Solitary mammals live shorter lives than social species

Mammals exhibit a wide variety of social structures, including solitary, pair-bonded and group-living species. They also show an extreme 100-fold range in longevity, from around two years in shrews and mice, to 200 years in bowhead whales. This led Professor ZHOU Xuming from the Institute of Zoology at the Chinese Academy of Sciences (CAS) to wonder whether these two variables were linked. 

Along with other researchers from the Institute of Zoology at the CAS, Professor ZHOU undertook an analysis of various traits found in 1,000 mammalian species, such as their maximum lifespan, body mass, sociality, lifestyle, ecological factors, life history traits, and phylogenetic relationships. The scientists searched published literature for information on these traits, and used the data to determine whether there is a link between sociality and longevity, while taking into account the possible influence of the other factors.

Previous research has usually focused on the effect of sociality on longevity within a single species. For example, chacma baboon females with strong and stable social bonds within a troop live longer than those with weaker bonds, due to better health, reduced levels of stress and increased vigilance against predators. Similar results have also been found for rhesus macaques. 

The researchers used only three categories of sociality, dividing the 974 mammalian species into three types of social organization: solitary, pair-living, and group-living. They then compared models with different evolutionary conjectures between social organization and longevity across the mammalian species, using a Bayesian framework. 

The results of the analysis, published in the journal Nature Communications, showed that group-living species live longer than solitary species. These results suggest a correlated evolution between social organization and longevity. Even species that live in pairs had greater life expectancies than the solitary species. And this relationship held, even when the researchers controlled for phylogeny, body mass and factors related to external mortality, such as activity (diurnal, nocturnal or others), lifestyle (terrestrial, aerial, arboreal, semi-arboreal, freshwater, marine, or terrestrial-marine), and fossoriality (living underground or above ground). 

The researchers then conducted a comparative brain transcriptomic analysis of 94 of the mammal species to identify genes that could underpin the correlated evolution of social organization and longevity. They hoped to detect candidate genes and pathways associated with both social organization and longevity, in order to elucidate the molecular mechanisms involved in this relationship. 

The experts found that hundreds of genes were significantly associated with solitary living, pair-living, or group-living, and a total of 262 genes were significantly correlated with longevity. However, they crucially identified 31 genes whose expression levels were significantly associated with both social organizations and longevity. Some of these genes were involved with hormone regulation, but many were associated with immunity pathways. The researchers propose that “hormonal regulation and immunity constitute the mechanistic foundation for the association between social organization and longevity.”

The findings indicate that mammals living in groups have better innate immune responses, perhaps precisely because diseases and parasites can be spread so easily between group members. Indeed, previous research has shown that in captive group-living macaques, individuals with more social connections have enhanced immune responses when compared to those living in isolation or with few social ties.

“Our study provides insights into the correlated evolution of social organization and longevity and serves as a basis for experimental validation and follow-up studies on the mechanistic drivers of this correlated evolution,” concluded the researchers.

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By Alison Bosman, Earth.com Staff Writer

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