Intraspecific variation in social organization by genetic variation, developmental plasticity, social flexibility or entirely extrinsic factors

Abstract

Previously, it was widely believed that each species has a specific social organization, but we know now that many species show intraspecific variation in their social organization. Four different processes can lead to intraspecific variation in social organization: (i) genetic variation between individuals owing to local adaptation (between populations) or evolutionarily stable strategies within populations; (ii) developmental plasticity evolved in long-term (more than one generation) unpredictable and short-term (one generation) predictable environments, which is mediated by organizational physiological effects during early ontogeny; (iii) social flexibility evolved in highly unpredictable environments, which is mediated by activational physiological effects in adults; (iv) entirely extrinsic factors such as the death of a dominant breeder. Variation in social behaviour occurs between individuals in the case of genetic variation and developmental plasticity, but within individuals in the case of social flexibility. It is important to study intraspecific variation in social organization to understand the social systems of species because it reveals the mechanisms by which species can adapt to changing environments, offers a useful tool to study the ultimate and proximate causes of sociality, and is an interesting phenomenon by itself that needs scientific explanation.

Figure 1.

The four processes that can lead to intraspecific variation in social organization. To explain the variation observed at the population level, we need to consider variation at the individual level. Only the two processes in (b,d) represent phenotypic plasticity. (a,b) Different individuals differ in social behaviour. (a) Genetic variation: individuals of the same species but two different populations differ genetically, leading to differences in social organization. (b) Developmental plasticity: depending on the environment in which individuals grow up, environmental cues will activate developmental pathways for social behaviour that either leads to the same social organization as observed in their parent generation (environment 2) or to a different kind of social organization (environment 1). (c,d) The same individuals occur in different forms of social organization. (c) Entirely extrinsic factors: the unpredictable death/disappearance of some breeding individuals is the only cause of changes in social organization. This itself will cause behavioural flexibility in the survivors, such as mate searching, to return to the original state, but flexibility is the result, not the cause of the observed intraspecific variation in social organization. (d) Social flexibility: if the environment changes, social tactics of individuals change, which as a consequence will change the social organization of the population. This is reversible, as the same individuals can switch their tactic again, if the environment changes again. (Online version in colour.)