bioRxiv preprint doi: https://doi.org/10.1101/652677; this version posted May 30, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 1 DIVISION OF LABOR INCREASES WITH COLONY SIZE, REGARDLESS OF GROUP COMPOSITION, IN 2 THE SOCIAL SPIDER STEGODYPHUS DUMICOLA 3 4 Authors: Colin M. Wright1*, James L. L. Lichtenstein2, C. Tate Holbrook3, Justin Pretorius2, Noa 5 Pinter-Wollman4 and Jonathan N. Pruitt2,5 6 7 1. Department of Biology, The Pennsylvania State University, University Park, PA 16801, 8 USA 9 2. Department of Ecology, Evolution and Marine Biology, University of California Santa 10 Barbara, Santa Barbara, CA 93106-2015, USA 11 3. Department of Natural Sciences, College of Coastal Georgia, Brunswick, GA 31520 12 4. Department of Ecology and Evolutionary Biology, University of California Los Angeles, 13 Los Angeles, CA, 90095-1606, USA 14 5. Department of Pyschology, Neurobiology & Behaviour, McMaster University, Hamilton, 15 ON, Canada L8S 4K1 16 17 *To whom correspondence should be addressed. 18 Email: [email protected] 19 bioRxiv preprint doi: https://doi.org/10.1101/652677; this version posted May 30, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 20 ABSTRACT 21 Division of labor (DOL) is a pattern of work organization where individual group members 22 specialize on different tasks. DOL is argued to have been instrumental for the success of eusocial 23 insects, where it scales positively with group size both within and across species. Here we evaluate 24 whether DOL scales positively with group size in a society of cooperative breeders (social spiders) 25 and whether this pattern is impacted by the behavioral composition of the group. To do this we 26 engineered experimental colonies of contrasting group sizes and behavioral compositions and 27 tracked individuals participation in two colony maintenance tasks: prey capture and web 28 construction. As with some eusocial insects, we found that larger groups exhibited DOL metrics 29 up to 10-times greater than smaller groups, conveying that individuals specialize on particular 30 tasks more in larger colonies. This scalar relationship did not differ by a groups behavioral 31 composition, though groups composed of only bold spiders exhibited reduced DOL relative to all- 32 shy or mixed groups. We also found that per capita participation in prey capture, but not web 33 construction, decreased as a function of group size. This suggests that individuals in larger groups 34 may save energy by reducing their involvement in some tasks. Together, our results convey that 35 similar scalar relationships between DOL and group size can emerge both inside and outside the 36 eusocial insects. Thus, theory developed for understanding DOL in eusocial societies may inform 37 our understanding of group function in a larger swath of animal social diversity than is broadly 38 appreciated. 39 40 41 42 43 bioRxiv preprint doi: https://doi.org/10.1101/652677; this version posted May 30, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 44 SIGNIFICANCE STATEMENT 45 46 Division of labor (DOL) has been a major area of research in the eusocial insects for 47 decades, and is argues to underlie their ecological success. Only recently have other social 48 arthropods, such as social spiders, been considered for studies concerning DOL. Given their 49 smaller colony sizes, and absence of morphological castes, DOL was not thought to be an 50 important facet of spider societies. However, we found that spider societies do indeed exhibit high 51 degrees of DOL that is positively correlated to colony size, as seen in many eusocial insects. These 52 findings suggest that the scalar relationship between group size and social organization seen in 53 social insects is likely generalizable to a larger diversity of social taxa, and that cooperative 54 breeders can show levels of division of labor equaling or exceeding those of eusocial systems 55 evaluated to date. 56 57 Keywords: division of labor, collective behavior, personality, sociality, spider, social behavior 58 59 60 bioRxiv preprint doi: https://doi.org/10.1101/652677; this version posted May 30, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 61 INTRODUCTION 62 63 The concept of Division of labor (DOL) was first articulated by the economist Adam Smith in his 64 book The Wealth of Nations (1776). In it, he described a process whereby workers can increase 65 group productivity and output