Semen Quality, Female Choice and Multiple Mating in Domestic Sheep: a Test of Trivers' Sexual Competence Hypothesis

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Abstract
TRIVERS (1972) proposed that females may choose to mate with males of high sexual competence (the ability to supply sufficient sperm for fertilisation). This hypothesis was tested by allowing ewes in oestrus to choose between four tethered adult rams, two of high and two of low semen quality. Contrary to the hypothesis, ewes did not prefer rams of higher semen quality and, even though frequent mating can depress the fertilising ability of ram semen, the attractiveness of each ram to oestrous ewes was not lowered by frequent ejaculation. Also, in contrast to TRIVERS' suggestion, semen quality and male courtship vigour were not consistently related either between or within individual rams. Ewes mated repeatedly during oestrus, receiving nearly six ejaculates each on average, and over two thirds of ewes mated with more than one ram. Repeated mating, rather than a preference for sexually competent males, might function to ensure fertilisation in female sheep and perhaps in other ruminants. TRIVERS (1972) proposed that females may choose to mate with males of high sexual competence (the ability to supply sufficient sperm for fertilisation). This hypothesis was tested by allowing ewes in oestrus to choose between four tethered adult rams, two of high and two of low semen quality. Contrary to the hypothesis, ewes did not prefer rams of higher semen quality and, even though frequent mating can depress the fertilising ability of ram semen, the attractiveness of each ram to oestrous ewes was not lowered by frequent ejaculation. Also, in contrast to TRIVERS' suggestion, semen quality and male courtship vigour were not consistently related either between or within individual rams. Ewes mated repeatedly during oestrus, receiving nearly six ejaculates each on average, and over two thirds of ewes mated with more than one ram. Repeated mating, rather than a preference for sexually competent males, might function to ensure fertilisation in female sheep and perhaps in other ruminants.