Social Organization of Mountain Lions: Does a Land-Tenure System Regulate Population Size?

Abstract

Mountain lions (Puma concolor) are thought to regulate their populations via social behavior. The proposed mechanism is a land-tenure system that results in exclusion of individuals from the population through territoriality and temporal avoidance. In the absence of mortality from intraspecific aggression, social behavior can regulate a population only by limiting reproduction. Successful reproduction among large mammals is related to the availability of food. Four states of nature must hold if a population is regulated by social behavior via a land-tenure system in mountain lions: (1) individuals should not be distributed randomly, but each should have its own distinct distribution, and those individuals should maintain regions of exclusivity; (2) use of food within the distribution of an individual should not be random, but should be clumped as individuals try to exclude each other from access to prey; (3) those clumps of prey must not be simply the result of prey distribution, but of social interactions among lions; and (4) social interactions and defense of food should occur in regions where distributions of individuals overlap; therefore, prey use by individual lions in areas of overlap should be less than expected based on the distribution of prey. We tested hypotheses regarding social regulation for a population of mountain lions that co-occurred on a winter range with a population of mule deer (Odocoileus hemionus) in the eastern Sierra Nevada, California, from 1991 to 1997. Individual mountain lions (n = 10) exhibited distinct distributions, and deer killed by individuals (n = 112) were not distributed randomly within the distribution of the lion that did the killing. Furthermore, the nonrandom distribution of lion-killed deer could be explained by the distribution of live deer alone, but that result was marginally not significant (P = 0.06) and indicated that something else affected the locations of kills made by lions. Results from tests of whether the presence of another mountain lion affected where individuals chose to kill prey indicated that social interactions had no effect. The distribution of deer killed by individual mountain lions in areas of exclusive use and areas of overlap was identical to that expected based on the distribution of live deer alone. That outcome indicated social behavior was not regulating the population of mountain lions via partitioning of prey, and temporal differences in use of space could not explain the distribution of mountain lions we observed. A system of land-tenure and mutual avoidance did not limit the population of mountain lions in Round Valley via partitioning of prey. Our results are concordant with other studies of large mammalian carnivores, which reported that populations were not limited primarily by territoriality but by the supply of food.