Spotted hyenas, just like Facebook users, often prefer to form bonds with friends of friends.

Scientists call this type of clustering “triadic closure.”

“Cohesive clusters can facilitate efficient cooperation and hence maximize fitness, and so our study shows that hyenas exploit this advantage,” says Amiyaal Ilany, a postdoctoral researcher in the biology department at the University of Pennsylvania.

“Interestingly, clustering is something done in human societies, from hunter-gatherers to Facebook users.”

Ilany and colleagues collected more than 55,000 observations of social interactions of spotted hyenas during a 20-year period in Kenya, making it one of the largest studies to date of social network dynamics in any non-human species.

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.

Hyenas can live as long as 22 years, typically in large, stable groups known as clans, which can comprise more than 100 individuals. They are socially sophisticated predators that can discriminate maternal and paternal kin from unrelated hyenas.

They are also selective in their social choices and tend not to form bonds with every hyena in the clan, rather preferring the friends of their friends.

Lasting relationships

Published in the journal Ecology Letters, the research shows that 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 allow rainfall amounts to influence her choice.

“In spotted hyenas, females are the dominant sex and so they can be very flexible in their social preferences,” says Kay E. Holecamp, professor of zoology at Michigan State University.

“Females also remain in the same clan all their lives, so they may know the social environment better. 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.

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The researchers used a new, more comprehensive method than those used in earlier studies, a type of mathematical modeling typically found in sociology, to arrive at their findings about the social world of the spotted hyena.

This more dynamic approach allowed them to evaluate the simultaneous effects of multiple factors—environmental, individual, genetic and structural—on network dynamics. It also gave the researchers insight into how or why the social structure changes over time and to isolate the factors that shape the structure. The method represents a major advance over methods used in previous studies of animal social networks where more static approaches have typically been applied.

The National Science Foundation, US Department of Homeland Security, US Department of Agriculture, University of Tennessee, Knoxville, and National Institutes of Health supported the study.

Doctoral candidate Andrew S. Booms of Michigan State was a coauthor of the study.

Source: University of Pennsylvania