Intersexual Resource Partitioning in Black-Tailed Deer: A Test of the Body Size Hypothesis

To understand deer-habitat interactions important to making management decisions, it is necessary to ask questions in an evolutionary framework. Then, patterns that are detected can be evaluated in terms of how they potentially influence fitness, and to make more accurate predictions about changes in habitat use with changes in the environment. Intersexual resource partitioning in Odocoileus spp. is considered to be affected by sexual dimorphism in body size. Body size influences metabolic requirements which in turn presumably influence feeding behavior. Thus, I tested 5 predictions with black-tailed deer (O. hemionus columbianus) on Hopland Field Station, Mendocino County, California, 1989-91, to determine if body size and its presumed impact on feeding behavior cause intersexual resource partitioning. Predictions, tested using radio telemetry and by measuring feeding behaviors of free ranging animals, ranged from general expectations about spatial distribution to specific predictions about feeding behavior. Males had larger (P < 0.001) homerange sizes than females, except in summer. Based on body size considerations female home ranges should be 0.75 the size of males. Mean ratios mostly varied from 0.30 to 0.40. However, because of large variation, confidence intervals overlapped 0.75 during most seasons. Females exhibited a higher (P < 0.05) degree of site fidelity among seasons than males. The sexes differed (P < 0.001) in use of habitats among months, however, the pattern was not consistent. Generally, deer used more open habitats (grassland, chaparral grassland, and oak grassland habitats) in the wetter winter months, and more closed habitats (oak woodland, chaparral) in the drier summer months. There was considerable monthly variation in percent of time active (P = 0.004) but no differences (P = 0.59) between the sexes. Type of forage and percent of time a deer's head was in the feeding position were the only variables correlated (P < 0.001) with number of bites taken in 7- to 10-minute feeding sessions. Type of forage accounted for 92% of the variation associated with number of bites. No differences were detected between the sexes in number of bites taken (P = 0.34) or percent of time head was in the feeding position (P = 0.39) on any forage type. There was no (P > 0.42) relationship between incisor breadth and body mass for each sex. Deer on Hopland Field Station exhibit sexual segregation, but the reason they segregate cannot be explained by diff