Trap-induced mass declines in small mammals: Mass as a population index

Abstract

Body mass is routinely used as an index of physical condition for comparing small-mammal populations. However, trapping effects on animals may undermine the effectiveness of body mass as an index of population health. We examined the effects of live-trapping on body mass of 3 small-mammal species: deer mice (Peromyscus maniculatus), southern red-backed voles (Clethrionomys gapperi), and red-tailed chipmunks (Tamias ruficaudus). We found that live-trapping resulted in significant trap-induced body mass declines (TMDs) that varied by species, age, and sex. Longer rest intervals between captures reduced TMDs in deer mice, but not in other species. The TMDs were more dramatic for deer mice that died in traps than for those that did not, suggesting that TMDs may induce trap mortality and reduce survivorship in small mammals that are live-trapped. The proximate causes of TMDs remain unknown, but dehydration and hypothermia are likely causal agents. Since TMDs vary by species, age, and sex, and exposure to heating and cooling vary in space and time (e.g., by habitat and season), body mass as measured by live-trapping appears to be a negatively biased index whose deviation from true mass may vary among study populations and demographic groups. Researchers can attempt to bias correct indices of body mass using regression approaches, but such adjustments require large samples to be robust and so may not necessarily improve the raw indices. Additional research is needed to identify the causes of TMDs to better understand biases associated with using body mass as a population index and minimize impacts on small mammals.

Citation

Pearson, Dean E.; Ortega, Yvette K.; Ruggiero, Leonard F. 2003. Trap-induced mass declines in small mammals: Mass as a population index. Journal of Wildlife Management. 67(4): 684-691.