Mammalian Photoperiodism

Abstract

Many organisms have evolved physiologic and behavioral adaptations that are presumed to increase reproductive fitness in highly seasonal environments. This review will focus on mammals, the group in which perhaps the most progress has been made in understanding mechanisms of seasonal adjustments at the neuroendocrine level. Seasonal modulation in a typical mammal may involve several traits, including reproductive capacity and related behaviors, increases and decreases in energy storage, and changes in pelage density. Food availability, precipitation, and ambient temperature vary seasonally in a more or less predictable fashion, and are potential environmental zeitgebers. Day length (DL) is, however, the most noise free and probably the most frequently used cue for phasing seasonal responses among mammals in mid and higher latitudes. This use of DL is termed photoperiodism and should be distinguished from the use of photic cues for the entrainment of circadian rhythms. Several review articles summarize recent progress in understanding mammalian photoperiodism (Bartness & Goldman, 1989; Goldman & Elliott, 1988; Goldman & Nelson, 1993; Karsch et al., 1984; Nelson, Badura, & Goldman, 1990). A recapitulation of the extensive corpus of findings is beyond our present scope; instead, we selectively review a few extensively studied model systems. We emphasize ways in which the natural progression of DLs in nature provides information used by animals to achieve seasonally appropriate adjustments. Our emphasis is on species, e.g., hamsters, mice, and voles, in which seasonal transitions do not recur spontaneously in the absence of seasonal changes in DL. These Type I rhythms (Zucker,Lee, & Dark, 1991) are not fully endogenous, and their recurrence in mammals is contingent on seasonal variations in DL and associated changes in the pineal melatonin rhythm. Several species with fully endogenous circannual rhythms are considered in Chapter 19. It should be emphasized that despite the emphasis in this review on Type I rhythms, there is no evidence to suggest that the fundamental mechanisms of photoperiodism are different in Type I and Type II rhythms. Indeed, both types of rhythms appear to depend on a circadian mechanism to measure DL, and the pineal gland is an important part of the photoperiodic mechanism in both (see below).