers show evidence of conflict over the handling of ma­ terials at the nest. The frequency with which workers took portions of nectar loads from forgers was positively correlated with their frequency of aggressive dominant behavior, and with their fre­ quency of taking other foraged materials. Compared to polyethism in P. occidentalis P. instabilis showed less individual specialization on foraging tasks and less partitioning of foraged materials with nestmates, suggesting that these characteristics of polyethism have been modified during the evolution of advanced societies.

Introduction

Wasps in the family Vespidae exhibit many grades of social development, from solitary nesting to advanced (swarm-founding) (Carpenter 1982). The vespid genus Polistes occupies an intermediate position of primitive eusociality, where colonies are founded independently by reproductive females, and reproduc­ tive division of labor is not based on morphological differences. Polistes social structure may represent a condition similar to that present in ancestors of modern eusocial species (West Eberhard 1978; Carpenter 1989). Comparisons of division of labor in Polistes species with that in more derived wasp societies may allow us to assess how the organization of work has been modi- O'Donnell 18 Divison of labor in Polistes instabilis 19 fied during the evolution of advanced eusocial . In this paper I compare divi­ Colony C: 10 days, 27 August to 6 September, 22: 04 h; Colony D: 6 days, 17 to sion of labor in post-emergence colonies of the primitively eusocial wasp, Polistes 27 September, 19: 15 h. I recorded behavioral data while seated within 0.5 m of the instabilis, with a sympatric advanced eusocial wasp, . colonies. Data were not collected during periods of heavy rain. Most observations The nature and extent of division of labor among workers, or polyethism, has were made in the morning to early afternoon (6.30 h to 12.30 h); additional after­ rarely been measured in Polistes species (Reeve 1991). Previous studies have docu­ noon observations were made at Colonies A and C. Observation periods were mented quantitative behavioral differences and division of labor among Polistes continuous except when interrupted by heavy rain. The mean duration of observa­ workers (Strassmann et al. 1984; Post et al. 1988). I focused on two aspects of tion periods was 2:00h (range 0:22min to 4:45h); the mean total duration of division of labor in P. instabilis that have been studied in advanced eusocial : observations per day was 2:30 h. forager specialization (O'Donnell and Jeanne 1992) and task partitioning (Jeanne During observation periods I recorded all occurrences of forager arrivals, noting 1986a; 1986b). forager identity, of arrival to the nearest min, material carried (if visible), iden­ tity of wasps that took portions of the foragerfs load, and how the forager handled Materials and methods her load at the nest within 5 min of arrival. If an arriving wasp made no transfers to brood or to adult wasps within 5 min, material was recorded as unknown. Forager Subject colonies departures were recorded when possible at Colonies B through D. Wasp foragers collect wood pulp (for building) and insect prey, which are carried Four post-adult emergence P. instabilis colonies were observed in situ in Guana­ externally in the mandibles, and liquids, which are carried internally in the crop caste Province, Republic of Costa Rica, during the wet season of 1988. Colony A (West Eberhard 1969). I categorized liquid loads as water if the forager or recipients was located in Palo Verde National Park along the Tempisque River; Colonies B placed the load on the nest surface for cooling, and as nectar if the load was passed through D were located at Centro Ecologico la Pacifica, along the Interamerican to larvae or to recipient wasps and not placed on the nest. Some loads categorized Highway 5 km northwest of the town of Canas. as nectar may have comprised liquids from masticated prey, or water that was not The four subject colonies differed in size and/or in phase of colony development. used for cooling. Nectar and prey will be referred to collectively as food materials; The presence of opened pupal caps on cells, and the numbers of adult wasps wood pulp and water as nest materials. Trips where material was unknown were not present, indicated that workers had emerged in each of the four colonies before used in analysis of forager specialization and foraging rates. observations were initiated. Although the phases of colony development may be Every five or ten minutes, scan samples were performed at Colonies A, C, less discrete in tropical and subtropical than in temperate Polistes wasps (West and D, during which all adults present were identified and their behavioral acts Eberhard 1969; Strassmann 198la; O'Donnell 1994), a large number of males recorded using an ethogram similar to that presented by Post et al. (1988). Because emerged in Colonies B and C during the study, suggesting that these colonies had of the high rate of foraging and large adult population of Cofony B, only pre­ reached the reproductive phase. Nest construction and egg laying occurred in all sence (not behavior) of adult wasps was recorded during scans conducted every colonies except in Colony B, which was apparently abandoned by its queen on the 15 min. second day of observations and was declining. Colonies A, B, and C were located in open areas and were sometimes exposed to direct sun during observations; Colony Data analysis D was located under a large tree and was fully shaded. Foraging trip were calculated as time elapsed from observed forager Marking of individuals and caste identification departures to subsequent arrival at the nest (field times), and from arrival to sub­ Adult wasps were uniquely marked with model airplane paint (Colony A) or paint sequent departure (nest times). Only field and nest times which fell completely pens (Colonies B through D). Wasps were marked starting several days before within observation periods were analyzed; estimates of field and nest time distri­ observations, and in the evenings of observation days on which they emerged. butions are therefore biased toward those of short duration. Waiting time was Queens were identified by performance of egg laying; only one female was seen defined as the time elapsed between arrivals of loads of a given material at the laying eggs in each colony. Other females will be referred to as workers, although nest. some may have been future reproductives (gynes) in Colonies B and C. Males were Frequencies of performance of on-nest activities (scan-sampled behavior and identified by their distally curved antennae and by the bright yellow frontal regions frequencies of taking foraged loads) were divided by the total number of scan (frons and clypeus) of their faces. samples and by the number of scans during which the individual was present on the nest. The latter values will be referred to as scan-weighted frequencies. Behavioral observations Non-parametric statistical tests were used when the assumptions of parametric tests (e.g., normal distribution of data, equality of variance) were violated. Un­ The number of days, dates, and total duration of observations were Colony A: less stated otherwise, results of statistical significance tests were similar across 3 days, 19 to 21 July, 10:00 h; Colony B: 5 days, 7 to 11 September, 10:08 h; colonies. · 20 O'Donnell Divison of labor in Polistes instabilis 21

