The evolutionary process between longevity, sociality and cultural transmission

The maximum life span of mammals varies widely. The shorter-lived shrews, for example, survive approx two yearswhile the bowhead whales can reach 200 years of age. Sociability has long been thought to be a factor that correlates with longevity in mammals: for example, highly socialized and stable females of a baboon species live longer than marginalized females of the same species. Even when comparing different species, taking into account the body mass (which in turn is linked to longevity for metabolic reasons), striking examples were observed: if we compare animals of the same mass and which consume the same type of food, such as shrews and certain insectivorous bats, solitary shrews are found to live only a few years, while some highly social bat species may live for 30 or 40 years.

It has always been imagined, and sporadically verified, that this effect was linked to the protection offered by large groups of individuals from diseases and predators, to the community rearing of the chicks which decreases their mortality thanks to the adoption by the group and to the protection offered and also to the greater efficiency of collective foraging, which allows for better optimization of the exploitation of food resources by the social species. The point, however, is that group life also has many disadvantages: first of all, the inter-individual competition, above all of a sexual and alimentary nature, but also and above all the exposure to a much greater risk of infection by various pathogens, the transmissibility of which is obviously increased by the high number of interspecific contacts that life social entails.

To understand whether, in mammals, in social species the advantages or the risks prevailed in terms of the length of the average life of the individuals, new research has correlated the average lifespan in this class of vertebrates with the sociability of the various species. Specifically, the researchers collected data on the average longevity of 974 mammal species. They then divided these species into three categories: solitary, pair-living, and group-living. When the researchers compared these three groups with longevity data, they found that, after proper normalization for such confounders as body mass, social mammals tend to live longer than solitary species—almost twice as long—and this difference is statistically significant.

Not only that: analyzing the level of expression of comparable genes in 94 mammalian species belonging to the three groups formed according to social lifestyle, the researchers found 31 genes whose levels of relative activity were correlated with both longevity and with a of the three prescribed social categories. Many of these genes appear to have roles in the immune system, which may be of importance when countering pathogens that spread through social group. Other genes were associated with hormone regulation, including some thought to influence social behaviors. These genes, if the correlation is supported by future causal analyses, could therefore be regulated to favor sociability and buffer the negative effects of group lifesuch as increased exposure to infectious diseases, explaining how mammalian social habits and lifespan may have co-evolved.

Moreover, starting from the conclusions obtained in this study it is possible to go one step further. If social life is really causally connected to a greater individual longevity, and at the same time if it is true that it is also connected to a greater protection of the little ones (a fact that is now certain), the possibility opens up of a period of training and learning for these latter of greater extension, compared to what does not happen for the solitary species. The cultural and training phenotype that is handed down in social species such as killer whales, social great apes and humans themselves, therefore, could owe its very possibility of transmission precisely to the formation of sufficiently large and stable groups to lengthen the time window in where it is possible for the parents to train the young, and at the same time prolong the period of immaturity of these. Longevity, sociability and cultural transmission in ours and in other species they could therefore be inextricable parts of a common evolutionary process, at least initially under the aegis of Darwinian selection.