Size, life history and ecology in mammals

Abstract

Allometric laws which scale numerous biomechanical and physiological processes to size in mammals have long been recognized and widely used in biology. There is now sufficient evidence to suggest that those life history parameters such as growth and maximum rates of reproduction, which depend in part on metabolic rate, are also scaled to size. Data are presented which, coupled with a literature review, show that gestation time, growth rates, age at first reproduction, lifespan, the intrinsic rate of natural increase, birth rate, net reproductive rate and litter weight are allometrically scaled to size and are, in consequence, inter-related. The exponents of the scaling functions are similar in all mammalian orders but in some taxa such as the primates, the species grow slower, live longer and reproduce at a lower rate. For these taxa the differences in life history parameters can be explained by differences in brain size and an existing hypothesis that the maximum rate of neural tissue growth constrains the maximum rate of growth and development of the entire organism.

It is then argued that because size scales the main life history parameters of mammals it should also be a central theme in ecology from the individual to the community level of organisation. Examples are presented to show that size is ubiquitous in ecology and accounts for most of the variation in life history parameters between species. Size scaling offers a method of reducing species of varying size to similar dimensions of time, space and rates of action. It is therefore fundamental in distinguishing those life history parameters which arise as a consequence of size, the first order strategies, from those that vary between populations and according to environmental circumstances, the second order strategies. From this approach should develop a broader biological synthesis in which genetic and physiological determinants will inevitably feature more centrally than they do in ecological and behavioural theory.