Abstract
Eusociality is one of the most complex forms of social organization, characterized by cooperative and reproductive units termed colonies. Altruistic behavior of workers within colonies is explained by inclusive fitness, with indirect fitness benefits accrued by helping kin. Members of a social insect colony are expected to be more closely related to one another than they are to other conspecifics. In many social insects, the colony can extend to multiple socially connected but spatially separate nests (polydomy). Social connections, such as trails between nests, promote cooperation and resource exchange, and we predict that workers from socially connected nests will have higher internest relatedness than those from socially unconnected, and noncooperating, nests. We measure social connections, resource exchange, and internest genetic relatedness in the polydomous wood ant Formica lugubris to test whether (1) socially connected but spatially separate nests cooperate, and (2) high internest relatedness is the underlying driver of this cooperation. Our results show that socially connected nests exhibit movement of workers and resources, which suggests they do cooperate, whereas unconnected nests do not. However, we find no difference in internest genetic relatedness between socially connected and unconnected nest pairs, both show high kinship. Our results suggest that neighboring pairs of connected nests show a social and cooperative distinction, but no genetic distinction. We hypothesize that the loss of a social connection may initiate ecological divergence within colonies. Genetic divergence between neighboring nests may build up only later, as a consequence rather than a cause of colony separation.
Highlights
Understanding how and why animal societies are organized in the way they are has long been a focus of biological research
Within a social insect colony, the workers do not themselves reproduce and are behaving altruistically by helping the queens reproduce. This altruism can be explained by inclusive fitness theory, with indirect fitness benefits to the workers accrued via the enhanced reproduction of kin (Bourke, 2011; Hamilton, 1964)
Positive relatedness between interacting organisms is required for the evolution of altruism, and as such, members of a social insect colony are expected to be more related to one another than they are to other individuals within the population
Summary
Understanding how and why animal societies are organized in the way they are has long been a focus of biological research. Polydomous trail networks represent connections between cooperating nests (which we use as a shorthand to mean cooperating ant communities, each living within its own nest), sharing workers and resources, in line with the expectations of a social insect colony. Wood ant trails frequently persist in the same location for many years (Rosengren, 1971); during mapping of trail networks of F. lugubris in the UK over 3 years, separate trail networks were never observed to become connected by trails (Ellis, Franks et al, In Review; Ellis, Procter et al, In Review), indicating stability of distinct networks of connected nests. The social connections characterized by worker and resource movement appear cooperative; we predict that they will be reflected by increased relatedness between socially connected nest pairs
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