Domesticated ruminants have lived in close association with humans for thousands of years and knowledge of the behavior of these organisms has contributed to their successful domestication, as well as to the management of animals in captivity, both extensive and intensive. Here we describe the reproductive behavioral endocrinology of cattle, goats and sheep. These relatively large and tame animals provide opportunities to conduct a wide range of behavioral studies from short-term to longitudinal or developmental in nature. Highlighted is some of the work from our laboratory describing the social, environmental and endocrine factors that influence the expression of sexual behavior in male and female goats.
Animal domestication has been defined by Price (1984) as “that process by which a population of animals becomes adapted to humans and to the captive environment by some combination of genetic changes occurring over generations and environmentally induced developmental events recurring during each generation.” Genetic changes associated with animal domestication include: inbreeding, which results in increased homogeneity; genetic drift, in which genes may be fixed by chance in a small population; artificial selection, either deliberate or unintended (i.e., when genes for selected traits are closely linked to genes for other traits); and natural selection in captivity, which is all of the selection influencing captive animals not accounted for by artificial selection (Price, 1984). For those who study the behavior of domesticated livestock in particular, an intriguing problem arises as a result of the genetic changes from what Price described as ‘relaxation of natural selection’ (Price, 1984). This concept implies that some behaviors lose their adaptive significance in the captive environment. For example, livestock producers generally breed animals in what is known as single-sire groups, i.e., one male with multiple females for the duration of the breeding season. This permits all selected males, even those with low libido, equal opportunity to produce offspring. The heritability of serving capacity, a measure of mating competence, has been estimated to be 0.59 (Blockey et al., 1978). Permitting animals with low libido to reproduce without competition from males with high libido has thus increased the quantitative variation in sexual performance, the combination of mating competence and sexual motivation. Indeed, it is estimated that eight to 10% of male cattle and perhaps as many as 15 to 25% of male sheep have low sexual performance (Price, 1985). This phenomenon has stimulated basic and applied research on sexual behavior in male ruminants in ours and many other laboratories around the world. Many of the studies have focused on the endocrinology of the male describing the role of various hormones in the regulation of the development of sexual behavior, characterizing the influence of hormones on the adult expression of sexual behavior, or quantifying the concentrations of various hormones in an attempt to predict the sexual performance of potential herd sires. This latter rationale for behavior studies is based on the negative economic impact of low performing males producing either fewer offspring or offspring conceived late in the breeding season, as these males may not breed all the herd females at the first estrus of the season. As weaned animals are generally sold as a group at one time, younger animals are lighter in weight and therefore produce lower returns.
The historic rationale for studying the behavioral endocrinology of female ruminants, especially cattle, was based on the need for humans to detect estrus as part of artificial insemination programs. Some of these applied questions have been addressed or partially resolved with induced ovulation protocols that simplify or eliminate the need for estrus detection or electronic detection systems that automatically record changes in the estrus-related physical activity of females. Nevertheless, basic questions regarding sexual differentiation and sex dimorphism, or lack thereof, warrant vigorous study of attractivity, proceptivity and receptivity in female ruminants.
