MUELLER, P.J.*; STEYERMARK, A.S.: Constraint of small cages turns California mice into couch potatoes.

Although care is taken in maintaining proper temperature, lighting and feeding regimes for captive experimental animals, other facets of husbandry that can influence experimental results may be overlooked. For example, investigators routinely place animals in whatever size of cage they have at hand that seems remotely reasonable. We questioned whether the size of the cage in relation to the size of the animal may affect physiological variables. We measured 24-hour energy expenditure (VO₂) and ten day food intake in two species of Peromyscus mice that differed in body size: large Peromyscus californicus (body mass 45-55 g) and small P. eremicus (22-26 g). In a two-period cross over study, mice were individually housed in both small (25x15x12cm) and large (40x25x20cm) cages. While VO₂ and food intake of small mice (P. eremicus) were unaffected by cage size, VO₂ of large mice (P. californicus) was less when placed in a small cage compared to a large cage, despite the fact that the small cage size was within normal animal care guidelines. Californicus mice also ate less food when housed in small cages, but not enough less to offset their reduced energy expenditure and consequently body mass increased slightly. In large cages, we observed all mice frequently jumping and flipping; small cages did not afford sufficient room for large mice to jump and hence they likely spent more time stationary. Small mice were not similarly constrained. Our data suggest that more care needs to be given to considerations of size and type of animal housing, otherwise measurements thought to be unbiased may be reflecting factors other than applied treatments. In our case, choice of cage was unimportant when studying the smaller species, but could skew results for larger mice.