Cooperative Defense by Female Bighorn Sheep

Abstract

Observations of cooperative defense against predators by bighorn sheep (Ovis canadensis) are rare. Shank (1977) and Woolf and O'Shea (1968) observed cooperative anti-predator defense by O. canadensis rams against coyotes (Carlis latrans), and Hornocker (1969) reported group defense by three ewes against a bobcat (Lynx rutus). Here, we report cooperative defense of O. canadensis ewes against a larger predator, C. Iatrans. Our observations were made on Specimen Mountain, Rocky Mountain National Park, Colorado at a distance of 0.7 km with the aid of 8 power binoculars and a 25-60X telescope. Ovis canadensis congregate in the crater of Specimen Mountain (approximately 2 km2 in area) during early summer to use mineral licks derived from volcanic ash and to forage on adjacent tundra slopes. One hundred and one individuals were counted in the crater on 25 June 1987. Thirtyone sheep (including 11 lambs) were involved in the C. Iatrans-O. canadensis interaction we witnessed. We first observed group 1 (12 ewes, 2 yearlings, and 11 lambs) moving and resting on the tundra ridge that extends into the center of the crater. Group 1 included two marked ewes with lambs and one marked ewe without a lamb. Another group of 13 sheep fed below the ridge in Crater Gulch. This group split into group 2 (2 ewes, 3 yearlings, and a 2-year-old ram) that fed up into cliffs south of the Gulch; and group 3 (4 ewes and 2 young rams) that fed south up a side draw. Both groups were about 0.3 km from the ridgetop. One of the group 3 ewes was marked and had a lamb, that we assumed was resting in group 1. Group 2 included one marked ewe that did not have a lamb. Suddenly, group 3 ran north across the Gulch and up slope to join group 1 which quickly dropped off the ridge to an area of cliffs on the south-facing slope. All of the lambs and several larger sheep (ewes and/or yearlings) stood in the cliffs. The rest of the larger sheep (15-18 individuals, predominantly ewes but including at least one 3-year-old ram) formed a tight group and moved up over the top of the ridge. They walked in stiff alarm posture (Geist 1971: plate 23) across the flat alpine tundra ridgetop, approximately 100 m wide. We then saw a large C. Iatrans running across the ridgetop directly toward the advancing sheep. Seeing the sheep, the predator reversed direction and ran out of view followed by the sheep. Of interest is the fact that group 3, that was known to include a maternal ewe, rushed to join the lamb group when it was alarmed. In contrast, group 2 which probably did not include maternal ewes (one of two was known not to have a lamb), fled into cliffs in the opposite direction. The sheep segregated by vulnerability to attack; all lambs and only a few larger sheep remained in the cliffs, while adults advanced toward the predator. Caching of lambs is a common behavior of O. canadensis ewes but it seldom occurs as a direct response to a predator threat (Geist 1971:251). More often we have observed alarmed ewes rush to join their lambs, lead them to secure terrain, and stay with them until the threat has passed. Although defense or attempted defense of lambs by individual ewes against C. Iatrans (Ashcroft 1986, Berger 1978, Woolf and O'Shea 1968), or golden eagles (Aquila chrysaetos) (Geist 1971: 248) has been reported, the typical response is flight into steep, rugged terrain. In contrast, much larger elk (Cerrus elaphus nelsoni) cows are known to mob C. Iatrans in defense of calves (Geist 1982). We believe that the large number of O. canadensis present in the crater accounts for this atypical response. Predation by C. Iatrans may be an important cause of mortality for O. canadensis in Rocky Mountain National Park; ten successful attacks by coyotes on lambs or yearlings have been documented by Park Service personnel (Stevens and Goodson, unpubl. data). Our observation increases the evidence for active anti-predator defense in O. canadensis; and suggests that, in large groups, medium-sized ungulates may adopt anti-predator defenses typical of larger ungulates.