• R. Terry Bowyer
  • Dale R. McCullough
  • Gary E. Belovsky


We tested hypotheses concerning evolution of sociality in mule deer (Odocoileus hemionus) by examining effects of resources and risk of predation on group size of these large herbivores. Groups of deer were largest during rut and smallest at parturition. Likewise, habitat has a profound effect on group size with groups being largest in open meadows and smallest in densely vegetated chaparral. Parch size of habitats, however, was unrelated to group size. Availability of preferred forage correlated with number of deer but not their group size. Limited and concentrated sources of free water affected distribution but not group size of deer. Group size increased, however, with distance from concealment cover. Steepness of ruggedness of terrain was not correlated with mean size of deer groups, but maximum group size was related inversely to both variables. With increasing group size, deer increased percentage of time spent feeding and decreased alert and alarm behaviors. Feeding and alert-alarm behaviors of deer also were affected by the likelihood of encountering a predator. Feeding was reduced and alert- alarm postures increased near concealment cover, where deer would encounter stalking predators such as bobcats (Lynx rufus) and mountain lions (Puma concolor), and also far from cover, where coursing predators such a coyotes (Canis latrans) were more effective hunters. Overall rate of aggressive interactions increased with group size, but per capita rate for deer declined as groups increased in size. That outcome ostensibly was a result of larger groups of deer spacing themselves further apart, perhaps because individuals in larger groups were less vulnerable to predators than deer in smaller groups. Resources likely constrained the upper limits to group size by regulating the number of deer available to form groups. Deer balanced the need to acquire food against risk of predation and altered feeding and alert-alarm behaviors accordingly. Groups of deer formed as an adaptation against predation while trying to acquire necessary resources: the evolution of sociality in mule deer is best explained as a tradeoff between those life-history strategies.




How to Cite

Bowyer, R. T., McCullough, D. R., & Belovsky, G. E. (2001). CAUSES AND CONSEQUENCES OF SOCIALITY IN MULE DEER. Alces: A Journal Devoted to the Biology and Management of Moose, 37(2), 371–402. Retrieved from