Marmots, ground squirrels found throughout the Northern Hemisphere, are a social animal often used in animal studies of behaviour – most recently, they revealed that agonistic individuals are generally more successful than those with a more temperate disposition
Credit: Wikimedia
SYDNEY: Seemingly harmful behaviours of some social animals are actually advantageous to their reproductive success and longevity, according to evolutionary biologists who now suggest the concepts of beneficial social relationships are incomplete.
A long-term study of a natural population of marmots – a stocky rodent found in the Northern Hemisphere – found that ‘popular’ marmots were more successful than their less connected peers, regardless of whether their popularity was defined by friendly or aggressive relationships.
“This is the first study to demonstrate that agonistic relationships may increase fitness, and will hopefully extend current discussions on the adaptive value of social relationships,” said evolutionary biologist Daniel Blumstein, a co-author of the study in the Proceedings of the National Academy of Sciences.
Social interactions: agonistic or affiliative?
Blumstein and his colleagues used social network statistics to quantify 12 social attributes in a natural population of yellow-bellied marmots Marmota flaviventris observed over a six year period.
They measured individuals’ tendency to initiate, or be the target of, harmful ‘agonistic’ exchanges, such as aggression and physical displacement. They also measured beneficial ‘affiliative’ interactions, such as play, grooming and foraging in close proximity to one another.
As well as these social network statistics, the researchers made use of existing observations of longevity and reproductive success to estimate the fitness of individuals, together with a previously published DNA data set to estimate heritability.
Nature of interaction not relevant
As may be expected, their data revealed that the most integrated marmots within a population were the most successful. Surprisingly, it also suggested that the nature of social interactions – be they aggressive or friendly - was of no relevance.
“We found that ‘popular’ marmots were more successful than individuals with fewer social connections. Individuals that were well integrated and centrally positioned in their social group lived longer and had more pups than more isolated individuals,” said Blumstein.
“Amazingly, this trend held for affiliative and agonistic relationships, suggesting that traditional views of aggressive interactions as harmful and costly are rather incomplete. We need to know more about the role of agonistic interactions in social dynamics and rethink traditional views of beneficial social relationships.”
Agnoistic interactions a by-product of popularity
“If there are benefits from being in a group and having lots of strong relationships, animals must be able to tolerate aggressive interactions. Agonistic interactions are a likely by-product of being a very social/popular animal, and it therefore makes sense that agonistic interactions do not negatively influence fitness.”
Or as lead-author Amanda Lea, also of the University of California, puts it, “popular individuals are at the centre of the social group in terms of friendly and unfriendly interactions, and they seem to benefit from this position despite traditional ideas about aggressive interactions carrying a large cost.”
They also found that a marmot’s tendency to be victimised – that is, receive aggressive interactions – had the largest variation of any trait that is a function of genetic differences, as oppose to environmental differences.
Whereas the behaviour of initiating agonistic relationships exhibited no genetic variation between individuals.
The researchers suggest this is likely due to strong selective pressures having fixed initiated aggressive traits within the population.
”Aggression has a selective advantage”
Evolutionary biologist Mark Elgar from the University of Melbourne told Cosmos online “the lack of genetic variation in initiated agonistic behaviour is consistent with the idea that aggression has a selective advantage.”
“The strong genetic correlation between affiliative and agonistic behaviours is not very surprising, and may reflect natural variation in the tendency of individuals to interact – either aggressively or affiliatively."
By way of explanation, Elgar places it in a human context, “shy people find it equally difficult to strike up a friendly conversation or rebuke someone who is acting offensively, while a garrulous person would not be bothered by either task.”
“Overall, it a very interesting study, and hopefully it will stimulate similar studies of other species, in which these kinds of data are available," Elgar concluded.
