R. Terry Bowyer

LANDSCAPE HETEROGENEITY AT DIFFERING SCALES: EFFECTS ON SPATIAL DISTRIBUTION OF MULE DEER

We quantified relationships between a suite of landscape metrics measured at different spatial scales and sizes of home ranges for female mule deer (Odocoileus hemionus) to test whether spatial heterogeneity played a major role in determining the distribution of deer. We collected data on female mule deer and their habitats at five diverse study sites that spanned the length of California, USA. Sizes of home ranges (95% adaptive-kernel analysis) for 80 adult female mule deer varied markedly among individual deer (39–2878 ha) and among the five study sites (range of means for each study site, 49–1138 ha). We measured landscape metrics within varying radii (250, 500, 1000, and 2000 m) from the center of the home range for each deer. Landscape metrics such as edge density, mean shape index, and double-log fractal dimension were inversely correlated with the size of loge-transformed home ranges in deer across all spatial scales. Mean nearest neighbor and contagion index were positively correlated with home-ra...

Habitat Selection and Survival of Mule Deer: Tradeoffs Associated with Migration

We examined tradeoffs related to migration in a population of mule deer ( Odocoileus hemionus ) in southern California from 1989 to 1991. All male deer that we radiocollared were migratory, whereas females exhibited a mixed strategy with both migrant and resident individuals. Increased movements of deer were associated with decreased temperatures and increased weekly precipitation. No differences within seasons in the sizes of home ranges occurred for either migrant or resident deer. The size of home ranges of deer, however, was smaller in summer than in winter. Sizes of home ranges were associated positively with proximity to human development and to the amount of avoided (use was less than availability) habitat in the home range. Deer avoided human developments in all seasons. Further, males and resident females used areas farther from water in summer than expected by chance, whereas migratory females selected areas nearer to water. In summer, migratory females selected meadows, riparian habitats, and pine forests, whereas resident females avoided riparian habitats and used pine forests less than did migratory females. Migratory males used habitats in a way similar to migratory females, although they avoided meadows and riparian areas. Clear tradeoffs existed for deer in montane southern California about whether they migrated. Migratory females were farther from human disturbance and used habitats of high quality more often than did nonmigratory conspecifics. Nonetheless, deer were at increased risk of predation during migration and, in years of low precipitation (low snow), had higher rates of mortality than did resident deer. Thus, in areas with extremely variable precipitation and snow cover, a mixed strategy for migration can be maintained in populations of mule deer.

SEXUAL SEGREGATION IN DIMORPHIC DEER: A NEW GASTROCENTRIC HYPOTHESIS

Abstract Why do sexes of polygynous ruminants segregate spatially outside the mating season? Existing hypotheses for differences in niche partitioning among species are not sufficient to explain temporal patterns of segregation and aggregation between sexes. Moreover, other hypotheses, including risk of predation, do not explain why females of some species inhabit sites with higher-quality forage while segregated from males, although competitive exclusion of males by females has been proposed. We offer a new hypothesis to explain this conundrum in sexually dimorphic deer (Cervidae) based on an allometric model of metabolic requirements, minimal food quality, and digestive retention. The model predicts that male deer consume abundant forages high in fiber because ruminal capacity prolongs retention and permits greater use of fiber for energy than in nonpregnant females. Low density of animals, high abundance of food, and adaptations of ruminal microflora keep large males on fibrous forages until quantity of food declines. Compared with males, smaller-bodied females are better suited to postruminal digestion of food, especially when intakes increase concomitantly with requirements for energy and protein during reproduction. High demands for absorption of nutrients during lactation and growth stimulate investment in intestinal and hepatic tissue in females, increasing the cost of maintenance and reinforcing differential use of habitats and forages when sexes are segregated. This new model explains sexual segregation without invoking predation or competitive exclusion of males by females.

Sexual Segregation in White-Tailed Deer: Density-Dependent Changes in Use of Space, Habitat Selection, and Dietary Niche

