Evaluating body condition in small mammals
Body condition (energy reserves) can have important fitness consequences. Measuring condition of live animals is typically done by regressing body mass on measures of body size and using the residuals as an index of condition. The validity of this condition index was evaluated by determining whether it reflected measured fat content of five species of small mammals (yellow-pine chipmunks (Tamias amoenus Allen), bushy-tailed wood rats (Neotoma cinerea Ord), deer mice (Peromyscus maniculatus Ord), red-backed voles (Clethrionomys gapperi Vigors), and meadow voles (Microtus pennsylvanicus Ord)). We also determined whether body water could predict fat content, enabling the use of hydrogen-isotope dilution for estimating condition. For all five species, condition estimates weakly predicted fat content and more accurately predicted variation in lean dry mass and water content. The relationship between body water and fat content was inconsistent among the five species, discouraging against the general use of isotope dilution in these animals. Although ecologically important, these indices are best interpreted as explaining variation in all constituents of body composition.