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On the behaviour and evolution of American mountain sheep

University of British Columbia

Behaviour studies were undertaken on three populations of free living American sheep, one being a Stone's sheep (Ovis dalli stonei) one a Dall's sheep (0. d. dalli) and one a bighorn sheep population (0. canadensis canadensis). The habitat-, body care- and social behaviour patterns - except those of mother and young - were described in detail. Each social interaction recorded was transferred to a computer card. 3800 interactions were analysed and form the basis of the quantitative data reported. American sheep show only minor quantitative differences in their social behaviour. However, behavioural differences within populations of the same species occur and appear to reflect population quality. Sheep evolution is conceived as follows: Sheep evolved from rupicaprid ancestors. They evolved large horns, pneumated skulls, lost or reduced display hairs, increased the size of the rump patch and increased sexual dimorphism. The damaging fighting forms of the rupicaprids were replaced by a ritualised form of combat. Sheep lost the thick hide as a defence mechanism, and defend themselves by catching the horn blow with their horned head. Skulls evolved to absorb concussion. The broadside display was replaced by horn displays. Ram horns function not only as weapons, but also as guards, display organs and rank symbols. Sheep appear to have evolved rapidly whenever they colonized new habitat in the wake of retreating glaciers. The expanding populations would experience intense selection for forceful clashing and larger horn size. Large horn size is a function of neoteny in rams. Rams mature sexually at 1.5 - 2.5 years of age, but do not mature behaviourally or reach ultimate growth form until they are 7-8 years old. Females are paedo-genic forms which remain similar in appearance and behaviour to sexually mature yearling rams. Neoteny also explains the long body growth of rams, the reduction and loss of the neck ruff, increased sexual dimorphism, the intense gre-gariousness of females and of juvenile rams, the increasing independence and leadership of rams as they grow older, and the more frequent use of aggressive patterns by bighorn as compared to thin horn sheep. Rams prefer to interact with rams of their own horn size and with females. They treat subordinates, irrespective of sex, much the same as they treat females. Rams change their behaviour not with the sex of the companion but with its dominance rank. The more that subordinate rams resemble females in appearance, the more they are treated sexually by dominant rams. Subordinate rams may act like females to the extent of urinating, or, assuming lordosis when mounted. Non-estrous ewes withdraw from rams, but estrous ewes remain and act like subordinate rams. Ewes are guarded and defended singly as they come into heat. They are followed by a group of rams. The largest horned rams guard and mount estrous ewes. Rams fight throughout the years, not for females, but for dominance. Despite intense competition, the dominant ram takes the estrous ewe away from the subordinate uncontested. Sheep changed primarily in social adaptations, which evolved quite independently of habitat - or body care adaptation.

Text

2011-10-04

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http://hdl.handle.net/2429/37753

Zoology

University of British Columbia

Graduate