Non-photoperiodic inputs into seasonal breeding in male ruminants.

In contrast with ruminants from higher latitudes, feral cashmere bucks and Merino rams do not exhibit rigidly-timed sexual seasons driven by photoperiod, but are “opportunistic” breeders with a reproductive strategy that includes short-term responses to proximate environmental cues (Fig. 12). Among the nonphotoperiodic cues are nutrition and social factors. Nutrition can exert profound effects on testicular size in the mature male and appears to operate through both GnRHdependent and GnRH-independent mechanisms. In the ram, nutritional changes initially affect gonadotrophin secretion through an action on the hypothalamic centres controlling pulsatile GnRH secretion, and this response appears to be triggered by volatile fatty acids, rather than the glucose–insulin system. The buck does not seem to display this response. However, in both bucks and rams, the long-term changes in testicular growth that are effected by nutrition are poorly correlated with changes in gonadotrophin secretion, and they do not seem to lead to long-term changes in the responsiveness of the testis to LH (as measured by testosterone secretion). The components of the GnRH-independent mechanisms are not yet known, but probably include the somatotrophic axis (growth hormone and insulin-like growth factors). By contrast, social cues appear to exert their effects through the same final common pathway as photoperiodic cues. The presence of oestrous females exerts powerful effects on the secretion of GnRH, and in particular LH, leading to an increase in testosterone secretion within minutes. This extra testosterone may be important for mating behaviour.
Thus, the final expression of breeding activity in animals adapted to nontemperate environments depends on interactions between these three major environmental cues, and at least two endocrine control mechanisms, one of which appears to be essentially metabolic rather than reproductive. This is not really surprising, considering the critical role that nutrient supply or reserves play in determining the success of a reproductive strategy.