Sexual performance of twin ram lambs and the effect of number and sex of contemporary siblings

The objectives of this investigation were to determine the degree of correspondence in the sexual performance of twin male sheep and to compare the sexual performance of ram lambs born co-twin to another male (M-M), co-twin to a female (M-F) and as single (S) offspring. Individual ram lambs (N=117), including 13 M-M twin sets, were exposed to four estrous ewes for 30 min at 7-day intervals over a 4-week period at 8 to 9 months of age.Frequencies of ejaculation (serving capacity) and mounts without ejaculation were recorded. M-M males averaged 2.9 ejaculations and 13.1 mounts per test. Individuals within 9 of the 13 M-M twin sets (69%) differed by an average of 0.5 or fewer ejaculations per test. Although variability within M-M twin sets was very low, variability between the 13 M-M twin sets was also low (coefficient of variation=10.6%). Consequently, frequencies of both ejaculation and mounting (without ejaculation) within the 13 twin sets were not correlated (P=0.50 and P=0.10, respectively). When the progeny of individual sires were compared, mounting frequencies were correlated (P<0.02) but not ejaculation rates (P=0.72). M-M, M-F and S ram lambs did not differ for either ejaculation frequency (P=0.26) or mounts without ejaculation (P=0.98). It was concluded that the sexual performance of related individuals is likely to be very similar and that the number and sex of contemporary siblings does not necessarily influence rates of mounting and ejaculation.