hood ratio chi-squared test for association of worker identity and trips per materi­ 20 NECTAR PULP al; Colony A: X 2 =117.39, df = 39, p < 0.001; Colony B: X 2=208.84, df = 33, p< 0.001; Colony C: X 2 =129.95, df=21, p<0.001; Colony D: X 2 =193.36, df=34, p<0.001; ~ 15 :!! 6 • Colony A restricting the same analysis to foragers that made eight or more trips does not • • ColonyA ..• .. f2 Colony B !! fJ ColonyB • D ColonyC change the results for any colony). However, few individuals that made four or more ~ 0 ColonyC £? !:;I ColonyD - 10 B Colony D c;• trips specialized exclusively on a single material (Colony A: one pulp forager; 0 ~ Colony B: six nectar foragers; Colony C: four nectar foragers, one water forager; ...~ ... E E Colony D: one nectar forager, one prey forager), and at least half of the foragers in :I ~ 2 z • each colony collected both food and nest materials. Foraging by queens and males is addressed below.

0.2 0.4 0.8 0.8 1 1.2 1.4 1.6 1.8 0.1 0.2 0.3 0.4 0.5 o.& o. 7 0.8 Foraging rate (trips/hour) Foraging rate (trips/hour)

Tablel. Overall colony foraging rates (trips/hour) and mean (±sd) waiting times (elapsed time in minutes PREY WATER between arrivals) for foraged loads (see Methods)

• ColonyA Colony A Nectar Prey Pulp Water .. .. UI ColonyB ~ ~ D ColonyC • ColonyA .. Foraging rate 6.20 2.30 3.30 4.30 :• IZJ ColonyB !! £? ~ 0 ColonyC N=62 N=23 N=33 N=43 0 8 ColonyD 0 1 ~ Waiting time 7.98±8.40 23.83 ± 31.64 17.59±25.85 7.04± 10.46 •~ ..... ~ 2 E N=58 N=18 N=28 N=40 :I :I z z p<0.005"

