Packs of African wild dogs run down impalas and other fleet-footed prey for a living. But that lifestyle is energetically precarious: running takes a lot of work, and food must be divvied among pack members. Moreover, small stomachs, an adaptation to running, mean the dogs must sometimes abandon their leftovers.

What pack size lets wild dogs maximize their hard-won calories?

The magic number is ten, according to a study by Gregory S.A. Rasmussen, of the University of Oxford, and three colleagues. From 1994 through 2002, Rasmussen tracked twenty-two wild-dog packs in and around Hwange National Park in Zimbabwe, monitoring their activity level, the distance of their chases, their hunting success, and the size of their prey.

From those data, the team calculated that packs ten strong posted the greatest caloric intake per dog. Any other number, and the calories dropped; in packs smaller than five, they plummeted.

The team also found that smaller packs breed fewer pups. They posit that in packs of four or fewer wild dogs, lack of food limits the number of offspring, further reducing pack size—a downward spiral toward oblivion. Most packs at the study site numbered just six, too close to that limit for comfort, particularly given that the species is endangered. The team says populations with small packs should get priority in conservation measures, such as the introduction of new members or special protection from hunters.

The finding was detailed in The American Naturalist.