Survival in wild‐living mice *

The survival of a mouse in the wild depends on its reaction to the environment (food, climate, social conditions, cover, disease and predation), and this depends on the condition or ‘vigour’ of the animal itself. In general an animal increases in vigour until adulthood, and then begins to decrease, in House mice, at an age of approximately 100 days. These changes can be described as ageing; they lead to the characteristic U‐shaped mortality curves deduced from life‐tables in a wide variety of mammalian species. Another factor that has to be taken into consideration is genotype, since animals of different genotype may respond differently to environmental stresses. A large number of deaths in wild‐living House mouse populations in Britain are temperature‐dependent. Animals may respond to cold either physiologically or behaviourally, but it is possible to distinguish between winter survivors and non‐survivors with a fair degree of accuracy on the basis of physiological measurements made in the autumn (i.e. before the time of environmental stress). Nevertheless, any general survival model must incorporate environmental, ontogenetic, and genetic factors and their interactions. A simple model is proposed. Animals are not always struggling for existence. They spend most of their time doing nothing in particular. But when they do begin, they spend the greater part of their lives eating (Charles Elton).