Mating in bighorn sheep: frequent male reproduction via a high-risk “unconventional” tactic |
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Authors: | John T Hogg Stephen H Forbes |
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Institution: | (1) Craighead Wildlife-Wildlands Institute, 5200 Upper Miller Creek Road, Missoula, MT 59803, USA Tel.: (406) 251-3867; Fax: (406) 251-5069; e-mail: cwwi@mssl.uswest.net, US;(2) Division of Biological Sciences, University of Montana, Missoula, MT 59812, USA Tel.: (406) 243-5503; Fax: (406) 243-4184; e-mail: sforbes@selway.umt.edu, US |
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Abstract: | Rocky Mountain bighorn rams use three distinct tactics in competition for mates. Two tactics (tending and blocking) feature
defense and cooperative mating over a relatively prolonged consort period (up to 3 days). In the coursing tactic, subordinate
rams fight dominants for temporary copulatory access (lasting seconds) to defended ewes. By combining population-wide genetic
(microsatellite) exclusion of paternity, behavioral data and a model of bighorn reproductive competition, we estimated that
coursing rams fathered 44% of 142 lambs assigned paternity in two natural populations. In one population, the probability
of successful defense against coursing was lowest among rams that had many female consorts and held highest dominance rank.
Even so, per capita annual male reproductive success was positively associated with social rank in both herds when measured
in terms of fall conceptions. The proportion of coursing versus defending ram matings in each population (0.36 and 0.39) was
similar to the corresponding fraction of lambs (0.43 and 0.47) fathered by coursing rams, suggesting that sperm competition
approximated a fair lottery. Male traits important in gaining social status and obtaining cooperative consorts with ewes were
different and potentially in conflict with those needed to defend against (and practise) coursing. Although the concussive
weapons (horns) of rams are less dangerous than, for example, the piercing weapons of other bovids, injury from falls and
horn blows during coursing brawls may cause death, handicap future mating competition or increase the risk of predation. Coursing
is a rare example of an unconventional alternative mating tactic that is high-gain and high-risk.
Received: 17 April 1996 / Accepted after revision: 15 March 1997 |
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Keywords: | Mating tactics DNA microsatellites Paternity Bighorn sheep |
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