Not all size measures are created equal: different body size proxies are not equivalent fitness predictors in the bat Carollia perspicillata

Body size variation can have important evolutionary, physiological, functional, and ecological consequences. Body mass is a widely used size measure across different taxonomic groups, but it combines skeletal size with nutritional and reproductive status. In bats, forearm length is commonly used as a measure of skeletal size. However, body mass and forearm length are poorly correlated within species. This suggests that the two size variables are measuring different biological attributes. Here, we tested this hypothesis by evaluating the association between body mass, forearm length, and fitness components (survival and reproduction), derived from mark-recapture models, as well as their trends over a nine-year period in a population of short-tailed bats ( Carollia perspicillata ). Results showed a direct relationship between body mass, survival, and reproduction, and an inverse relationship between forearm length, survival, and reproduction. Different temporal trends in the size variables were observed according to sex and age. Males showed a trend of increasing average mass over the years. In adults, average forearm length decreased over the years, whereas juveniles showed an increasing trend. Our results showed that body mass and forearm length have distinct evolutionary dynamics and proximal mechanisms of change. Forearm length is a measure of wing size and should not be used as a proxy for body size in intraspecific studies.