The Chitty effect: A consequence of dynamic energy allocation in a fluctuating environment

An important biological feature of cyclic populations of voles and lemmings is phase-related changes in average body mass, with adults in high-density phases being 20-30 % heavier than those in low-density phases of a cycle. This observation, called the "Chitty effect," is considered to be a ubiquit ous feature of cyclic populations. It has been argued that understanding th e Chitty effect is fundamental to unraveling the enigma of population cycle s. However, there exists no agreement among biologists regarding the causes of the Chitty effect. Here, I propose a simple hypothesis to explain the C hitty effect, based on phase-related, dynamic allocation of energy between reproductive and somatic effort. The essence of the hypothesis is that: (1) reproduction is suppressed in animals born or raised in the later part of the increase phase by environmental factors, including social influences; ( 2) suppression of reproduction limits the amount of energy that is diverted for reproductive effort, and forces a disproportionately greater amount of surplus power (the energy left after the energetic costs of standard and a ctive metabolism are met) to be allocated for somatic effort; (3) the surpl us energy, above and beyond what is required for routine biological activit ies, will allow continuous growth and deposition of additional body mass, w hich causes an increase in body mass; and (4) animals grow to a larger size as a population enters the peak density phase, causing an increase in the average body mass. The Chitty effect is predicted to be most pronounced at the late increase or peak phase of a population cycle. Possible causes of r eproductive suppression include direct or indirect influences of the enviro nmental factors. The Chitty effect may be a consequence, not a cause, of po pulation cycles in small mammals. (C) 1999 Academic Press.