Biology of Reproduction

In this paper some of the major variations in mammalian reproductive cycles are discussed from the viewpoint of their broad adaptive values. The variations involve qualitative differences in various aspects of the reproductive cycle and seem to be genetically fixed within a species. Research in the area of reproductive physiology has chiefly been confined to a few laboratory and domestic species which have been studied in great detail. As a result we have much information about these few forms, but perspectives may be distorted by this same wealth of information. It is often forgotten that these studies have been made on highly specialized forms which are usually the resultant of many years of domestication and artificial selection. Furthermore, the work has been done without regard to ecological or social context. For example, so much emphasis has been given to studies of the nonpregnant cycle, that we tend to lose insight into its significance in natural populations. In natural populations the nonpregnant cycle is a rarity, and it is essentially a pathological luxury which cannot be tolerated. Even in relatively long-lived animals with low mortality rates a nonpregnant cycle is an exception. In a study done under the somewhat artificial conditions existing in the Cayo Santiago Island colony in Puerto Rico, only 6 of the 28 fertile mature female rhesus monkeys (Macaca mulatta) in a free ranging social group failed to conceive on the first estrus of the breeding season (Conaway and Koford, 1964). The remaining six conceived during their second estrous period. For short-lived prey species the occurrence of a nonpregnant cycle is a disaster, which must be avoided if the individual is to contribute significant numbers of offspring to the population. Many small mammals in this category have only a few months of reproductive life. Any portion of this which is lost through a nonpregnant cycle can be critical. The only acceptable alternatives are either to safeguard against the occurrence of a nonpregnant cycle or to recover and recycle as quickly as possible. One of the basic variants in mammalian reproductive cycles seems to be in the methods which have evolved for handling the nonpregn ant cycle or preventing its occurrence. Full understanding of these variations is impossible since we have detailed information about the nonpregnant cycle in fewer than 50 of the 1000 mammalian genera. This is not surprising since information must come from laboratory studies; however, it does indicate the importance of obtaining basic information on a wide variety of forms.