Body condition can have important fitness consequences, but measuring body condition of live animals from wild populations has been the subject of much recent debate. Using the residuals from a regression of body mass on a linear measure of body size is one of the most common methods of measuring condition and has been used in many vertebrate taxa. Recently, the use of this method has been criticized because assumptions are likely violated. We tested several assumptions regarding the use of this method with body composition and morphometric data from five species of small mammals and with statistical simulations. We tested the assumptions that the relationship between body mass and body size is linear, and that the proportion of mass associated with energy reserves is independent of body size. In addition, we tested whether the residuals from reduced major axis (RMA) regression or major axis (MA) regression performed better than the residuals from ordinary least squares (OLS) regression as indices of body condition. We found no evidence of nonlinear relationships between body mass and body size. Relative energy reserves (fat and lean dry mass) were generally independent or weakly dependent on body size. Residuals from MA and RMA regression consistently explained less variation in body composition than OLS regression. Using statistical simulations, we compared the effects of violations of the assumption that true condition and residual indices are independent of body size on the OLS, MA, and RMA procedures and found that OLS performed better than the RMA and MA procedures. Despite recent criticisms of residuals from mass–size OLS regressions, these indices of body condition appear to satisfy critical assumptions. Although some caution is warranted when using residuals, especially when both interindividual variation in body size and measurement error are high, we found no reason to reject OLS residuals as legitimate indices of body condition.

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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.Résumé : La condition physique (réserves énergétiques) peut avoir une influence importante sur le fitness. La mesure de la condition physique d'animaux vivants se fait généralement par une régression de la masse du corps en fonction de mesures de la taille du corps, les résidus servant de coefficient de condition. La validité de ce coefficient de condition a été évaluée en déterminant s'il reflète bien le contenu lipidique mesuré chez cinq petits mammifères (le Tamia amène (Tamias amoenus Allen), le Néotoma à queue touffue (Neotoma cinerea Ord), la Souris à pattes blanches (Peromyscus maniculatus Ord), le Campagnol-à-dos-roux de Gapper (Clethrionomys gapperi Vigors) et le Campagnol des champs (Microtus pennsylvanicus Ord). Nous avons également tenté de déterminer si le contenu hydrique du corps permet de prédire le contenu lipidique, ce qui justifierait l'utilisation d'une dilution des isotopes d'hydrogène pour estimer la condition physique. Chez les cinq espèces, les indices de la condition physique se sont révélés des indicateurs mé-diocres du contenu lipidique; ils permettent toutefois de prédire plus exactement la masse du corps sans les graisses, la masse sèche et le contenu en eau. La relation entre le contenu hydrique et le contenu lipidique n'est pas la même chez les cinq espèces, ce qui nous empêche de recommander la méthode de dilution des isotopes chez ces animaux. Malgré leur intérêt écologique, c'est comme facteurs explicatifs de la variation de toutes les composantes du corps que ces indices semblent le plus utiles.[Traduit par la Rédaction] 1029Schulte-Hostedde et al.

Vertebrate gastrointestinal tracts have co-existed with microbes over millennia. These microbial communities provide their host with numerous benefits. However, the extent to which different environmental factors contribute to the assemblage of gut microbial communities is not fully understood. The purpose of this study was to determine how the external environment influences the development of gut microbiome communities (GMCs). Faecal samples were collected from deer mice (Peromyscus maniculatus) born and raised in captivity and the wild at approximately 3-5 weeks of age. Additional samples were collected 2 weeks later, with a subset of individuals being translocated between captive and wild environments. Microbial data were analysed using 16S rRNA next-generation Illumina HiSeq sequencing methods. GMCs of deer mice were more similar between neighbours who shared the same environment, regardless of where an individual was born, demonstrating that GMCs are significantly influenced by the surrounding environment and can rapidly change over time. Mice in natural environments contained more diverse GMCs with higher relative abundances of Ruminoccocaceae, Helicobacteraceae and Lachnospiraceae spp. Future studies should examine the fitness consequences associated with the presence/absence of microbes that are characteristic of GMCs of wild populations to gain a better understanding of environment-microbe-host evolutionary and ecological relationships.