Socio-Sexual Stimuli and Reproductive Function: Emerging Perspectives of the Male Effect in Sheep and Goats

Abstract

Producers are facing increasing public scrutiny of the methods used in the production of animal products. Concerns over hormone residues in meat and milk may lead to restricted use of exogenous hormones in the reproductive management of farm animals in the future, so it is important to develop non-pharmacological methods of oestrus synchronisation. The ‘male effect’ was first reported in sheep in the 1940s and describes the capacity of the male to increase the secretion of reproductive hormones and induce ovulation in the female. It is widely accepted that olfactory signals (ostensibly ‘pheromones’) are primarily responsible for the profound shift in the activity of the reproductive centres of the female brain, though the chemical nature of this signal is yet to be conclusively identified for either sheep or goats. On the other hand, our understanding of the mechanism through which socio-sexual stimuli stimulate the hypothalamic-pituitary-gonadal axis of females has been greatly improved through a combination of histological, electrophysiological and endocrinological techniques. It is clear that olfactory stimuli from males are primarily transmitted through the main olfactory system, with audio and visual stimuli playing a synergistic but relatively minor role. In spite of over 60 years of research in this field, there are several areas of current and potential research that should improve our understanding of this remarkable phenomenon and its application to farm animal management.