Correlates of life-history variation in Clethrionomys and Microtus (Microtinae)

We investigated how life history traits in the microtine genera Clethrionomys and Microtus were related to four major correlates (seasonality, habitat, metabolic rate, and geographic range). We found that the degree of seasonality as measured by latitude, elevation, or length of the breeding season was not strongly associated with litter size within each of seven species. Correlations between litter size and measures of seasonality are much stronger when analyses are carried out at higher taxonomic levels. Species of New World Microtus were placed in five different vegetation habitat types (graminoids, forbs, deciduous, coniferous, and mixed). Species associated with coniferous forests were heavier, had larger litter sizes, and occurred in short season environments compared to those in other vegetation types. Species placed in two other habitat categories (soil moisture regimes and habitat-patch configurations) revealed no differences in any life history trait. Data from the literature showed that basal metabolic rates were negatively correlated with body weight. Neonate weight, weight at weaning, and growth to weaning were also negatively correlated with metabolic rate, but were not significant when the effects of body weight were removed by partial correlation. North American species with large geographic ranges had young which were weaned later and grew less rapidly to weaning than species occupying smaller ranges. However, when the two species that only occupy islands were excluded, geographic range was positively correlated with litter size. At the species level, vegetation habitat type along with seasonality seem to be the most important factors associated with life history variation in these microtines.