Reproduction in Deer:Adaptations for Life in Seasonal Environments

Abstract

Deer became a distinct family in the early Miocene era, evolving in high latitudes at a time of global cooling. Many species that now live in temperate latitudes have well-developed physiological mechanisms to suit them to a seasonally changing environment. In addition, some species that live at low tropical latitudes with a less marked seasonal environment also show a degree of inherent rhythmicity. Deer living in temperate latitudes show overt seasonal fluctuations in reproduction, metabolism, appetite, growth, and pelage growth and moult. These seasonal cycles are closely linked temporally and are difficult to separate experimentally. The predictable changes in the length of the photoperiod have been shown to be the major environmental factor controlling seasonal cycles via the secretion of the hormone melatonin during the night. Treatment with melatonin induces phase shifts in all of these rythms. Recent studies have shown, however, that the seasonal red deer (Cervus elaphus) exhibits an overt rhythms in its metabolism, coat growth, and associated endocrine parameters when kept on constant midsummer day lengths from birth. This rhytym is similar to that of animals on natural day lengths and indicates that the rhythm is endogenous. In some tropical species such as the Axis deer (Axis axis), stags exhibit a circannual cycle of the antler growth, testis growth and regression, and body weight change. These cycles are not synchronized to the environment, even when animals are kept in temperate latitudes or treated with melatonin. Thus, tropical species may retain may retain many of the endogenous rrhythmicity evolved by their temperate ancestors.