The Role of Reproduction and Life Histories in Models of Small Rodent Population Dynamics

Abstract

Mammalian reproduction consists of internal fertilization, a period of intrauterine development, parturition, a period of postnatal dependence (lactation), and weaning. Quantitative aspects of this process include the time for development (both intrauterine and postnatal), the number of offspring raised, the age at maturation and the frequency of breeding. These characteristics are governed, within the constraints of morphology, physiology and genetics, by natural selection. Life historians have attempted to understand how organisms are adapted to their environments by considering age specific breeding patterns and the probability of reproductive success in relation to the survival costs of reproduction. Their studies have emphasized the constraints on reproductive patterns, the coevolved nature of reproductive characteristics, and the relationships between these patterns and nutritive resources or social systems (see Clutton-Brock and Harvey 1979, Zeveloff and Boyce 1980, Stearns 1983, Gittleman 1986, Harvey 1986 for examples). Population processes represent the cumulative effects of life history patterns. Many of the consequences of life histories (e.g. population growth) are too far removed from the immediate environment of organisms to be of great interest to life historians. Similarly, some aspects of life histories (e.g. neonatal weights and development) have no direct effects on population growth or density and are of little interest to population ecologists. Nevertheless, there are areas of mutual interest where improved communication may be beneficial. These comments will be restricted to life history principles that may be useful to population ecologists, particularly those who incorporate individual responses into their hypotheses. Reproductive processes are fueled by energy, with specific requirements dictated by the rate of metabolism and number of offspring raised. Among small mammals, reproduction is energetically expensive because basal metabolic rates are generally high (McNab 1986) and several young are produced per litter. Small mam-