Pregnancy Determination in Uncaptured Feral Horses

Abstract

The urinary excretion of estrone sulfate (ES) by 25 free-roaming feral horses (Equus caballus) was measured by radioimmunoassay applied to extracts of urine-soaked soil. Twelve of 15 mares having ES concentrations >1.0 Ag/mg creatinine (f = 2.64 + 1.02 [SD]) produced foals. All 10 mares with ES concentrations <1.0 gg/mg creatinine (t = 0.44 ? 0.26) did not foal. Extracting urine from soil and measuring ES and creatinine can be used to determine pregnancy in free-roaming feral horses without the stress of capture or immobilization. J. WILDL. MANAGE. 52(2):305-308 The ability to determine pregnancy in freeroaming animals is poorly developed. Birth rates are an inadequate index because they do not provide information about fetal losses, which are vital in understanding nutrition, general health, or stress. To accurately assess pregnancy in wild animals, individuals are usually trapped or immobilized, blood samples taken, and pregnancy determined by measuring estrogen or progestin concentrations or some other specific pregnancy test (Kirkpatrick 1980). These methods, though adequate for domestic animals, frequently are cumbersome, expensive, and disruptive or severely disturbing when applied to wild animals (Seal et al. 1985). The concentrations of urinary steroids or their conjugated metabolites that change during pregnancy have been measured in many species. Captive animals in which pregnancy has been determined by measuring urinary estrogen or progestin metabolites include the gorilla (Gorilla gorilla) (Hopper et al. 1968, Czekala et al. 1983), ruffed lemur (Lemur variegatus) (Shideler et al. 1983a), vervet monkey (Cercopithecus aethiops) (Setchell et al. 1980), lion-tailed macaque (Macaca silenus) (Shideler et al. 1983b), rhesus monkey (Macaca mulatta) (Liskowski et al. 1970, Liskowski and Wolfe 1972), baboon (Comopithecus hamadryas) (Hodges et al. 1986), Indian rhinoceros (Rhinoceros unicornis) (Kassam and Lasley 1981), and horse (Raeside and Liptrap 1975, Evans et al. 1984, Kasman et al. 1984). Poole et al. (1984) measured testosterone concentrations in elephant (Loxodonta africana) urine samples deposited on the ground; urine samples were collected by aspirating the pool of urine minutes after urination occurred. Steroid analysis of urine-soaked soil samples could be an easy, accurate, and inexpensive method to determine pregnancy in fre -roaming species with a minimum of disturbance to the population being studied. Ou objective was to develop a test of pregnancy in free-roaming feral horses that is accurate, inexpensive, does not require capture or handling, and does not disturb the population being studied. Specifically, we tested for pregnancy by measuring a urinary estrogen metabolite conjugate, estrone sulfate, in urine-soaked soil. We thank L. R. Taylor, Bureau of Land Management, Billings, Montana, for providing age data for the mares in this study, and S. M. Kruemmling for assistance in collecting urine samples.