Mechanisms for Seasonal Control of Reproduction in Small Mammals

Abstract

Reproduction at times of the year when survival of offspring is questionable implies a large risk for survival not only for offspring but for adults as well. Cessation of reproduction during harsh seasons can contribute to the survival of individuals due to reassignment of energy sources. If we accept the notion that small mammals usually live at the cutting edge between thriving and just surviving, we can appreciate the importance for a small mammal to economize on the energy required for both reproduction and thermoregulation. The energy saved due to cessation of reproduction during winter could be used for improvement of thermogenetic capabilities. Reproduction requires a lot of energy, and of all phases of the reproductive cycle, lactation is energetically by far the most expensive one [5]. A female house mouse must more than double her energy intake during late lactation to nourish herself and her pups [6, 7]. Consequently, there is strong competition between the energy needs for reproduction and thermoregulation especially in small mammals. In this competition energy allocation for thermoregulation has a high priority relative to the allocation of energy for reproduction [2, 8]. This, of course, makes sense because only a mammal which survives can reproduce when environmental conditions improve. Although cold exposure per se causes a large energy drain especially in small mammals, an associated effect of low ambient temperatures has to be considered in the framework of breeding success. In the natural habitat cold seasons are also seasons with low food availability and/or poor quality that in itself can have profound effects on breeding success. The effect of low ambient temperature on survival of females or incidence of ovulation is magnified if the energy drain for thermoregulation cannot be compensated by an increased food intake. These effects are less pronounced if food intake can be increased during times of cold stress. Cold exposure and limited food availability act synergistically in preventing successful breeding [9]. In Djungarian hamsters, reproduction is usually restricted to late spring and summer. During winter these hamsters show gonadal regression and do not reproduce. If, however, due to some experimental manipulation, gonadal regression is prevented at times of low ambi- ent temperature, breeding success is dramatically reduced compared to hamsters of the same gonadal state but kept at thermoneutrality [10].