Advances in photoperiodic and bio-stimulations of seasonal reproduction in small ruminants

Abstract

In small ruminants, photoperiod synchronizes reproductive seasonality, and this environmental factor has been used to stimulate the sexual activity of males and females in months of seasonal sexual rest. However, socio-sexual interactions between males and females can also be used to stimulate the sexual activity of females in the seasonal anestrus. In this review, we describe how photoperiodic treatments can stimulate the sexual activity of bucks and rams in the months of sexual rest. In addition, we describe how sexually active males can be used to stimulate reproduction in goats and sheep in seasonal anestrus or to prevent seasonal anovulation through the classic “short-term male effect”, and or through the newly discovered so-called “long-term male effect”. We conclude with a description of the very recently documented “male-to-male effect”. The “male-effect” is a socio-sexual stimuli caused by the sudden introduction of males and close contact with anestrous females in sheep and goats. After introduction of the male, the levels of pituitary luteinizing hormone (LH) in the blood of females increase quickly (< 10 min), and remain high for at least 4 h. That stimulus triggers the resumption of ovulatory activity in females, which occurs within 48 h. Prior separation of male and females, the intensity of anestrus, and the age, diet, and body condition of females can affect the response of the females to the male introduction. The long-term “male effect” phenomenon consists of a permanent contact of females with males that had been made sexually hyperactive in the usual period of sexual rest by light treatments applied in winter. This stimulation causes strong activity of the hypothalamic-pituitary axis, which releases the gonadotropic hormone LH at a frequency that allows a succession of ovulatory cycles during the anoestrous period as it does in the usual estrous period. The introduction of a male into a group of males in sexual rest stimulates the secretion of LH and testosterone, and their sexual behavior; this has been called the "male-to-male effect" ("buck-to-buck effect" or "ram-to-ram effect"). The intensity of sexual behavior displayed by the stimulatory males influences the responses of bucks and rams in sexual rest to the "male-to-male effect". Moreover, bucks that had been stimulated by the "buck-to-buck effect" are as effective in inducing high ovulatory and estrous activities in females as bucks that had been made sexually hyperactive by the photoperiodic treatment. In conclusion, these effects illustrate the power of socio-sexual relationships in controlling seasonal reproduction in sheep and goats, and shifts their importance in the final control of seasonal reproduction throughout the year compared with photoperiod.