The life history of a mammal species can be defined by variables such as the mean length of gestation, the size and number of offspring at birth, the interval between births, the age at which weaning takes place and body size at that time, the age at which males and females first breed successfully, their size as adults and the age at death. Some species ‘live fast and die young’ and, at the other extreme, some species ‘live slow and die old’ (Promislow and Harvey, 1990; Smith, 1992). A ‘fast’ mammal might typically have a short gestation time, multiple small offspring, a short birth interval, early age of weaning and of sexual maturity, and a short lifespan. It would be small in body size at each stage. A ‘slow’ animal would have a long gestation time, after which one or a few large babies would be born, a long interval for each female between successive births, a late age of weaning and maturity, and a long lifespan. Offspring would be large at all stages and born from large mothers. Body size is thus strongly related to the pace at which life history unfolds but, when the mean values for life history variables in different species are plotted against one another, they remain correlated even when the effect of size is removed.
Models of life history emphasise the switch between growth and reproduction and suggest that the age of sexual maturity in a given species represents a balance between the two. Fast mammals, on one hand, minimise their risk of dying without reproducing themselves by maturing young and giving birth to many, but they are vulnerable to predators because of their small size. Slow mammals have an increased risk of dying before they can give birth, but their larger size decreases their overall risk of predation and increases their ability to protect their offspring through a longer period of development.