Can you summarise the theme issue for us?
This theme issue summarises our current understanding of the multiple relationships between sociality and health, as well as their consequences for individual fitness, in various social species, ranging from ants to primates and humans. In particular, it looks at two types of social factors that have direct health consequences: social stress arising from unavoidable competition among group members and pathogen transmission through social contacts. This focus on sociality adds a new perspective to understanding variation in health and fitness outcomes.
How do we define sociality?
Sociality, in the most fundamental sense, refers to life in permanent groups. In the majority of animal species, adult individuals only meet for mating and spend the rest of their time as solitary individuals. However, associating permanently with several conspecifics in stable groups has a number of advantages, especially with respect to improved protection from predation. It is therefore no surprise that group living has evolved repeatedly in all major animal lineages. Because the evolutionary switch from a solitary to a social way of life comes with so many fundamental consequences, it is considered as one of the major evolutionary transitions.
Does greater social interaction mean greater susceptibility to disease or better immunity?
Actually both. One the one hand, living in permanent association with other individuals of the same species engenders competition for resources and mates, which is often mitigated by establishing dominance relationships. It turns out that both, being at the top and at the bottom of a dominance hierarchy can be stressful, which in turn enhances susceptibility to infectious and non-infectious disease. In addition, having frequent close social contacts facilitates transmission of some parasites and pathogens among group members, also challenging individuals’ health. On the other hand, some social interactions counteract these unavoidable costs of group living. For example, in ants and other social animals mutual grooming serves to remove pathogens or ecto-parasites whereas in primates strong social bonds with friends and kin can enhance survival and reproductive success.
As humans, what can we learn from the behaviour of other animals regarding sociality, health and disease?
Humans are subject to the same costs of group living and their detrimental effects on health as other social animals. Comparative studies on a range of different animal species can therefore help identify general principles concerning links between sociality and health, and the underlying physiological mechanisms can often be better studied in suitable animal models. However, comparative studies can also identify aspects that may be uniquely human, such as negative health effects of loneliness. In addition, studies of natural animal populations can reveal which pathogens are socially transmitted and which types of social contacts are necessary for social transmission. This kind of information may contribute to better future containment of epidemics, such as the current Ebola outbreak.