Season, length of day and chemical exposure effects on the male

In the human, reproductive functions in both sexes continue throughout the year without any major or obvious changes in different seasons. However, it is important to remember that in the overwhelming majority of species inhabiting the earth, reproductive functions are restricted to a well defined and often quite short breeding season. The annual cycles of transitions between reproductive activity and quiescence are driven by environmental signals and assure arrival of the young at the time when conditions are optimal for their survival. Of the environmental signals that influence male reproductive activity in mammals inhabiting the temperate zone, the role of photoperiod is best understood and probably most important. Annual changes in the day length provide an organism with reliable information about progression of the seasons and thus, in effect, allow "prediction'' of upcoming changes in temperature and availability of food.

The golden (Syrian) hamster is a popular model for the study of the effects of photoperiod on reproduction. Exposure of adult male hamsters to short photoperiod inhibits the release of prolactin (PRL) and the gonadotropins luteinizing hormone (LH) and follicle-stimulating hormone (FSH) (Fig. 1). There is an associated loss of testicular LH, FSH and PRL receptors, suppression of spermatogenesis, inhibition of testicular testosterone production and sexual behavior, a drastic reduction of testicular mass and sterility which persists for several months or until the animals are again exposed to long photoperiod. These effects are mediated by the action of photoperiod on the pineal gland, altering the diurnal pattern of melatonin release. The effects of short photoperiod can be completely prevented by prior removal of the pineal gland, and mimicked by appropriately timed injections or infusions of melatonin. Melatonin acts primarily within the hypothalamus by altering the release of neurotransmitters which control pituitary hormone release.

In other species of small animals, photoperiod controls not only adult testicular function but also sexual maturation. Typically, increasing day lengths of the spring promote early onset of puberty while shortening photoperiod of the late summer leads to postponement of puberty until the next spring. Interestingly, regulation of puberty in these species involves transfer of information about photoperiod from the pregnant female to the developing fetuses. In effect, the juvenile animal obtains precise information about the season of the year by "comparing'' the photoperiod it is exposed to after birth with the photoperiod to which its mother was exposed during pregnancy.

In large species in which pregnancy lasts for several months rather than several weeks, arrival of the young in the spring is assured by breeding taking place in the fall. Thus, in male deer annual recrudescence of the testes, increase in plasma testosterone levels, conspicuous growth of neck muscles, and appearance of aggressive and sexual behavior take place in the late summer and fall in response to a short, rather than a long, photoperiod. Similar annual changes, but of lesser magnitude, occur in males of most breeds of domestic sheep.

Seasonal fluctuations in male reproductive functions do not depend solely on annual changes in photoperiod. Both temperature and food availability can exert important effects and either dampen or amplify the effects of photoperiod. Reproductive functions can also be influenced by specific, often strictly seasonally available, diet components and by chemical messages received from other members of the same species. For example, male puberty in some rodents is hastened by the presence or proximity of adult females. Chemical (pheromonal) communication between the members of the same species is among many social and density dependent factors that can affect reproduction. These include territorial behavior and aggressive interactions between males that can impose major stress and interfere with access to food sources. Both stress and malnutrition can suppress reproductive development and function.

In the human, seasonal fluctuations have been detected in sperm count, motility and morphology, blood levels of LH and testosterone, as well as sexual activity and are believed to be related primarily to the effects of photoperiod. However, these fluctuations are relatively subtle and fertility continues throughout the year. Of much greater clinical significance are effects of environmental influences unique to our own species such as occupational or accidental exposure to chemicals, use of alcohol, psychotropic drugs, prescription and over-the-counter medication, and illicit use of androgenic and anabolic steroids. Each of these factors is capable of exerting profound suppressive effects on the production of spermatozoa and androgens by the testis, on libido and on potency.

Suggested Reading

Reiter RJ. The pineal and its hormones in the control of reproduction. Endocr Rev 1980;1:109-131.

Gilmore DP, Cook B, eds. Environmental factors in mammal reproduction. Baltimore: University Park Press; 1981.

Bronson FH. Mammalian reproduction: an ecological perspective. Biol Reprod 1985;32:1-26.

Steger RW, Bartke A. Environmental modulation of neuroendocrine function. In: Gass GH, Kaplan HM, eds. Handbook of Endocrinology, Vol. II. Boca Raton: CRC Press; 1987:111-141.

Bartke A, Steger RW. Seasonal changes in the function of the hypothalamic-pituitary-testicular axis in the Syrian hamster. Minireview, Proc Soc Expr Biol Med 1992;91:139-148.