, , ColonyB Nectar Prey Pulp Water 0.2 0.4 0.8 0.8 1.0 1.2 1.4 1.8 1.8 9.0 0.2 0.4 0.8 0.8 1 1.2 Foraging rate (trips/hour) Foraging rate (trips/hour) Foraging rate 8.68 0.10 1.68 11.05 Figure L Distributions of foraging rates among four materials for P instabilis workers. E~ch individua~ appears N=88 N=l N=17 N=112 in the histogram for every material she collected. X-axis values represent the up~er limit of foragmg rates Waiting time 6.53±5.99 14.26± 17.17 3.21±4.25 for each interval; note differences among materials in numbers of foragers and m maximum rates of for­ N=81 N=13 N=109 aging p<0.001

Colony C Nectar Prey Pulp Water Results Foraging rate 3.44 0.36 0.18 2.99 N=76 N=8 N=4 N=66 Forager specialization Waiting time 15.79 * 19.83 41.70± 29.27 10.37± 15.50 The numbers of adult wasps that had been marked or recorded when observations N=65 N=2 N=57 terminated (q =queen, w=workers, m=males, u=unmarked) were Colony A: q = 1, p<0.005 w = 23, u = 3; Colony B: q = 1, w = 73, u = 4 (several males were present but were not marked or counted); Colony C: q = 1, w = 22, m = 12; Colony D: q = 1, w = 44, u = 9. ColonyD Nectar Prey Pulp Water The numbers of workers (percent of marked workers) that foraged were Colony A: Foraging rate 4.47 3.58 2.44 18 (78.3 % ); Colony B: 31 ( 42.5 % ); Colony C: 18 (81.8 % ); Colony D: 32 _(72.7 % ). Waiting time 11.75±13.78 10.07±9.51 22.86 ± 21.92 The low proportion of foraging workers in Colony B may have been due m part to N=80 N=63 N=41 the presence of future reproductives, which rarely forage (Post et al. 1988). Most p<0.005 foragers devoted part or all of their effort to nectar collection. Fewer foragers

collected other materials, and water foraging was performed primarily by one or a Kruskal-Wallis test for differences among materials in the distribution of time elapsed between two workers in each colony (Fig. 1 ). Foragers differed significantly in the distribu­ arrivals of foraged loads (in some colonies, times between arrivals could not be calculated for tion of their trips among materials (for workers making four or more trips: Likeli- some materials because of low rates of foraging for those materials 22 O'Donnell Divison of labor in Polistes instabilis 23

Foraging rates Table 3. Partitioning of foraged materials among nestmates

Colony A The colony-wide rates of foraging differed among materials; waiting times for water Degree of partitioning• and nectar were typically shorter than for other materials within colonies (Fig. 1; Used all Shared Gave all Total Table 1). Field time foragers spent collecting loads differed significantly among materials (Table 2). Nest materials usually required less field time for collection than Material did food materials; nectar trips required the greatest amount of time in the field in Nectar 23 (34%) 26 (38%) 19 (28%) 68 Prey 5 all colonies. Time spent on the nest before leaving for another trip also differed sig­ (22%) 11 (48%) 7 (30%) 23 Pulp 4 (12%) 24 (73%) 5 (15%) 33 nificantly among materials (Table 2). Nest times were shortest for liquid loads, and Water 26 (70%) 10 (27%) 1 ( 3%) 37 longer for pulp and prey, which were often masticated at the nest before being parti­ p<0.001 h tioned with nestmates, fed to larvae (prey), or used in building (pulp) by the forager. An exception is seen for pulp foraging in Colony B (Table 2), where pulp foragers ColonyB Degree of partitioning apparently used their loads to build daughter nearby after first landing on the observation nest where no construction took place (pers. obs.). Used all Shared Gave all Total Material Table 2. Mean (± sd) trip times and nest times in minutes for foraged loads. Trip and nest times could not be Nectar 7 (8%) 30 (34%) 51 (58%) 88 calculated for Colony A because forager departures were not recorded Prey 0 1 (100%) 0 1 Pulp 16 (100%) 0 0 16 Colony B Water 112 (99%) 1 (1 %) 0 113 Material X field time N X nest time N p<0.001

