According to a team of scientists led by Dr Amiyaal Ilany of the University of Pennsylvania, spotted hyenas (Crocuta crocuta) prefer to have friendship relations with friends of their friends.
Spotted hyenas (Crocuta crocuta). Image credit: Steve Jurvetson / CC BY 2.0.
The scientists collected more than 55,000 observations of social interactions of spotted hyenas (also known as laughing hyenas) over a 20 year period in Kenya, making this one of the largest to date of social network dynamics in any non-human species.
They found that cohesive clustering of the kind where an individual bonds with friends of friends, something scientists call ‘triadic closure,’ was the most consistent factor influencing the long-term dynamics of the social structure of these mammals.
Individual traits, such as sex and social rank, and environmental effects, such as the amount of rainfall and the abundance of prey, also matter, but the ability of individuals to form and maintain social bonds in triads was key.
“Cohesive clusters can facilitate efficient cooperation and hence maximize fitness, and so our study shows that hyenas exploit this advantage. Interestingly, clustering is something done in human societies, from hunter-gatherers to Facebook users,” said Dr Ilany, who is the lead author on the study published in the journal Ecology Letters.
Hyenas, which can live up to 22 years, typically live in large, stable groups known as clans, which can comprise more than 100 individuals.
According to the scientists, hyenas can discriminate maternal and paternal kin from unrelated hyenas and are selective in their social choices, tending to not form bonds with every hyena in the clan, rather preferring the friends of their friends.
They found that hyenas follow a complex set of rules when making social decisions. Males follow rigid rules in forming bonds, whereas females tend to change their preferences over time. For example, a female might care about social rank at one time, but then later choose based on rainfall amounts.
“In spotted hyenas, females are the dominant sex and so they can be very flexible in their social preferences. Females also remain in the same clan all their lives, so they may know the social environment better,” said study co-author Dr Kay Holekamp of Michigan State University.
“In contrast, males disperse to new clans after reaching puberty, and after they disperse they have virtually no social control because they are the lowest ranking individuals in the new clan, so we can speculate that perhaps this is why they are obliged to follow stricter social rules.”
Knowing why and how these animals form lasting relationships can help scientists better understand cooperation patterns and the consequences of sociality in other species.
_____
Amiyaal Ilany et al. Topological effects of network structure on long-term social network dynamics in a wild mammal. Ecology Letters, published online May 14, 2015; doi: 10.1111/ele.12447
