Within a species, motor patterns associated with courtship and copulation are species-specific and relatively invariant, although the sequence of behaviors exhibited varies within and among individuals. Large variation in the frequency or intensity of displays of sexual behavior does occur in farm animals and is likely due to relaxation of natural selection.[2,3] For example, single-sire mating systems enable animals with poor sexual performance to produce large numbers of offspring. This would not occur normally in wild animals, because competition among males would continue to exert intense selective pressure for high sexual performance. The number of males displaying inadequate sexual performance is significant, and this lack of performance has direct economic consequences.
This variation in sexual performance has contributed to the need for animal scientists and producers to develop tests of sexual behavior in males. With animal agriculture the need still exists for males that express high levels of sexual activity. This is true for those used for semen collection and for those used for natural service. In natural breeding systems, males with high sexual performance and a combination of high sexual motivation and superior endurance, strength, and coordination will likely breed more females early in the breeding season. Males with lower sexual motivation and/or low sexual performance will inseminate fewer females, thus either reducing herd production or extending the breeding season, as more estrous cycles will be required to impregnate the group. Sexual performance tests, also referred to as serving capacity tests, have been developed to identify and select high performers and to cull low performers.[3-5] These tests measure copulation frequency within a short period, but they are inconsistent in predicting either field reproductive performance or how well young males will perform when they are mature. High sexual performance depends on a male having high motivation to mate and also possessing the physical ability to do so. Low sexual performance may be caused by low sexual motivation, or by physical attributes such as poor structural conformation or genital abnormalities.1-3-1 Current tests identify physical limitations but not necessarily motivational ones. New behavior tests to measure sexual motivation in farm animals are necessary to develop better behavioral tools for assessing reproductive performance and thus enhance animal productivity.
Research on the sexual performance of males has yielded interesting findings that remind us that the domesticated animal is a social animal with species-typical perceptual abilities and responses. Providing sexual arousal for males improves the efficiency of sexual performance tests. Male pigs, horses, goats, and cattle but not sheep are sexually stimulated by the opportunity to view estrous females or other sexual activity. Male sheep are aroused, however, when exposed to other male sheep that have either recently mated or recently contacted estrous females. Promiscuous farm species, if given the opportunity, will form social systems consisting of groups of mature females with their offspring, bands of juvenile males, and individual adult males. Cows and female goats, respectively, will engage in female-to-female sexual activity. This activity provides a visual signal that attracts and arouses the males. In contrast, estrual sheep do not form groups of mounting females. Instead, female sheep seek the male. Thus the selective forces shaping the development of arousal mechanisms differ among farm species.
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