Ecological Factors and the Breeding Season: Introduction

Abstract

Mammals reproduce successfully over a wide range of latitudes and climatic conditions. In the course of mammalian reproduction, the period from late pregnancy through birth to lactation is a period when the mother and infant are both susceptible to detrimental effects from external environmental factors. It therefore seems logical to assume that mammals will tend to produce their young at a period of the year when environmental conditions are optimal for survival of both mother and young. Baker (1938a) has called the conditions which exist at such an optimum period the ultimate cause of the timing of the breeding season. Many writers have pointed out that the mechanism for selecting this timing of the birth period is fairly obvious, in that offspring which are not born at the optimal period would naturally have a lesser chance of survival. Species living in environments which alter so that at certain times survival is better than others will therefore tend to concentrate their birth periods relative to this time, and selection will work to fix such a breeding time in the individual genotypes. However, although most of the mammalian species studied live in temperate regions, where the progression of the seasons is such that the period of time for optimal reproduction is fairly well defined, it must not be forgotten that many more species of mammals live in tropical or semi-arid climates where the optimal season does not hold the same position in the annual cycle.