Foraging Behavior by Mule Deer: The Influence of Cattle Grazing

We studied the effects of different cattle stocking rates on activity patterns of female mule deer (Odocoileus hemionus) on a summer range in the Sierra Nevada of California. Using an automated telemetry system, we determined that deer averaged 32 ? 2.2 (SE)% of the time feeding, 8 ? 1.1% traveling, and 60 ? 2.4% resting per 24-hour period. Deer spent more time feeding and less time resting with increased cattle stocking rates. During 1984, a year of average precipitation, deer spent more time feeding per day in late summer than in early summer in range units grazed by cattle but did not do so in ungrazed range units. In 1985, a drier year, deer spent less time feeding per day in late summer in grazed range units. Time spent feeding by deer was negatively correlated with standing crop of herbaceous forage in meadow-riparian habitats. Deer increased their time spent feeding by shortening the length of resting bouts and including more feeding bouts each day, not by increasing the length of each foraging bout. Companion studies indicated that with cattle grazing, deer home-range sizes were larger (Loft 1988), and hiding cover for fawns was reduced (Loft et al. 1987). The results are consistent with the hypothesis that cattle competed with deer, particularly at high stocking rates and during a year of below-average precipitation. We suggest that female mule deer were acting as time-minimizers to meet the high energetic demands of lactation while minimizing their exposure to predators. Management options to reduce adverse effects include reducing or eliminating cattle grazing during early summer on all or part of the grazing allotment. J. WILDL. MANAGE. 55(4):665-674 The debate over competition between livestock and native ungulates is centuries old (Stover 1985). Competition between 2 species occurs in the broad sense if an increase in either one harms the other (MacArthur 1972). Specifically, 2 species compete when they both use a resource in short supply, or if the resource is not in short supply, when 1 species seeking the resource harms the other in the process (Birch 1957). Competition between large herbivores is difficult to measure at the population level because of logistics associated with manipulating numbers of 1 species and measuring changes in another. However, studies of foraging behavior in a multi-species system can be used as a first step in examining interspecific competition, particularly where numbers of one or both species can be manipulated. Traditional optimal foraging theory predicts that animals either maximize the amount of energy gained over time or minimize the amount of time needed to obtain some fixed level of energy (Schoener 1971, Belovsky 1984, Pyke 1984, Stephens and Krebs 1986). However, some of the underlying simplifications of optimal foraging theory have been criticized (Stephens and Krebs 1986, Pierce and Ollason 1987). Mangel and Clark (1986), McNamara and Houston (1987), and Lima and Dill (1990) argued that an animal must not only maximize its rate of energy intake, but also must avoid predators and reproduce. When those factors are considered, optimal foraging theory can provide a framework for understanding the foraging behavior of mule deer. We examined activity patterns of free-ranging female mule deer on a summer range stocked with varying numbers of cattle. Unlike studies using tame deer, deer in our study were freeranging, and as such, subject to constraints on foraging behavior imposed by predator-avoidance strategies and by the need to bear and raise fawns. We attempt to show how mule deer must balance those sometimes conflicting needs and how potential competition with cattle affects that balance. Specifically, we discuss changes in the amount of time spent feeding by deer, number and duration of feeding bouts, and changes in the times of the day that foraging occurs, and use optimal foraging theory to suggest reasons behind those changes. This 2-year study was part of a series of investigations conducted during 1983-85 on the Present address: California Department of Fish and Game, 1416 Ninth Street, Sacramento, CA 95814.