Ecological adaptation and mammalian reproduction.

Abstract

Some of the major variations in mammalian reproductive cycles are discussed from the viewpoint of their broad adaptive values. In natural populations a nonpregnant cycle is an exception in long-lived animals and a disaster in short-lived prey species. Acceptable alternatives are either to safeguard agaisnt the occurrence of a nonpregnant cycle or to recover and recycle as quickly as possible. Mammalian female cycles are categorized as Type I Type II and Type III. In Type I both ovulation and pseudopregnancy are spontaneous. Subtype A includes medium length cycles and Subtype B long cycles. In Type II ovulation is induced and pseudopregnancy is spontaneous with the similar subtypes. In Type III both ovulation and corpus luteum formation are spontaneous but pseudopregnancy is induced via the release of luteotropin following copulation. Induced and spontaneous ovulation delayed implantation seasonal breeding and postpartum estrus serve different functions in animals in natural population. It appears that induced ovulation is the more widespread phenomenon and that spontaneous ovulation occurs in a more restricted number of species. Delayed implanatation occurs in widely scattered species and groups within many orders but also the physiological controls vary. At least some degree of seasonality in reproductive activity is almost universal in natural.a Postpartum estrus seems to have developed independently many times and its distribution is sporadic throughout many mammalian speices. The necessity of trying to understand the significance of variations seen in reproductive cycles is stressed. After some basic patterns are established appropriate species can be selected to test some of the hypothesis rather than haphazardly studying forms just because they are available.