Sex is more important than prey when it comes to explaining why cheetahs eschew the typically solitary feline lifestyle to live in groups, says a new book by UC Davis behavioral ecologist Tim Caro. Most cats are loners; among the 37 kinds of wild cats, only lions and cheetahs live in groups. One popular explanation says that carnivores live together to enjoy the fruits of hunting in groups. Not so, says Caro in "Cheetahs of the Serengeti Plains: Group Living in an Asocial Species." Cheetahs live in small groups to increase their success in mating, defending territory and fending off predators, argues Caro, an associate professor of wildlife, fish and conservation biology with the UC Davis Center for Population Biology. Drawing from 10 years of study and 5,000 hours of field observations, Caro combines a detailed natural history with evolutionary theory, ending with a controversial chapter on conservation of this endangered species.