Benefits of Group Living Include Increased Feeding Efficiency and Lower Mass Loss during Desiccation in the Social and Inbreeding Spider Stegodyphus dumicola.

Bram Vanthournout,Trine Bilde,Jesper Bechsgaard,Anne Bruun,Johannes Overgaard,Michelle Greve

doi:10.3389/fphys.2016.00018

Bram Vanthournout, Trine Bilde + Show 4 more

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https://doi.org/10.3389/fphys.2016.00018

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Abstract

Group living carries a price: it inherently entails increased competition for resources and reproduction, and may also be associated with mating among relatives, which carries costs of inbreeding. Nonetheless, group living and sociality is found in many animals, and understanding the direct and indirect benefits of cooperation that override the inherent costs remains a challenge in evolutionary ecology. Individuals in groups may benefit from more efficient management of energy or water reserves, for example in the form of reduced water or heat loss from groups of animals huddling, or through reduced energy demands afforded by shared participation in tasks. We investigated the putative benefits of group living in the permanently social spider Stegodyphus dumicola by comparing the effect of group size on standard metabolic rate, lipid/protein content as a body condition measure, feeding efficiency, per capita web investment, and weight/water loss and survival during desiccation. Because energetic expenditure is temperature sensitive, some assays were performed under varying temperature conditions. We found that feeding efficiency increased with group size, and the rate of weight loss was higher in solitary individuals than in animals in groups of various sizes during desiccation. Interestingly, this was not translated into differences in survival or in standard metabolic rate. We did not detect any group size effects for other parameters, and group size effects did not co-vary with experimental temperature in a predictive manner. Both feeding efficiency and mass loss during desiccation are relevant ecological factors as the former results in lowered predator exposure time, and the latter benefits social spiders which occupy arid, hot environments.

Highlights

Elucidating the underlying factors that give rise to group living remains a challenging task because many selective forces could potentially affect the cost and benefits
Because we found that larger spiders extracted more fly mass (Supplementary Results 2), we calculated the fly mass extracted per unit spider mass (EpSM) by dividing the extracted fly mass (FM) by the total spider mass in a petri dish (SM)
The initial SMR experiment comparing individuals and groups of five did not reveal a significant effect of group size on SMR (F = 1.59; P = 0.431, N = 72), but the standard metabolic rate was much higher at 30◦C than at 22◦C (F = 1.49; P < 0.0001, N = 72, Figure 1)

Summary

Introduction

Elucidating the underlying factors that give rise to group living remains a challenging task because many selective forces could potentially affect the cost and benefits. The maintenance of group living depends critically on these net benefits outweighing the costs that an individual experiences from being in a group. There is evidence that cooperation can increase fitness by lowering energy and/or resource requirements for certain tasks (Muradian et al, 1999; Tojo et al, 2005), such energetic benefits of group living are arguably less wellexplored, for ectotherms. In endotherms, group living behavior such as huddling allows animals to conserve heat (Gilbert et al, 2010), thereby providing a distinct energetic advantage, especially in cold environments. While huddling behavior is displayed in tropical endotherms, it does not always provide a distinct energetic advantage in these environments, which already have an ample supply of heat energy. As ectotherms do not expend energy to maintain a constant body temperature huddling has been suggested to provide benefits in the form of a reduced rate of water loss (Broly et al, 2014) and a reduced metabolic rate (Tojo et al, 2005)

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