The Relationship between Agonistic Behaviour and Population Changes in the Deermouse, Peromyscus maniculatus (Wagner)

Abstract

The deermouse, Peromyscus maniculatus (Wagner), of North America has been studied frequently, but there has been little attempt to determine the mechanisms of population control of the species. Both Blair (1948) and Howard (1949) assumed that predation was the most important factor, although Howard considered that cold weather combined with food shortage was also a possible mechanism. McCabe & Blanchard (1950) and Brant (1962) documented predator raids on their study areas and believed that P. maniculatus populations can be decimated or eliminated by predators. However the same authors present data which show that in geographically diverse areas of the United States there is a fairly regular annual cycle in numbers of deermice (Blair 1940, p. 279; Howard 1949, p. 33; McCabe & Blanchard 1950, p. 108; Brant 1962, p. 162). In these studies numbers were at their lowest in spring or early summer. They increased only slightly until autumn when there was a sudden rise to a winter high. After this there is a gradual decrease to the spring low. This annual cycle suggests that there is some mechanism other than predation which affects the numbers of deermice. Since the pattern of annual changes occurs in areas with different predators, it seems unlikely that predation can be a common factor, although it is theoretically possible for different predators to cause similar levels of predation pressure. McCabe & Blanchard (1950, p. 127) have suggested that some intrinsic intraspecific mechanism is important in the population regulation of Peromyscus. Recent work (Eisenberg 1962) on the behaviour of the species has shown that P. maniculatus has a wide behavioural repertoire which includes a considerable amount of agonistic activity. Seasonal changes similar to those in Peromyscus populations have been described by Chitty (1955) for Microtis agrestis and he suggested that the severe juvenile mortality in the early breeding season was an effect of adult hostility. The present paper describes an investigation into the effects of agonistic behaviour on population phenomena in Peromyscus maniculatus.