Nectar 29.97±15.12 26 1.62±3.44 40 ColonyC Degree of partitioning Pulp 10.80± 10.13 6 0.88±0.83 13 Water 1.91 ± 4.97 22 2.31±3.42 21 Used all Shared Gave all Total Field times p < 0.001 • Material Nest times p < 0.05 h Nectar 11 (14%) 27 (36%) 38 (50%) 76 Prey 1 (13%) 3 (38%) 4 (50%) 8 ColonyC Pulp 1 (25%) 2 (50%) 1 (25%) 4 Water 60 (91 %) 6 (9%) 0 66 Material X field time N X nest time N p<0.001 Nectar 40.78±40.41 44 5.97± 12.96 51 Prey 18.90± 7.44 4 12.10±9.94 6 ColonyD Degree of partitioning Pulp 5.80± 0.92 3 16.20 1 Water 6.26±21.61 44 6.61± 10.53 42 Used all Shared Gave all Total Field times p < 0.001 Material Nest times p<0.05 Nectar 10 (12%) 41 (48%) 35 (41 %) 86 Prey 7 (10%) 14 (20%) 48 (70%) 69 ColonyD Pulp 7 (15%) 36 (77%) 4 (9%) 47 Material X field time N X nest time N p<0.001

Nectar 46.71±39.87 55 4.12± 10.77 71 a Partitioning cartegories: Used all= foraged load not partitioned with nestmates. Shared= a por­ Prey 26.72 ± 30.26 52 4.26±4.82 56 tion of the load given up to nest wasps. Gave all= entire load given to nestmates 2 Pulp 19.16±24.27 34 35.72±22.74 31 b Likelihood ratio X test for association of material type and degree of partitioning Field times p < 0.001 Nest times p < 0.001 a,b Kruskal-Wallis tests for differences in the distributions of field times and nest times among foraged materials (in some colonies, field and nest times could not be calculated for some ma­ terials because of low rates of foraging for those materials) 24 O'Donnell Divison of labor in Polistes instabilis 25

Load partitioning difference was significant only in Colony C (Table 4). In two colonies, indivi­ duals' scan-weighted frequencies of taking portions of foraged loads were There were significant differences among materials in the degree to which foraged positively correlated among nectar, prey, and pulp (Spearman rank correla­ loads were partitioned with nestmates (Table 3). Food materials were more likely tion, Colony A: r>0.48, df=22, p<0.05 in all cases; Colony D: r>0.31, df=42, to be partitioned than nest materials. In many cases, prey loads were given up p < 0.05 in all cases; these rates were not significantly correlated in Colony C; completely to nestmates; in contrast, water loads were almost always used entirely in Colony D too few loads other than nectar were partitioned to test this relation­ by the forager. For loads that were shared or given up, the number of recipients ship). per load was generally higher for food materials (nectar and prey), but this Eight workers in Colony C and nine in Colony D were observed physically dominating (e.g., chasing or ) other workers. In these colonies, scan-weighted frequencies of dominance were positively correlated with scan-weighted fre­ quencies of taking nectar loads (Spearman rank correlation; Colony C: r = 0.42, Table 4. Number of wasps taking portions of foraged loads df=21, p<0.05; Colony D: r=0.54, df=42, p<0.001). Colony A Labor by males and queens N range mean (±sd)

