Caring for the sick is not a recent human invention. Diseased wolves living in packs live far longer than those that are solo, because they have the support of those around them, new research has found.
The reintroduction of gray wolves (Canis lupus) into Yellowstone National Park has been a revelation, providing environmental benefits far beyond what was anticipated, and inspiring “rewilding” campaigns worldwide. It has also been of great benefit to scientific researchers.
Lone wolves in the park that become infected with the parasite Carcoptes scabiei, which causes the mange, are five times more likely to die in a month than those that are healthy, a team led by authors from Penn State University reports in Ecology Letters.
On the other hand, when the mangy wolves are part of a pack containing at least five healthy wolves, their death rate drops to match their companions. "Our hypothesis is that pack-mates are able to offset the survival costs of infection with mange -- and perhaps other infections -- by assisting with food acquisition and territory defense," says senior author Professor Peter Hudson. As the number of infected wolves in a pack rises, the risk of death is unchanged for healthy wolves and rises only slightly for those with mange, unless the number free from mange gets very small.
It might be expected that contagious diseases like mange would be a threat to the health of the entire pack. Nevertheless, lead author, PhD student Emily Almberg, says that there is no relationship between pack size and risk of infection.
Almberg acknowledges that this would not be true for every infectious disease, but says, "What we've under-appreciated in the past are the ways in which social species might compensate for this increased disease risk. In some cases, social species exhibit adaptive behavior to limit the spread of disease -- things like defending territories or having distinct social roles within the group that limit contact and therefore disease transmission, but our research has shown that group living can alleviate the actual cost of an infection as measured by survival rates."
The details of how healthy wolves support the sick are much harder to track than survival rates, but the fact that they paid no price, at least in terms of mortality, is striking.
The study is the first clear example of its sort in mammals, although something similar was proposed for lions with tuberculosis. Replication will require finding diverse species that live both socially and alone. The authors also note that mange is an easier condition to diagnose at a distance than most diseases.
However, if findings for other species prove consistent, they will provide an insight into why humans evolved to live in groups, as well as demonstrating that supporting the weak and sick is fundamental to our nature.