Sexual segregation, defined as the exclusive use of different areas by males and females at specified spatial and temporal scales, is common among polygynous ruminants and in cervids in particular. Underlying mechanisms for such segregation are not understood fully, and reports have included female cervids segregating into habitats of both poorer and better quality than those used by males. Furthermore, two competing hypotheses of sexual segregation (body-size hypothesis, reproductive-strategy hypothesis) predict different responses to changes in population density; an increase in degree of sexual segregation with increasing density in the former and a decrease in segregation in the latter. We examined patterns of sexual segregation among white-tailed deer (Odocoileus virginianus) on the Welder Wildlife Refuge in south Texas at moderate (39 deer/km 2 ) and high (77 deer/km 2 ) population densities during 1974-1977. At moderate density, females with young made greater use of chaparral-mixed grass habitat with dense cover than did males, where preferred herbaceous forage was less abundant, presumably for reasons of predator avoidance. At high density, which was a result of predator control, sexual segregation among male and females decreased during all seasons (P < 0.05). Males that otherwise used more open habitats increased their use of the chaparral-mixed grass as levels of intraspecific competition increased. As spatial segregation between males and females decreased at the high population density, diets of both sexes shifted toward more graminoids and browse, and shifts were more pronounced among males. The result was decreased dietary overlap between sexes when measured by principal-component analysis. Measures of fat reserves suggested that although both females and males were in poorer condition at high density, females were affected to a greater extent than were males. This outcome suggested that females were not driving patterns of spatial segregation by being better able to compete with males for closely-cropped forages. Rather, predator avoidance by females with young related to the reproductive-strategy hypothesis best explained patterns we observed, and competition between sexes was rejected as a cause of sexual segregation.

Temporospatial Distributions of Elk, Mule Deer, and Cattle: Resource Partitioning and Competitive Displacement

Abstract Rocky Mountain elk (Cervus elaphus nelsoni), Rocky Mountain mule deer (Odocoileus hemionus hemionus), and cattle frequently co-occur on landscapes in the northwestern United States. We hypothesized that niche overlap would be greatest between introduced cattle with either of the 2 native herbivores because coevolution between native elk and mule deer should have resulted in strong patterns of resource partitioning. We observed strong differences among species in use of space, especially elevation, steepness of slope, and use of logged forests. We used 2 temporal windows to examine both immediate (6 h) and long-term (7 days) effects of competition. We noted strong avoidance over a 6-h period among the 3 ungulates. That effect was weaker for the previous 7 days. Cattle were generalists with respect to habitat selection; the 2 native herbivores avoided areas used by cattle. Mule deer and elk avoided one another during the short temporal window (6 h), although spatial differences in habitat use often were not maintained over 7 days. Elk used lower elevations when cattle were absent and moved to higher elevations when cattle were present, indicating shifts in niche breadth and competitive displacement of elk by cattle. We demonstrated strong partitioning of resources among these 3 species, and presented evidence that competition likely has resulted in spatial displacement.

Sexual Segregation in Southern Mule Deer

Sexual segregation in southern mule deer ( Odocoileus hemionus fuliginatus ) was studied on East Mesa, Cuyamaca Rancho State Park, San Diego Co., California, from June 1977– January 1979. Spatial separation of the sexes occurred throughout the dry portion of the year (May–October), but was most pronounced during and immediately following the fawning period (June–August). Bucks occurred most frequently on dry meadows during sexual segregation, and does and fawns primarily in moister meadows. Bucks occurred farther from summer sources of water than did other sex and age classes of deer. Sexual segregation was not attributable to food habits or selection of vegetative types. However, the percent cover of Sisymbrium altissimum , a preferred food, was higher and in earlier phenological stages on ranges occupied primarily by does than in meadows where bucks predominated. The proportion does and fawns comprised of all deer in eight meadow systems was correlated positively with overall population density; an inverse relationship existed between the proportion of bucks and deer density. Consequently, the availability of forage per individual did not vary between ranges inhabited by bucks and does. Sexual segregation in southern mule deer may occur as a result of the greater needs for water of lactating does. Larger body size and rumen to body volume ratio of bucks reduce rates of water loss and may allow males to subsist on vegetation in drier phenological states.

SEXUAL SEGREGATION IN RUMINANTS: DEFINITIONS, HYPOTHESES, AND IMPLICATIONS FOR CONSERVATION AND MANAGEMENT

Abstract The concept of sexual segregation was 1st formally proposed by Charles Darwin. Among mammals, ruminants have been the focus of most research on this phenomenon. Sexual segregation has been defined traditionally as the differential use of space (and often habitat and forage) by sexes outside the mating season, but other hypotheses related to activity patterns of sexes recently have been forwarded. These new hypotheses, however, cannot explain the spatial separation of sexes or their differential use of habitats and forages. Sexual segregation should be considered in a niche framework wherein overlap on 1 niche axis is accompanied by avoidance on another, including space, diet, and habitat. Moreover, sexual segregation is scale sensitive, which limits the usefulness of a comparative approach in investigating differences among species or populations. Failure to discriminate between the potential evolutionary causes of sexual dimorphism in ruminants has led to confusion over whether polygyny or intersexual competition has led to sexual segregation. Neither exploitive nor interference competition, wherein males are competitively excluded by females, is a likely cause of sexual segregation. I suggest that the gastrocentric model, which invokes allometric and life-history differences between sexes, or risk of predation are the only hypotheses necessary to explain sexual segregation. Additional research, however, is necessary to verify some aspects of those hypotheses. The management and conservation of ruminants requires consideration that sexes behave as if they were different species, which holds consequences for estimating populations, manipulating harvest and habitats, and the potential spread of diseases and parasites and might have genetic consequences for some populations. This review attempts to resolve long-standing problems related to studies of sexual segregation, but much research remains to be accomplished, including more manipulative experiments.

