Optimal Foraging and Risk of Predation: Effects on Behavior and Social Structure in Ungulates

Optimal foraging theory predicts that animals will either attempt to maximize energy gained or minimize time spent to obtain a fixed amount of energy. A time-minimizing approach implies that an animal is attempting to maximize time spent in other behaviors such as reproduction or to minimize its exposure to temperature extremes, predators, or some other factor in the environment while foraging. Indeed, many ungulates must balance the need to obtain sufficient energy and other nutrients required for maintenance, growth, and reproduction while avoiding predation. Adopting social behavior that results in the formation of herds confers several advantages to the individual because of the difficulty a predator has in approaching large groups, or in capturing individuals in the confusion caused by a fleeing herd. Such behavior is often seen in ungulates occurring in open habitats where coursing predators are common. The problem becomes more acute, however, for ungulates living in closed habitats year-round, where predators commonly hunt by stealth, or for those sex and age classes such as females with young that exhibit solitary behavior. Such species or sex and age classes would be expected to exhibit a time-minimizing strategy at least seasonally. Use of linear-programming models of dietary choice have been successful in predicting classes of forages consumed by ungulates and other generalist herbivores and indicate that they often follow an energy-maximization strategy. Nonetheless, overwhelming evidence indicates that ungulates modify their behavior in the presence of predators. I suggest that decisions about when and how to forage are being made at different scales, and these differences may account for observed discrepancies between models and empirical evidence. Finally, new analytical techniques such as stochastic dynamic programming may allow development of more realistic models of foraging behavior and may better incorporate observed behaviors in ungulates.