Floyd W. Weckerly

Sexual Segregation in Ungulates: New Directions for Research

We reviewed the literature on sexual segregation in polygynous ungulates in an effort to clarify terms and concepts, summarize recent information that supports or discredits three broadly defined hypotheses, and suggest directions for future research that should help resolve when and why the sexes segregate in these large mammals. The hypotheses discussed include those based on intersexual differences in energetics and security (reproductive-strategy hypothesis), body size dimorphism and dietary requirements (sexual dimorphism-body size hypothesis), and social mechanisms (social-factors hypothesis). These hypotheses represent ecological, physiological, and social perspectives and are not mutually exclusive. Most evidence reviewed supported the reproductive-strategy hypothesis. Less support was available for either the sexual dimorphism-body size hypothesis or the social-factors hypothesis. Nonetheless, most available evidence is provided by field studies that contend with many confounding variables. We suggest several areas of future study that may serve as critical tests and are likely to be productive in resolving why sexual segregation occurs in polygynous ungulates.

Sexual-size dimorphism : Influence of mass and mating systems in the most dimorphic mammals

I examined influence of body size and mating systems on sexual-size dimorphism by summarizing characteristics and testing for associations among the most dimorphic mammalian taxa—Macropodidae, Primates, Mustelidae, Pinnipedia, Elephantidae, Ruminantia. The most dimorphic taxa were seals in Otariidae. On average, males were three times larger than females, and all otariids displayed extensive dimorphism. Except for the Strepsirhini, most taxa had dimorphism ratios (mass of males:mass of females) between 1.2–1.8. Extent of dimorphism increased with body size but the effect was slight (power function between masses of males and females, 1.04–1.05) for most taxa. Phocid seals and macropodid marsupials had power functions of ca. 1.2. Mating systems were associated with size dimorphism in simian primates and ruminants. Monogamous simian primates were less dimorphic than simians that had polygynous mating systems. Ruminants with tending and harem mating systems were more dimorphic than those with territorial polygynous and monogamous mating systems. Polygyny and how it was conducted were associated with the extent of sexual size dimorphism.

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

Sexual Segregation in Roosevelt Elk: Cropping Rates and Aggression in Mixed-Sex Groups

Abstract Few studies of sexual segregation in ruminants have tested widely invoked mechanisms of segregation in mixed-sex groups. In a sexually segregated population of Roosevelt elk (Cervus elaphus roosevelti), we examined if adult males had reduced intake of forage when in mixed-sex groups and if intersexual differences in aggression caused females to avoid males. Based on a mechanistic model of forage intake, animals with lower instantaneous feed intake should have higher cropping rates. Focal animal sampling indicated that adult males and females in summer and winter had similar cropping rates in mixed-sex groups, whereas males in male-only groups had lower rates of cropping than males in mixed-sex groups. Outside the mating season, males in male groups spent proportionally less time ≤1 body length of congenders than females in female groups, and the rate of aggression ≤1 body length was higher for males. Female–female aggression was higher in mixed-sex groups that contained more males than the median proportion of males in mixed-sex groups. Female and mixed-sex groups walked away when groups of males numbering >6 were ≤50 m but did not walk away when male groups ≤50 m had ≤5 individuals. Sexual segregation was associated with behaviors of sexes in mixed-sex groups: reduced intake of forage by males and increased female–female aggression with more males.

ARE LARGE MALE ROOSEVELT ELK LESS SOCIAL BECAUSE OF AGGRESSION

Abstract Large males are solitary or occur in small groups in numerous ruminants. I propose that large males may become less social because of differences in costs and benefits of aggression as they age and increase in size and social status. Outside the mating season, large males have little to gain from sparring and interacting aggressively with other large males. I examined if large male Roosevelt elk (Cervus elaphus roosevelti) were more solitary or in smaller groups than other size or sex classes, associated with other large males, directed more aggression to small males, and displayed behaviors that reduced aggression when in groups. Large males were in the smallest groups and solitary more than twice as often as small males and females, and groups with large males had ≥50% of the group composed of large males. In relation to small males, large males sparred less frequently, were more dispersed in groups, and directed more aggressive behaviors toward them. Differences in costs and benefits of aggression between small and large males may help explain the asocial nature of large males.