by assigning different jobs to different individuals. The increase in 66 productivity and efficiency produced by DOL is attributed to several factors: (1) time saved by the 67 elimination or reduction of task switching, (2) reduced task learning costs, and (3) increased task 68 efficiency associated with practiced motor learning and dexterity. In many social taxa, including 69 humans, individuals often differ innately in their aptitudes for different tasks. Thus, DOL and 70 within-group behavioral variation can allow workforces to adaptively align individuals with 71 contrasting aptitudes with the tasks that they each perform best (Oster and Wilson 1978). Although 72 DOL was originally formulated in the context of the human factory and assembly-line workers, it 73 has since been recognized as a ubiquitous phenomenon in biological systems from cells to social 74 insect colonies (Szathmary and Smith 1995). Many examples have been identified across the 75 animal kingdom, where DOL is defined as a pattern of non-random association between individual 76 worker identities and the tasks that they perform (Michener 1974; Rypstra 1993; Duarte et al. 77 2011a; Jeanson 2013; Jeanne 2016). 78 While research on division of labor has traditionally focused on eusocial insects, which 79 exhibit reproductive DOL between queen and worker castes (Oster 1978), other animal societies 80 further divide non-reproductive labor among workers or group members. Interindividual variation 81 in task performance can be associated with differences in morphology (e.g., majors vs. minors) 82 (Wilson 1980; Holldobler and Wilson 1990; Kay and Rissing 2005), age (temporal polyethism) 83 (Wilson 1971; Seeley 1982), innate response thresholds (Bonabeau et al. 1996; Bonabeau et al. bioRxiv preprint doi: https://doi.org/10.1101/652677; this version posted May 30, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 84 1998; Beshers 2001; Jeanson 2013), reproductive state (social spiders: (Junghanns et al. 2017), or 85 consistent individual differences in behavioral tendencies (Dornhaus 2008; Holbrook et al. 2014; 86 Wright et al. 2014). This last element of individual variation can arise from a variety of 87 mechanisms including individual differences in genotype, gene expression, hormonal changes, and 88 learning/experience (Duarte et al. 2011b). In some cases, such differences can cause individuals 89 to perform better at some tasks than others (Oster 1978; Holldobler and Wilson 1990). 90 DOL, however, is not a simple, static trait. Rather, it is a dynamic emergent property of a 91 group. Within a species, for instance, DOL tends to increase with colony size (Holbrook et al. 92 2011). This social scaling effect is consistent with a general principal known as the size-complexity 93 rule (Bonner 2004) that applies to many biological entities, including human societies. This rule 94 states that, in general, larger collective entities have higher DOL than smaller entities, and these 95 “entities” can refer to individual organisms composed of various types of cells or cooperative 96 societies composed of various types of individuals. The scaling of DOL in functionally integrated 97 societies could be an evolved response that allows them to overcome logistical challenges 98 associated with increasing group size, and thus, helps to increase performance efficiency. 99 Alternatively, the positive relationship between DOL and group size could be an emergent 100 consequence of social dynamics, as predicted by self-organizational models (Gautrais et al. 2002; 101 Merkle and Middendorf 2004; Jeanson et al. 2007; Ulrich et al. 2018). This latter hypothesis has 102 received some support from empirical tests in forced associations of normally solitary ant 103 foundresses (Jeanson and Fewell 2008) and sweat bees (Holbrook et al. 2013). 104 The DOL index developed by Gorelick et al. (2004) and Gorelick and Bertram (2007) 105 provides researchers with a rigorous and generalizable method to measure DOL and make 106 comparisons across social groups and taxa. In short, their approach quantifies the degree to which bioRxiv preprint doi: https://doi.org/10.1101/652677; this version posted May 30, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 107 individuals specialize on a subset of tasks instead of dividing their time evenly across all tasks, 108 and the degree to which different individuals specialize on different tasks.