The Characteristics of Rat Populations

Division of Vertebrate Ecology, School of Hygiene and Public Health, The Johns Hopkins University T HE characteristics of a particular group of animals (a population) is the subject of this review. Although biologists have long investigated the characteristics of a group of cells (a metozoan), the study of population is in its infancy. The species used as the experimental nimal for this review is the wild Norway or Brown Rat (Rattus norvegicus). Millions of domesticated rats of this species have been used for scientific work, but until recently few studies of the life history and ecology of wild rats were conducted. However, because of the economic and epidemiologic importance of the rat, much time and money have been spent in efforts to reduce the numbers of many species of rats. These expenditures, unfortunately, rarely provided data of value for science and also rarely reduced the pest except temporarily. The subject of the review, the species Rattus norvegicus, is the best known of more than 400 named forms in the genus Rattus. Some comments will be also included here about the Roof Rat (R. rattus), which is represented by numerous ubspecies in tropical and subtropical regions. With the exception of the notable studies by McDougall (1944-46) on populations of R. conatus in Australia, only miscellaneous observations of the life histories of other species of rats exist. It seems likely, however, that the characteristics of the populations of the various species in the genus Rattus differ only in degree and that the characteristics of the populations of Norway Rats will be found largely true for other species, even though there may be great differences in their physical environments. Since this review is concerned with populations of wild rats, it will exclude a discussion of studies of the individual rat or of domestic rats except when such work clarifies the population phenomena. Thus the vast literature on the anatomy, physiology, genetics, endocrinology, nutrition, human diseases, rat control methods, economic losses, distribution, behavior, and taxonomy will rarely be mentioned. It seems at the start desirable to indicate briefly the plan of this review. Any population at a given time is the result of the interaction of three general forces. Reproduction tends to increase the population, mortality tends to decrease it, and movements end to increase it or decrease it depending upon the net result of immigration and emigration. Our present knowledge of the equilibrium of these forces will be reviewed in the first section. The second section will describe the changes in population that result from the interaction of these three primary forces. The third section will describe the action of the environmental factors (habitat, predation, and competition) on the three forces. The inherent capacities of the organism will be mentioned briefly, merely for the sake of completeness. Methods will be discussed where appropriate. It is also desirable at this point to comment on the conflict between the impossibility of logically dividing the ecosystem (population and its environment) into parts and nevertheless, the necessity of dividing it for analysis and discussion. The above division into forces and factors is artificial because each group interacts with the others. But for an understanding of the population, it is necessary to look at it from different points of view. Before the detailed data for rat populations are presented, a summary of population theory may be desirable. The simplest situation is a population in a constant environment. Under such circumstances, the population will increase to the capacity of the environment and then remain at that level. At first births are greater than deaths, but at capacity births equal deaths. When the population is below capacity it will increase to capacity along a sigmoid curve. The increase at first is slow, then rapid and then slow again, till at equilibrium it