EXTRA-LARGE BODY SIZE IN CALIFORNIA VOLES - CAUSES AND FITNESS CONSEQUENCES

Extra-large body size in microtine rodents is a ubiquitous feature of peak population densities, and it has been hypothesized that these giant individuals represent a genetically based morphotype that has high fitness under increasing and high densities. and may represent a key element of a genetic polymorphism driving multi-annual cycles (Chitty/Krebs model). We examine this large-size phenomenon (Chitty Effect) in the California vole utilizing three approaches: analyzing the weight distribution in a non-cyclic population over a 13 yr period with comparisons to cyclic populations, analyzing body composition, especially fat content, as a function of body size, and observing the microhabitat distribution of extra-large males in two populations, one of which cycles. Extra-large body sizes occur in all populations, and can occur in any year except in the year following a cyclic peak. They are most likely to be found when favorable conditions prevail for an extended period. Analysis of covariance demonstrates that body composition (fat, water, fat-free dry weight) is influenced by season, sex, reproductive condition, as well as by body size. Fat accounts for only 45% of variation in body weight, and extra-large voles are sometimes fat and sometimes lean. Contrary to expectations of the Chitty/Krebs model, extra-large males tend to be in marginal microhabitats and to be negatively associated with reproductively active females. We conclude that the Chitty Effect in California voles is the result of prolonged periods favorable to growth and survival, that these giant individuals do not have enhanced fitness under conditions of rapid population growth and high densities, and that large size may or may not be associated with obesity. Collectively, our data do not support the hypothesis that the Chitty Effect is caused by a genetic polymorphism, and instead we view it as an interesting epiphenomenon of vole density regulation.