Age and sex differences of white-tailed deer diet composition, quality, and calcium

We examined differences between age groups (fawns, ad) and sexes in diet composition, quality (crude protein and fiber content), and calcium content in white-tailed deer (Odocoileus virginianus) in Tennessee, 1983-86. We used a factorial design with body mass as a covariate to determine that diet composition varied between sexes in October and between age groups in January. Females consumed more acorns than males did in October, but males compensated by eating more agricultural crops. Diet quality varied between the sexes in October but was not consistent among years. For most time periods, diet composition, quality, and calcium content varied among years. Diets of fawns and adults do not agree with predictions based on body mass, and male and female diets do not follow patterns reported for deer of the George Reserve, Michigan

Inferring Absence of Houston Toads Given Imperfect Detection Probabilities

Abstract

Sources of inaccuracy in black-tailed deer herd composition counts

Herd composition counts to establish sex and age structure are commonly used to assess population status for deer (Odocoileus spp.) and other large ungulates. Such counts are frequently biased, which compromises their usefulness. We evaluated composition counts of black-tailed deer (O. hemionus columbianus) for biases by comparing known behavior of a sample of radio-telemetered animals with the outcome of systematic dawn and night spotlight herd composition counts. Three sources of error (differential use of habitats, activity schedules, and deer behavior relative to observers) varied (P ≤ 0.036) by diel period; only activity schedules did not vary (P ≤ 0.521) by sex and month

A comparison of effects of radiotransmitter attachment techniques on captive white-winged doves

Abstract We experimentally evaluated alternative techniques of attaching radiotransmitters to captive white-winged doves (Zenaida asiatica) in Kingsville, Texas during 1998. Our evaluation consisted of monitoring physiological, pathological, and behavioral parameters in doves subjected to 6 radiotransmitter attachments (backpack harnesses, adhesive, subcutaneous implants, intracoelomic implants, subcutaneous surgeries without implantation, intracoelomic surgeries without implantation). We analyzed physiological parameters across 2 pretreatment and 4 post-treatment periods using a model-selection approach of mixed-effect models. Birds did not differ in physiological variables among treatment groups and a control. Time-activity budgets analyzed using nonparametric Friedmans tests did not differ in any activity category among treatment groups and a control. Subcutaneous implants were the most effective method of attachment based on retention rates, lack of mechanical difficulties associated with external attachment techniques, and minimum levels of pathology reported following necropsies.

RELIABILITY OF OCCUPANCY AND BINOMIAL MIXTURE MODELS FOR ESTIMATING ABUNDANCE OF GOLDEN-CHEEKED WARBLERS (SETOPHAGA CHRYSOPARIA)

ABSTRACT. Reliable estimates of population parameters derived from survey methods are essential for decision making in management of endangered species. We evaluated whether point-count surveys used in conjunction with occupancy and binomial mixture models (BMMs) constituted a reliable approach for monitoring the federally endangered Golden-cheeked Warbler (Setophaga chrysoparia) on a preserve in central Texas. Occupancy and abundance were estimated using point-count surveys conducted on each of five 113-ha detection grids in 2008 and seven grids in 2009. Single-season occupancy models and BMMs were used to estimate occupancy and abundance, respectively. Occupancy estimates per grid ranged from 0.48 to 1.0 in 2008 and from 0.52 to 1.0 in 2009. Estimates of abundance were compared with territory densities independently estimated using spot mapping, the standard by which all other avian survey methods are often compared. Abundance estimates produced by BMMs were significantly higher than territory density estimates at all but one site in 2008 and two sites in 2009. While estimation techniques incorporating detection probabilities should be considered in monitoring programs, our results suggest that BMMs deserve careful scrutiny before being used to estimate abundance or to monitor population trends.