Demography of Feral Burros in the Mohave Desert

Abstract

Age, sex, and body and reproductive condition of 631 burros (Equus asinus), and sex of 79 burro fetuses were determined from the Mohave Desert, California. The age distribution was skewed to young age classes in both sexes. Twelve individuals lived > 10.5 years, and males lived longer than females.The postnatal sex ratio (61 M: 100 F) was skewed, whereas the prenatal sex ratio (68.1:100) did not differ significantly (P > 0.05) from parity. Males had better body condition than females. Poor female body condition was associated with costs of pregnancy and/or lactation. Females of reproductive age (?1.5 years) had a 61.9% pregnancy rate and a 35.2% lactation rate. Sixty percent of lactating females were also pregnant, which indicated that burros can give birth in consecutive years. J. WILDL. MANAGE. 51(4):916-920 Introduced burros (McKnight 1958, Carothers et al. 1976, Woodward and Ohmart 1976, Hanley and Brady 1977), goats (Capra hircus) (Spatz and Mueller-Dombois 1973), and European wild boars (Sus scrofa) (Bratton 1975) have become management problems because expanding populations have resulted in altered habitat. Federal protection for burros under the Wild Free-Roaming Horses and Burros Act of 1971 resulted in population increases throughout the western and southwestern United States, which made management actions necessary by the late 1970's. Lack of demographic data, however, has inhibited effective management of burro populations. Most of the available demographic information on burros was gathered ancillary to studies conducted for other reasons (Moehlman 1974, Woodward 1976, Norment and Douglas 1977) or from studies limited in scope (Morgart 1978, Walker 1978). The objective of our study was to quantify age distribution, sex ratio, body condition, and female reproductive condition of an undisturbed feral burro population in the Mohave Desert, California. The area is a secured military reservation, and California's 1952 moratorium on capturing or killing free-roaming burros was in effect for almost 30 years prior to the study. We thank R. Heinz, B. Kohfield, and S. Stockton for help in the field and J. Berger, J. G. Blake, C. L. Douglas, P. R. Krausman, P. C. Marsh, W. L. Minckley, R. D. Ohmart, and A. T. Smith for valuable comments on drafts of this manuscript. The project was supported by the U.S. Dep. Navy (Purchase Orders N60530-81W-700C and N60530-81-M-01DA).