Seasonality of reproduction in ungulates : the effect of photoperiod on reproduction in a seasonal breeder, the sheep (Ovis aries) and a putative aseasonal breeder, the springbok (Antidorcas marsupialis)

Abstract

I investigated reproduction in springbok (Antidorcas marsupialis) to determine whether the basic physiological system that underpins seasonal reproduction exists in this putatively aseasonal ungulate. In addition, I examined the transition from reproductive quiescence to activity in female mammals using the sheep (Ovis aries) as a model species. My analysis of reproduction in wild and captive springbok populations suggested that springbok, unlike seasonal breeders, do not use photoperiod or other climatic variables to cue reproduction. This was reinforced by the finding that the reproductive characteristics of a captive herd of springbok ewes are not the same as typical seasonal breeders. However, the pineal-melatonin system of springbok functions in the same way as in photoperiodic species. Thus, I suggest that springbok ignore photoperiodic information that could potentially be used to cue reproduction, which suggests that the physiological basis of aseasonality in springbok lies downstream from the pineal-melatonin system. The results of the investigation of the sheep suggested that the neurotransmitters that have been implicated in the regulation of reproduction might have differential roles that are determined by the reproductive state of an animal. The results supported the contention that there is a complex system of interneurons that link melatonin to the modulation of gonadotrophin secretion. In conclusion, the physiological system that regulates reproduction in ungulates is complex but nevertheless seems to be fundamentally similar in seasonal and aseasonal species. Thus, it seems that aseasonal breeders may not be physiologically bound to reproduce aseasonally, but reproduce in the manner that they do because it is an adaptive advantage.