Costs and Benefits of Group Living in a Recently Social Ungulate: The Alaskan Moose

Risk of predation is considered a major influence on foraging behavior and the evolution of social groups. We measured foraging efficiency (percentage of active time spent foraging) for Alaskan moose ( Alces alces gigas ) in various categories of predation risk in Denali National Park, Alaska, in 1991. For a particular group size, foraging efficiency declined significantly with distance from cover. Foraging selectivity by moose for diamondleaf willow ( Salix planifolia pulchra ) also decreased as distance from cover increased. Adult females accompanied by young foraged less efficiently than either females without young or adult males. Group size was positively correlated with distance from cover, which suggests that social grouping in moose is an adaptation to increased predation risk. Surprisingly, group size was inversely related to foraging efficiency because of the overriding negative effects of aggressive behavior. This finding contrasts with behavioral trends in more social ungulates, which display increased foraging efficiency with increasing group size. We suggest that the lowered foraging efficiency in Alaskan moose is a result of the recent evolution of gregarious behavior in this subspecies and that increased foraging efficiency within groups of social ungulates may be a subsequent adaptation rather than an a priori selective advantage that led to the evolution of social behavior.

Birth-Site Selection by Alaskan Moose: Maternal Strategies for Coping with a Risky Environment

We studied birth-site selection in Alaskan moose ( Alces alces gigas ) from 1990 to 1994 in Denali National Park and Preserve in interior Alaska. Twenty percent of preparturient females made extensive movements (≥5 km) immediately before giving birth. Females selected (use was greater than availability) sites for giving birth ( n = 39) that were on southerly exposures with low soil moisture and high variability in overstory cover. Moose selected birth sites based on micro-site characteristics rather than on broad types of habitat, which were used in proportion to their availability. Spatial distribution of birth sites did not differ significantly from random locations. We hypothesize that such unpredictable behavior by females is a strategy to avoid predators. Parturient females also selected sites with high visibility that were located at high elevation, which ostensibly allowed them to see and then hide from approaching predators. We rejected the hypothesis, however, that moose in this population spaced themselves away from predators or avoided habitat types favored by large carnivores. Likewise, we rejected the hypothesis that moose gave birth close to human developments to avoid predators; random sites were >100 m closer to human developments than were birth sites. Cover of forage, especially willows ( Salix ), was more than twice as abundant at birth sites than random sites. Forage quality, as indexed by nitrogen content and in vitro dry matter digestibility, was slightly but significantly higher at birth sites. An inverse relationship between visibility and availability of forage indicated that female moose made tradeoffs between risk of predation and food in selecting sites to give birth. Thus, maternal females coped with a risky environment; they gave birth at sites that helped them minimize risk of predation but exhibited risk-averse behavior with respect to the forage necessary to support the high cost of lactation. We hypothesize that risk of predation prevented moose from seeking birth sites with more forage and, hence, a greater nutritional reward, which reduced the variance in forage availability at birth sites.

Habitat Use by Female Caribou: Tradeoffs Associated with Parturition

We compared habitat use, forage characteristics, and group size among preparturient, parturient, and nonparturient female caribou (Rangifer tarandus) during and after the birthing season to test hypotheses involving acquisition of forage and risk of predation, We monitored 39 radiocollared females from the Mentasta caribou herd, Alaska, in 1994 and 40 animals in 1995. Group size of females giving birth at higher elevations was smaller (P 0.5). During peak parturition, females with young used sites with fewer predators (P < 0.05), a lower abundance of forage (P < 0.05), but with variable forage quality compared with those sites used by females without young. We hypothesized that parturient females used birth sites that lowered risk of predation, and traded-off forage abundance for increased safety. Nonetheless, few differences existed between parturient and nonparturient females in composition of diet or in indices of diet quality; we could not demonstrate a nutritional cost to maternal females from our analyses. We suggest that increasing population density might intensify intraspecific competition among females for birth sites, and thereby increase nutritional costs of using high-elevation areas with less forage but fewer predators.