Nectar 41 1-8 1.93 ± 0.47 Male behavior was recorded only in Colony C. Two males brought single nectar Prey 18 1-3 1.39±0.61 loads to the nest, one of which was shared with a worker before being offered to Pulp 29 1-4 1.66±0.94 larvae. Five males fanned the nest on several occasions. One male was twice observ­ Water 15 1-3 1.47±0.64 ed offering pieces of prey (collected by workers) to larvae. Two males took portions p>0.50• of pulp loads, but how these loads were handled was not recorded. All queens spent time away from the nest. The numbers of observed queen ColonyB arrivals were Colony A: 3; Colony C: 10; Colony D: 2. Queens were not observed N range mean (±sd) returning with solid loads, engaging in material exchange, or entering cells within 5 min of their returns to the nest. Queen absences may not represent foraging trips; Nectar 81 1-5 2.23±1.16 their activities while away from the nest are unknown. Prey 1 2 As indicated by their presence in scan samples, queens spent a higher proportion Water 1 1 of time on the nests (Colony A: 74%; Colony C: 69%; Colony D: 100%) than all p>0.25 workers except two in Colony C. Queens interacted with brood more frequently than did all workers, but four workers had higher scan-weighted brood interaction Colony c frequencies in Colonies A and D. Queens also ranked high as recipients of pulp N range mean (±sd) loads (number of workers with equal or higher frequency, Colony A: 1; Colony C: 4; Colony D: 0) and nectar loads (number of workers with equal or higher fre­ Nectar 65 1-5 2.06± 1.04 quency, Colony A: l; Colony C: 4; Colony D: 6; each queenfs scan-weighted Prey 7 1-4 2.57±1.27 frequency of material receiving ranked lower). Dominance interactions (such as Pulp 3 1-3 1.67± 1.15 Water 6 1 1.00 chasing or biting nestmates) involving the queen were observed twice in Colony C and never in other colonies. p<0.05

