A study of social networks in rock hyraxes has tweaked a conventional wisdom of the Facebook age: that a large network leads to better health and longer life.

Among hyraxes, a small herbivorous mammal that lives in colonies across Africa and the Middle East, one’s network size matters less than its hierarchies.

“Hyraxes that live in more ‘egalitarian’ groups, in which the social ties are spread more evenly, survive longer,” said Amiyaal Ilany, a Tel Aviv University zoologist.

In a study published July 27 in Public Library of Science One, Ilany and colleagues analyze 11 years of painstakingly gathered behavioral data from the hyraxes of Israel’s En Gedi Nature Reserve, turning huddles and cuddles and group sleeps into mathematics of relationship configuration and node centrality.

The hyraxes, which are highly social and can live up to 12 years, proved to have communities of variable character. In some, each animals tended to have networks of similar size and relationships of similar strength. In others, just a few animals had far larger, stronger networks than their peers, some of whom were peripheral and relatively lonesome.

Lifespans proved shortest in the latter colonies. In colonies with more equal networks, which Ilany tongue-in-cheek called “socialist,” hyraxes on average lived several years longer.

The researchers don’t yet have an explanation, but the pattern doesn’t appear to involve isolated hyraxes falling victim to predators or being excluded from shared colony resources. Even highly-connected hyraxes tend to die sooner when living in less-egalitarian groups. As for sheer group size, it mattered little.

According to Ilany, the prime suspect is stress. “We can only speculate,” he said, but “it is known from many studies in other species that stress affects health and longevity.” It’s plausible that tension is highest when a few strong individuals control a group.

Other research shows that stress in hierarchical societies affects both weak and strong individuals. “In a more egalitarian group things may be more calm, as there is less harassment,” said Ilany.

While it’s far too soon to extrapolate to humans a possible relationship between network hierarchy and longevity in hyraxes, the findings do suggest new angles of investigation. So far, studies of social relationships and health — in humans and in other mammals — have focused almost exclusively on individual relationships.

“Contrary to previous studies, we suggest that it is not the number or strength of associations that an adult individual has that is important,” conclude the researchers. Instead, what’s important may be “the overall configuration of social relationships within the group.”

Image: Tambako the Jaguar/Flickr

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Citation: “Variance in Centrality within Rock Hyrax Social Networks Predicts Adult Longevity.” By Adi Barocas, Amiyaal Ilany, Lee Koren, Michael Kam, Eli Geffen. Public Library of Science One, July 27, 2011.