Social Factors in Adult Reproduction in Natural Populations

Abstract

It is possible roughly to categorize animals into cyclic and non-cyclic species. In certain mammals repeated observations have shown that the numbers present in any one locality fluctuate from year to year in a more or less regular manner. Examples include the 3- or 4-year cycle in numbers of certain microtine rodents (Lemmus, Dicrostonyx, Microtus) and the 10- or 11-year cycle of some lagomorphs (Lepus). Although there are changes in numbers present inside each year in these species, there is a fairly regular cycle of high-density years and years of lower density. These marked changes in numbers have been the subject of lengthy field studies and are now beginning to be well documented. This does not apply to many other species of mammals whose changes in numbers from year to year are not so obvious. Therefore, in by far the majority of mammal species there are no field studies involving estimates of density of more than three years duration. This lack of data means that although it is fairly easy to classify a species from the literature as being truly cyclic, the categorization of species as non-cyclic is usually fraught with presumption. Therefore the reader is warned that the categorization of species used below will probably prove inaccurate with further field research. The distinction is maintained, however, in the belief that the regularity of density changes in cyclic species should mean a biological system at work which may be of a different nature than mere density changes would suggest — whereas in ‘non-cyclic’ species other mechanisms, probably more closely related to the density alone, may operate on the reproductive processes.