ColonyD Discussion N range mean (±sd) Forager specialization and foraging rates Nectar 76 1-4 1.86±0.74 Prey 62 1-4 1.94±0.83 Pulp 40 1-4 1.83±0.90 P. instabilis foragers showed evidence of specialization on materials over the course of several days. However, the specialization was less than in P. occidentalis foragers, p>0.50 which rarely switch among materials (O'Donnell and Jeanne 1990). Unlike P. in­ a Kruskal-Wallis test for differences among materials in number of stabilis, foraging in P. occidentalis is divided into two nearly discrete task sets, food recipient wasps and nest material foraging (O'Donnell and Jeanne 1992). In both species many 26 O'Donnell Divison of labor in Polistes instabilis 27 foragers collected nectar, most at low rates, while few collected water, with one or Labor by males and queens two individuals working at high rates (O'Donnell and Jeanne 1992). The relative ranks of field time foragers expended on different materials was similar to P. occi­ P. instabilis males contributed to colony labor at low rates. Task performance by dentalis (O'Donnell and Jeanne 1990), and to other Polistes spp. (Gamboa et al. males such as fanning and brood care has been noted in other Polistes species (West 1978; Strassmann 1981 b ). Eberhard 1969; Hunt and Noonan 1979; Cameron 1986), but male labor is generally presumed to be of minor importance (Starr 1985). The observation that P. instabilis males performed tasks such as fanning, foraging, and feeding larvae is significant to Load partitioning studies attempting to assess relative investment in sexes by Polistes and other insect societies (Noonan 1978). The costs of producing males are offset in part by any P. instabilis foragers partition their loads with nestmates much less often than do investments males make in colony through their labor. P. occidentalis P. occidentalis foragers (Jeanne 1986a). This difference holds for all materials, but males have been observed performing tasks such as removal of rain water from is strongest in the case of building materials (Jeanne 1986b; Jeanne 1991). Time nests (R. L. Jeanne, pers. comm.). spent on the nest between trips by foragers was longest for materials that required More than in many other insect societies, Polistes queens are behavioral regula­ processing (i.e. prey and pulp) before being used or partitioned with nestmates. In tors of colony activity (Reeve and Gamboa 1987). Queens are at the top of the contrast, P. occidentalis foragers typically spend little time on the nest because they colony . Hughes and Strassmann (1988) claimed that P. in­ transfer their loads without processing. Reduction in waiting time spent on the stabilis queens in post emergence colonies frequently engaged in aggressive inter­ nest is one reason why partitioning of loads may increase the efficiency of foraging actions in subtropical populations; in this study queens were infrequently or never (Jeanne 1986a, 1986b ). However, effective task partitioning requires a match be­ observed engaging in dominance interactions. Reasons for this difference are tween numbers of foragers and load receivers, which is easier to achieve in larger unknown; one untested possibility is that reproductive options for subordinate colonies with a group of relatively inactive "reserve" workers. Mature Polistes females vary with latitude (e.g., with seasonality and population synchrony of nest colonies are typically small, rarely exceeding 150 adults in any species (Reeve 1991). founding) and may in turn influence population-typical levels of conflict within Increased task partitioning may have accompanied the evolution of larger colony colonies. size in swarm-founding wasps; even within P. occidentalis, smaller colonies partition Polistes instabilis queens left their nests on most days but never transferred loads less often (Jeanne 1986b). Large colony size may be necessary but not suffi­ materials to nestmates. Low rates of queen foraging, even after worker emergence, cient for increased task partitioning, however, because partitioning is not well devel­ were seen in Polistes metricus (Gamboa et al. 1978). As in this study, Hughes and oped in vespine wasps with large colonies (Jeanne 1991). It remains to be deter­ Strassmann (1988) found that P. instabilis queens ranked high among female wasps mined whether a high degree of load partitioning is typical of swarm-founding in proportion of time on the nest and in frequency of interactions with brood. , but observations of Polybia aequatorialis, P. emaciata, and P. sericea I found that queens play important roles as receivers of foraged material loads, colonies suggest that foragers of these species frequently partition loads (S. O'D., thereby potentially controlling use of materials at the nest. Activity levels of queens pers. obs.). (Reeve and Gamboa 1983, 1987) and task performance by nestmate workers (Dew The low degree of material partitioning in Polistes may indicate greater conflict and Michener 1981; Reeve and Gamboa 1987) have been shown to influence over the control of material use in primitively eusocial insects. Individuals that take foraging behavior in P. fuscatus and P. metricus workers. A fuller understanding of materials can control how they are used at the nest. Polistes instabilis workersf worker polyethism will require further studies of how individual differences are dominance behavior was positively correlated with their frequency of receiving regulated by interactions with queens and other nestmates. materials from foragers. Further evidence for conflict over material handling comes from the fact that the frequency of taking portions of loads was positively correlat­ ed among materials, suggesting that a subset of the workers were able to control Acknowledgements material use at the nest to a high degree. Bette Loiselle, Robert Jeanne, and Larry Phelps gave valuable advice, encouragement, and assis­ Relative dominance status among workers appears to have wide-reaching ef­ tance in the field. Thanks to Werner and Lilly Hagnauer for permission to work on their property. fects in organizing polyethism, as well as on opportunities for personal reproduction Peter Nonacs and two anonymous reviewers made helpful comments on the manuscript. Finan­ (Strassmann and Meyer 1983; Hughes and Strassmann 1988). In a study of worker cial support was provided by the Organization for Tropical Studies, a University of Wisconsin dominance and queen supersedure in P. instabilis, Hughes and Strassmann (1988) Graduate School summer field biology scholarship, NSF grant BNS-8517519 to Robert Jeanne, found that position on the dominance hierarchy correlated with performance of and a postdoctoral fellowship from the NSF funded Behavior Research Training Grant at the University of California at Davis. brood care, but not with foraging rate. Worker dominance interactions are not restricted to Polistes, and also appear to play a role in polyethism in P. occidenta­ lis (O'Donnell and Jeanne in press) and in some highly eusocial vespine wasps (Montagner 1966; Akre et al. 1976). 28 O'Donnell Divison of labor in Polistes instabilis 29

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