Resource Consumption Variance Within and Among Individuals: On Coloniality in Spiders

Abstract

Several models for the economics of sociality demonstrate that the probability distribution of an individual's resource consumption could depend on the size of its social group. When groups share food clumps discovered by any member, expected resource consumption may increase or decline with group size, but the individual's resource consumption variance will ordinarily decline as group size increases. Hence, the formation and dissolution of social groups may represent risk—sensitive responses to foraging success. A series of field studies report greater mean prey consumption and decreased prey consumption variance as group size in colonial spiders increases. Several hypotheses attempt to explain this effect. Coloniality may promote acquisition of information concerning temporal variation in prey availability. Colony membership may permit individuals to steal food when the number of captured prey varies spatially. Finally, colonality can enhance food acquisition because prey (that might otherwise escape) ricochet from one spider's web to another group member's web. Our purpose was to quantify each hypothesis and ask if the resulting models predict reduced resource consumption variance as group size increases. We model each mechanism and conclude that stealing prey is the simplest explanation for variance reduction in spider colonies. We emphasize that variance among individuals need not provide a good estimate of the within—individual resource consumption variance, and it is the latter variance to which risk sensitivity responds.

The relation between maternal phenotype and offspring size, explained by overhead material costs of reproduction

“…1), through which the mother can increase offspring growth efficiency and resistance to starvation, thus excluding too vulnerable stages (body sizes) from the lifecycle (the 'safe harbor' hypothesis of Shine, 1989). Moreover, parental care, group living and sociality can similarly reduce the stochasticity in intake rate (Caraco et al, 1995) and boost the growth efficiency experienced by offspring, which then allows to keep the optimal propagule size relatively small (Fig. 1).…”

Evolutionary drivers of group foraging: A new framework for investigating variance in food intake and reproduction

“…We further present results from a case study on colonial spiders using this new framework. Furthermore, colonial spiders have been the focus of several previous studies on variance reduction as a driver of group living (Uetz 1988;Caraco et al 1995;Uetz 1996). Females feed and breed in groups but do so without direct cooperation (Bilde and Lubin 2011).…”

“…In colonial spiders, sharing of surpluses in the form of prey stealing is thought to reduce variance between group members: successful foragers may store surplus prey in their webs that may be stolen by less successful neighbors (Caraco et al 1995;Uetz 1996). Indeed, we had to conclude that there was no support for CLT in this system despite finding support for both of its key predictions: both per-capita food intake and per-capita reproduction varied less and were more normally distributed in larger colonies.…”

Eusociality outcompetes egalitarian and solitary strategies when resources are limited and reproduction is costly

“…Food resources, specifically the mean amount of resources as well as resource variance, affect foraging decisions as suggested by risk‐sensitive foraging theory (reviewed in Bednekoff, ; Kacelnik & Bateson, ; Smallwood, ; Bateson, ). In this context, group formation has traditionally been seen as a risk‐averse mechanism reducing the variance in resource supply (Caraco, ; Caraco, Uetz, Gillespie, & Giraldeau, ; Clark & Mangel, ; Uetz, ; Uetz & Hieber, ; Wenzel & Pickering, ). The simple idea behind these models is that foraging success may vary: individuals may find more resources than what they are able to fully utilize, or alternatively, they may not find any resources at all, which leads to certain death (Figure b).…”

“…. In this context, group formation has traditionally been seen as a risk-averse mechanism reducing the variance in resource supply (Caraco, 1981;Caraco, Uetz, Gillespie, & Giraldeau, 1995;Clark & Mangel, 1986;Uetz, 1996;Uetz & Hieber, 1997;Wenzel & Pickering, 1991). The simple idea behind these models is that foraging success may vary: individuals may find more resources than what they are able to fully utilize, or alternatively, they may not find any resources at all, which leads to certain death (Figure 1b).…”