Age-Specific Reproduction of Bighorn Ewes in Alberta, Canada

Most measures of female reproductive success are related to age among large mammals. Younger females are characterized by lower pregnancy rates (Clutton-Brock et al., 1982; Ozoga and Verme, 1982; Saether and Haagenrud, 1983; Schladweiler and Stevens, 1973), smaller litter size (Ozoga and Verme, 1982; Schladweiler and Stevens, 1973; Valdez, 1976), later parturition (Mitchell and Lincoln, 1973; Verme, 1985), and lower weaning success (Clutton-Brock et al., 1982; Ozoga and Verme, 1984; Reiter et al., 1981). Published data on age-specific reproduction of North American mountain sheep (Ovis canadensis and 0. dalli) are limited. For both species, pregnancy rate is lower for yearling than older ewes (Nichols, 1978; Smith and Wishart, 1978). Bunnell and Olson (1981) reported lower natality for young Dall sheep in a population in which ewes first bred at the age of 3.5 years. No information has been published on agespecific lambing dates or lamb survival. Herein, I address the question of whether age affects the probability of producing a lamb, lambing date, and lamb survival to autumn of bighorn ewes. The study was conducted in the Sheep River drainage in southwestern Alberta. The study area (approximately 50 km2), sheep population, and census techniques are described in Festa-Bianchet (1986). Ewes and lambs were captured by use of tranquilizing drugs (Festa-Bianchet and Jorgenson, 1985) or in a corral trap, and marked with plastic ear tags for individual identification. All lambs and 90% of the ewes were captured only once, and 77% of ewes were captured between October and March. Data reported herein refer only to tagged ewes > 1 year of age (n = 82 ewes; 234 ewe-years) and were collected from April 1981 to April 1986. A ewe was classed as lactating if it was seen to suckle a lamb (n = 199), or if the udder exhibited obvious swelling (n = 13) even though a lamb was not seen. Therefore, lactation could be detected also for ewes whose lambs died at or soon after birth, provided the ewes were sighted before the udder regressed. Lamb survival to early autumn was monitored by observing nursery herds until all lambs were seen to suckle or closely associate with an individual ewe: this procedure allowed identification of marked ewes whose lambs had disappeared. Between 23 and 68% (X = 46%) of surviving lambs of tagged ewes each year were captured and tagged in late August-early November. Survival of individual unmarked lambs could not be monitored past October, because they disassociated from their mothers. Survival to October is reported here. As nursing after this month is rare (Horejsi, 1976), survival rates in October provide an indication of weaning success.