A review of the Chitty Hypothesis of population regulation

Abstract

Populations do not increase without limit, and one of the central problems of population biology is to explain why. The self-regulation hypothesis states that indefinite increase in population density is prevented by a change in the quality of the population. Changes in quality may be physiological or behavioural, genotypic or phenotypic, and three different mechanisms of self-regulation have been proposed: (1) the Stress Hypothesis suggests that mutual interactions lead to physiological changes, phenotypic in origin, that reduce births and increase deaths. (2) The Behaviour Hypothesis suggests that mutual interactions involving spacing behaviour prevent unlimited increase and that spacing behaviour is not an inherited trait. (3) The Chitty Hypothesis, or polymorphic behaviour hypothesis, postulates that spacing behaviour limits population density and that individual differences in spacing behaviour have a genetic basis and respond to rapid natural selection.The testability of the Chitty Hypothesis is examined with regard to 13 predictions that are explicit in Chitty's writings or derived by subsequent workers. Many of the predictions are not unique to the Chitty Hypothesis and only a few difficult manipulations adequately test Chitty's proposed mechanism to the exclusion of all others.Four population studies are reviewed with reference to the Chitty Hypothesis. While the detailed mechanism proposed by Chitty is not yet adequately tested in any population, his general belief that both behaviour and genetics are relevant to understanding population problems is now assumed by the new generation of population biologists.