Seasonal Changes in Survival in Mixed Populations of Two Species of Vole

Abstract

The purpose of this study was to follow up the inference that certain kinds of loss among Microtus agrestis are due to the direct effects of behavior and not to its indirect effects through changes in susceptibility to local hazards. Between Aug. 1959 and July 1961 changes in survival were observed in mixed populations of M. agrestis and Clethrionomys glareolus on about 2 ac (0.81 ha) of grassland near Oxford. The area was trapped weekly, most animals on it were marked, and changes were found by direct counts. The numbers of both species had increased rapidly in 1958-59, but thereafter declined or remained about the same in spite of a satisfactory birth-rate. Overwintering Microtus suffered 2 types of loss: a steady drain on numbers in the autumn, winter, and greater part of the summer; and sudden severe losses, especially in spring, when the proportion lost in a single week might exceed that lost in several mo. at other times. The steady losses were presumably due to predation, disease, and other contingencies peculiar to the locality. The sudden losses were presumably due to intraspecific strife peculiar to the species. There was no direct evidence for this latter process or for its presumed selective effects, but there was no other plausible explanation for the suddenness of the changes and the way they affected particular groups at particular times only. Clethrionomys adults suffered no comparable losses in spring, and their numbers changed more gradually throughout the year. In both species the young seem to have suffered from the antagonism of their own adults, and in Clethrionomys to have suffered from interspecific attacks as well. Improved recruitment in July 1960 coincided with the adult male Microtus disappearing from most of the area. The study as a whole is consistent with the belief that some, at least, of the sudden changes in survival are due to changes in behavior. The next task will be to quantify behavior and design experiments to discover its supposedly selective effects.