Photoperiodism in Birds and Mammals

Abstract

Using the annual cycle of changing day length, photoperiodic animals restrict their reproductive efforts to a favorable time of year. Thus, the perception and measurement of day length are vital for maximal reproductive success. A clock mechanism to measure day length is necessary for photoperiodic responses. In one section, this chapter reviews the basic principles of the biological clock for measurement of and entrainment to a 24 h light: dark cycle. Some differences between birds and mammals and the ways in which they measure changing day length are highlighted; most notably, differences in photoreception and in the role for the pineal melatonin signal in the transduction of the light: dark signal. The mammalian clock has received a great deal of attention in recent years because of the identification of several clock genes and greater knowledge of how they interact. The clock in birds is less well understood. Birds measure day length in a circadian manner, but in contrast to mammals, pineal melatonin is not involved in photoperiodic time measurement. Seasonal breeding cycles of birds and the photoperiodic regulation of the hypothalamo-pituitary gonadal axis are discussed, as is a newly-identified role for melatonin in birds. Melatonin acts as an inhibitory hormone on seasonal neuroplasticity within the song control system of songbirds, acting in opposition to the stimulatory effects of gonadal steroids. Thus, there is a complex interaction between the circadian system, photoperiodic time measurement, activation and regulation of the neuroendocrine system regulating reproduction and hormonal actions upon the brain.