Abstract
Social insects provide good model systems for testing trade-offs in decision-making because of their marked reproductive skew and the dilemma workers face when to reproduce. Attaining reproductive skew requires energy investment in aggression or fertility signaling, creating a trade-off between reproduction and dominance. This may be density-dependent because the cost of achieving dominance may be higher in larger groups. We investigated the effect of group-size in B. terrestris queenless workers on two major reproduction-dominance correlates: between-worker aggression, and pheromone production, aiming at mimicking decision-making during the transition of worker behavior from cooperation and sterility to aggressive reproductive competition in whole colonies. Despite the competition, reproductive division of labor in colonies can be maintained even during this phase through the production of a sterility signal by sterile workers that has an appeasement effect on dominant nestmates. Worker-worker aggression, ovary activation, and production of sterility-appeasement signals may therefore constitute components of a trade-off affecting worker reproduction decisions. By constructing queenless groups of different size and measuring how this affected the parameters above, we found that in all groups aggression was not evenly distributed with the α-worker performing most of the aggressive acts. Moreover, aggression by the α-worker increased proportionally with group-size. However, while in small groups the α-worker monopolized reproduction, in larger groups several workers shared reproduction, creating two worker groups: reproductives and helpers. It appears that despite the increase of aggression, this was evidently not sufficient for the α-worker to monopolize reproduction. If we compare the α-worker to the queen in full-sized colonies it can be hypothesized that worker reproduction in B. terrestris colonies starts due to a gradual increase in the worker population and the queen's inability to physically inhibit worker oviposition. This may shift the trade-off between cost and benefit of worker reproduction and trigger the competition phase.
Highlights
The balance between cost and benefit is a powerful driving force underlying decision-making in animal societies
Among other factors, attaining reproductive skew in favor of one female requires her to invest energy either in aggression or in chemically signaling her fertility, creating a trade-off between reproduction and dominance [2]. This trade off may be quantity or density dependent since the cost of achieving dominance in larger groups is not equivalent to that in smaller groups, it is not surprising that group-size has a profound effect on the behavior and physiology of the individuals that compose it, especially among groups that cooperate together such as social insects
The results obtained in the present study demonstrate that reproductive competition among queenless B. terrestris workers develops along similar pathways as in whole colonies during the competition phase
Summary
The balance between cost and benefit is a powerful driving force underlying decision-making in animal societies. Among other factors, attaining reproductive skew in favor of one female requires her to invest energy either in aggression or in chemically signaling her fertility, creating a trade-off between reproduction and dominance [2]. This trade off may be quantity or density dependent since the cost of achieving dominance in larger groups is not equivalent to that in smaller groups, it is not surprising that group-size has a profound effect on the behavior and physiology of the individuals that compose it, especially among groups that cooperate together such as social insects. Use the phrase ‘‘quantity/density effect’’ to describe the effect of both, while using the term ‘‘group size’’ as to describe the factual quantity effect
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