Xperimental studies of competitive interaction in a two-species system II. The behaviour of Mkrotus, peromyscus and Clethrionomys species

Abstract

1. 1. The interactive behaviour of four species of microtine rodents was studied in the laboratory. The species were Clethrionomys gapperi and Peromyscus maniculatus from woodland habitat on the North American mainland; Microtus pennsylvanicus from grassland habitat on the same mainland and from a mixture of grassland and woodland habitat on Newfoundland; Clethrionomys glareolus from woodland habitat on mainland Britain, and from grassbracken habitat on a British island (Skomer). Two animals of the same group (controls) or of different groups (experimentals) were introduced simultaneously to an arena which contained grass in one half and maple saplings (woodland) in the other, and their behaviour was recorded for 30 min. 2. 2. Animals were aggressive to each other. Within each species the larger animal was almost always the dominant one. When animals of different species were tested together they interacted generally less frequently than did the controls, and the outcome in terms of dominance was not as predictable. 3. 3. When tested with Peromyscus or any of the Clethrionomys groups, both groups of Microtus from mainland and island occurred significantly more frequently in the grassland half of the arena than did the respective Microtus controls. Peromyscus occurred significantly more frequently in the woodland half when tested with mainland Microtus but not when tested with island Microtus. There were no significant differences in the distribution of the respective control and experimental animals of any of the Clethrionomys groups. But all the differences between controls and experimentais, whether statistically significant or not, were consistent with the hypothesis that two species have a mutually dispersive effect upon each other in relation to habitat occupied. 4. 4. In a separate experiment Peromyscus, born and reared in the laboratory in the presence of grass or maple or in the absence of both, were tested at a minimum age of 8 weeks either with Microtus or as controls. Although they did not differ from their controls, grass-exposed Peromyscus spent more time in the grass when tested with Microtus than did maple-exposed Peromyscus. This indicates that in young animals the early exposure to habitat features may be of over-riding importance. 5. 5. The relevance of the experimental results to events in the wild is discussed, and it is concluded that interactions at the individual level, involving aggressive encounters and possibly other factors, form the basis of the interactions which have been demonstrated at the population level in the wild.