Mammals that live in groups tend to have longer lifespans than solitary species, new studies show of nearly 1,000 different animals.
Scientists from China and Australia compared 974 species of mammals, analyzing their lifespan and their social organization.
Dividing mammals into three categories – solitary, paired and group – the researchers found that animals that live in groups, such as elephants and zebras, live longer on average than solitary species, such as aardvark and eastern chipmunk.
The correlation persisted even when the researchers took into account the relationship between larger species size and longer lifespans.
The maximum life span of mammals ranges from two years for shrews to over 200 years for bowhead whales.
Solitary northern short-tailed shrews and large horseshoe bats, which live in groups, are the same weight, for example, but live a maximum of about two and 30 years, respectively.
The researchers also conducted a genetic analysis of 94 species and identified 31 genes that are associated with both social organization and longevity.
Genes are primarily associated with immunity and hormones, the latter of which the study authors believe may play a role in social behavior.
The authors of the study suggest that “a group lifestyle reduces external mortality by limiting the risks of predation and starvation, and strong and stable social ties formed between group members can increase life expectancy.”
“These benefits are expected to outweigh the costs inherent in group life, such as competition for mating partners and food, stress from dignitaries, and the spread of infectious diseases through social contact,” they wrote.
Previous studies on specific species that live in groups, such as the Chakma baboon, have shown that people with strong social bonds lived longer than those with weaker, less stable relationships. Scientists have documented similar results in rhesus monkeys.
But sociality seems to play a different role in animals that don’t necessarily live in groups. A 2018 paper on yellow-bellied marmots, a “socially resilient” species, links strong social relationships to shortened lifespans.
Whether longevity provides any evolutionary advantage remains debatable. Associate Professor Celine Frere, an evolutionary biologist at the University of Queensland who was not involved in the study, said some short-lived animals reproduce much faster than long-lived species.
“In an evolutionary context, what a person seeks to do is pass on their genes to future generations,” she said. “An animal that lives for two years, compared to a whale that lives for 200 years, is more likely to have produced the same number of offspring in its entire life.”
Frere said the results of the study were interesting, but his classification of species as solitary, paired, or grouped was “a simplistic approach to considering social organization”.
“They want to show that species that live in groups live longer. But everything is much more complicated. [Lifespan] related to their ecology. It has to do with their reproductive biology, with their mating system.
“Whether you are a solitary animal or an animal living in a group, you must learn to live with others… and you will compete with others for access to resources.”
Frere added that social organization varies considerably among mammals living in groups, from highly structured dominance hierarchies to split-fusion societies in which the size and composition of social groups change over time.
The study was published in the journal Nature Communications.
