Endocrines, Behavior, and Population

Abstract

For several decades the spectacular increase and decrease of certain arctic mammals has stimulated research on populations. The crashes of rabbits were dramatized by Seton (1), and the suicidal movements of lemmings were publicized by many authors. However, as is so often the case, the conspicuous features turn out to be merlCy an cxtreme case of a very general phenomenon--namely, the fluctuations of a population. Investigators first sought an explanation for the "crash," but now most of them search for a description and understanding of the interaction and relative importance of the many factors that influence the ups and downs of populations.In this article we describe the current status of our understanding of population fluctuations, emphasizing the regulatory features that prevent populations from destroying the habitat. The research discussed is limited to work with mammals, since the mechanisms are best known for that class. It is assumed that the reader has knowledge of ecological principles such as density dependence and limiting factors.For many years it was assumed that epizootics, famine, and climatic factors terminated the explosive rises in population size and precipitated the often spectacular crashes (2). However, by the early 1940's it had become apparent that none of these mechanisms explained some of the observed declines in population, and it was suggested that factors intrinsic to the population were involved in its regulation Dr. Christian is affiliated with the Research Laboratories of the Albert Einstein Medical Center, Philadelphia, Pa.; Dr. Davis is professor of zoology at Pennsylvania State University, University Park. (3). The skepticism toward earlier explanations was reflected further in a review by Clarke in 1949 (4), as well as in Elton's classic earlier work (5).Probably the greatest shift in emphasis has occurred since 1949; there has been an upsurge of investigations in which density-dependent changes in the animals themselves have been explored, and of theories in which the observed phenomena of population growth and decline (6-9) are explained in terms of biological mechanisms intrinsic in the populations and not only as results of the action of external factors. It is clear that food, climatic factors, and disease may cause population change. Indeed, it would be foolish to state that these factors do not, under certain circumstances, limit population growth or produce spectacular decline. The early investigations of Emlen, Davis, and their co-workers (8) on populations of Norway rats demonstrated clearly that environmental factors can reduce a population. For example, a drought followed by excessive rain resulted in a notable decline in rats in Baltimore (8). However, as early as 1946 spectacular declines in rat populations were found to be coincident with social disturbances rather than with environmental changes.The suspicion that social phenomena were involved prompted a search for mechanisms that could regulate the growth of populations in a densitydependent manner. No lo...

Demographic processes in a confined population of the common vole

“…This is evidence of the low effectiveness of the immunological system in M. arvalis during this time. In accordance with the theory put forward by Christian & Davis (1964), such reaction may be a manifestation of the action of stress caused by strong social stimuli taking place with great density of rodents. In population R an additional factor causing prolongation of the time of increased mortality among voles was the drastic change in habitat conditions caused by harvesting the crop and ploughing the field towards the end of July, and re-sowing the field with rape.…”

“…On the one hand Schultz (1969), Birney et al (1976), Stenseth et al (1977 a, b) drew up models of the functioning of Microtinae populations based mainly on the use the individual makes of the advantages of the biotic habitat. On the other hand the role of habitat factors, including food factors, in formation of population cycles, is usually overlooked by supporters of the stress hypothesis (Christian & Davis, 1964) or geneticbehavioural hypothesis (Chitty, 1964;Krebs & Myers, 1974), on account of the small extent to which rodents make use of the accessible food supply (Grodziriski et al, 1970;Trojan, 1970 and others). Andrzejewski (1975), however, considers that in the case of Clethrionomys glareolus (Schreber, 1780) food conditions play an important part in shaping the dynamics of population numbers, but such effects cannot be explained on the basis of a simple energy balance.…”

Effects of Fasting on Oestrus and Ovulation in the Oestrous Rabbit

“…Failure of oestrus in situations of stress involving increased adrenocorticoid output has frequently been reported (e.g. Christian and Davis 1964). There was no evidence in this experiment of increased adrenal gland weights in response to fasting, though this does not conclusively exclude changed adrenocorticoid status as a factor affecting reproductive performance.…”

Effects of grouping and crowding on learning in isolation-reared adult rats

“…although reproductive maturation may be slowed by increased social contacts (Christian & Davis, 1964). Recently, Goeckner.…”