The ontogeny of body composition and gut morphology in free-ranging muskrats

Abstract

We quantified changes in body composition, external morphology, and digestive tract morphology during the growth and development of muskrats from two marsh environments in central Saskatchewan, Canada. A total of 141 individuals were analysed, with body masses ranging from 20 g (newborns) to >800 g (prior to first reproduction). The growth rate of the digestive tract in nursing young was rapid, and at weaning juveniles had attained 75% of adult large and small intestine length, but only approximately 20% of adult body mass. At birth, muskrats contained no fat tissue, but accumulated fat reserves during the early stages of nursing. Fat reserves were then exhausted during weaning. Protein content also exhibited marked variation during the nursing–weaning period. As juveniles matured, the proportion of protein and ash became relatively constant at approximately 25 and 4.5% of body mass, respectively. Conversely, the proportion of fat tissue continued to increase with body mass, and was inversely related to water content. The accumulation of fat reserves from autumn through early winter was similar to that exhibited by adults and was probably associated with the presence of energy-rich food. Growth generally ceased in mid-October. Prior to the start of the reproductive season, juveniles had attained 77% of adult body mass and 95% of adult skeletal size. The growth diapause in juveniles from mid-October through March may limit their access to primary breeding habitat the following spring.