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Social Isolation: Animals that break away from the pack can influence evolution

For some animals, such as beetles, ants, toads, and primates, short-term social isolation can be just as vital as social interaction to development and long-term evolution, according to a new study led by researchers from the University of St Andrews and the University of Georgia.

The research, led by evolutionary biologists, Dr Nathan Bailey, School of Biology, University of St Andrews and Professor Allen Moore at the University of Georgia, published in the journal Trends in Ecology & Evolution (17 June 2018), describes approaches for testing how an animal’s isolation might impact natural selection and evolution. This framework can help design more effective breeding, reintroduction, and conservation strategies.

Research on evolution typically focuses on the importance of social interactions, including parent-offspring bonding, competition for resources, and courtship and mating rituals  – realised that isolation must then be an extreme condition worthy of equal attention.

“The environment an animal experiences can influence which genes it expresses, when, and how much, so conditions of social isolation might cause expression of different traits,” says Dr Bailey. “This in turn could affect responses to natural selection in terms of survival and reproduction, which has evolutionary consequences. For some species, it might even mean that temporary social isolation is favourable.”

The invasive cane toad Rhinella marina of Australia, for instance, will venture off on its own to expand into new territory, but the isolation this causes drives an uncharacteristically strong attraction to members of the opposite sex upon the toad’s return to a social environment. This boosts the likelihood of both communication and successful mating, which are necessary for survival as the toads expand into new regions. This means that social isolation itself provides the conditions for natural selection to favour adaptations to cope with it.

Toad
Cane toads (Rhinella marina) at the leading edge of their range expansion in Australia are more cut off from other individuals – they are, by definition, the toads who have gotten furthest, and there do not tend to be very many of them in newly-occupied habitat. A recent study found that male toads from one such recently established, peripheral population display a greater tendency for social attraction than toads from longstanding populations. Their enhanced social behaviour might give them an advantage in conditions of relative social isolation, by increasing the chances that they will transfer useful information, be protected against predators, and mate. (Photo credit: Dr Nathan W. Bailey).

Likewise, when poisoned, the European ant Temnothorax unifasciatus secludes itself from its kin until death. This eliminates contact with its nestmates, protecting them from the infection, ensuring its relatives’ survival, and overall lessening some of the costs associated with social living, such as spreading disease.

Dr Bailey added: “Traits expressed during social interactions might exist because they’ve been shaped by selection, but at the same time, social interactions themselves represent a type of environment that can select and shape how individuals behave.”

This duality of social interaction as both trait and environment merits further study, and Bailey and Moore propose gaining a more complete understanding of social isolation’s effects using a measurement termed the “index of social isolation.” The index would allow researchers to compare an animal’s ideal amount of isolation with how much it is actually experiencing.

In Hawaii, some male crickets (Teleogryllus oceanicus) have lost the ability to sing, which protects them from deadly parasitoid flies that find hosts by eavesdropping on their sexual advertisement calls. In populations with many of these silent males, there may be a perception of social isolation that does not match reality: if you are a cricket in such a population, how do you react when you think nobody else is around because there are no songs detectable in the environment? Some evidence suggests that the ability to change behaviour after perceiving social isolation might have promoted the rapid evolution of protective silence among males. (Photo credit: Dr Nathan W. Bailey).

To do this, researchers must first measure the optimal balance of interaction and isolation by testing individuals with different levels of each to find the best possible outcome in terms of survival and reproduction. Comparing this ideal to real observations will help determine whether animals are more or less isolated than they should be and ultimately allow for more effective designs for conservation strategies, reintroduction models, and breeding programs.

“To understand how short-term social isolation experienced by individual animals translates into trans-generational evolutionary impacts for a larger population, we need a number, something measurable that we can compare across different species and contexts,” says Bailey, “after all, isolation that has negative effects for one species could in fact be beneficial for another.”

The paper Evolutionary Consequences of Social Isolation is published today (17 June 2018) in Trends in Ecology and Evolution and is available online

Issued by the University of St Andrews Communications Office.

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