Evaluating body condition in small mammals

Body condition (energy reserves) can have important fitness consequences. M easuring condition of live animals is typically done by regressing body mas s 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 cin erea Ord), deer mice (Peromyscus maniculatus Ord), red-backed voles (Clethr ionomys gapperi Vigors), and meadow voles (Microtus pennsylvanicus Ord)). W e also determined whether body water could predict fat content, enabling th e use of hydrogen-isotope dilution for estimating condition. For all five s pecies, condition estimates weakly predicted fat content and more accuratel y 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 exp laining variation in all constituents of body composition.