Seasonal variation in primate fertility with an emphasis on the male

Abstract

Seasonality of births occurs in a variety of animals including nonhuman primates and humans. Seasonal reproduction in humans is dampened by comparison with many animal species, and the factors controlling seasonality are more complex. It has proven difficult to unravel the multiplicity of variables controlling seasonal reproduction in humans. In this review, the seasonal variation in human reproduction is compared with that which occurs in freeranging troops of nonhuman primates in an effort to gain insight into the human phenomenon. This comparison shows that in both nonhuman primates and in humans, photoperiod is an important impeller of seasonal reproduction. Also, seasonal reproduction in nonhuman primates and humans is driven by a complex interaction between the animal and the social environment. This paper focuses on environmental impellers of seasonal reproduction in the male. Free-ranging male macaques exposed to a natural environment show a waxing and waning of sexual behavior, testosterone production, and spermatogenesis with a peak during mating season and a nadir during the birth season. Male macaques deprived of social cues from sexually receptive and/or attractive females undergo an incomplete or retarded sexual recrudescence when compared to male macaques exposed to a physical and social environment that initiates mating activity in a breeding group of male and female macaques. These latter results suggest that recrudescence of reproduction in male nonhuman primates during the mating season is dependent on cues from both the social and the physical environment. These results from nonhuman primates, together with results of the review of seasonal variation in human reproduction, suggest that the human female might play a key role in seasonality of births. Therefore, it is suggested that future research designed to unravel factors controlling seasonal patterns of human reproduction focus on factors regulating male-female interaction, female sexual behavior, and female fecundity.