Postnatal growth and development in the Mexican free-tailed bat (Tadarida brasiliensis mexicana: birth size, growth rates, and age estimation

Abstract

Based on mark-recapture data, we quantified changes in body mass, length of forearm, and lengths of the total, proximal, and distal epiphyseal gaps of individual Mexican free-tailed bats (Tadarida brasiliensis mexicana) from birth to peak lactation. We used these data to develop empirical growth curves, derive growth rates, establish age-predictive equations, and compare growth parameters based on three nonlinear growth models. Length of forearm and body mass of neonates averaged 18.5 mm ? 0.7 (SD) and 3.2 g + 0.3, respectively. Mean body mass of pups increased linearly for the first 3 weeks, by which time they had achieved ca. 80% of the mean postpartum mass of adult females. Thereafter, body mass remained relatively stable until a small decrease in body mass was observed at the beginning of the 6th week when young first began to fly. The growth trajectory of the forearm also was linear for the first 3 weeks, but, thereafter, increased more slowly until average adult dimensions were reached in the 6th week. Length of total epiphyseal gap increased to its maximum size during the first 2 weeks and subsequently decreased in a linear fashion. Secondary centers of ossification appeared in the metacarpal and first phalanx of the fourth digit at ca. 14 days, thus, making it possible to quantify age-related changes in the proximal and distal epiphyseal gaps. The equation for estimating age based on length of the forearm is valid when this dimension is -39 mm, whereas the equation for estimating age based on length of the total epiphyseal gap is valid when length of the forearm is >39 mm. Together, these two equations make it possible to estimate the age of pups from 1 to 42 days of age in T. brasiliensis. Of the three nonlinear growth models (logistic, Gompertz, and von Bertalanffy), the logistic equation provides the best fit to the empirical curves for length of forearm and body mass.