Ontogeny of body size and shape of Antarctic and subantarctic fur seals

Pre- and post-weaning functional demands on body size and shape of mammals are often in conflict, especially in species where weaning involves a change of habitat. Compared with long lactations, brief lactations are expected to be associated with fast rates of development and attainment of adult traits. We describe allometry and growth for several morphological traits in two closely related fur seal species with large differences in lactation duration at a sympatric site. Longitudinal data were collected from Antarctic (<i>Arctocephalus gazella</i> (Peters, 1875); 120 d lactation) and subantarctic (<i>Arctocephalus tropicalis</i> (Gray, 1872); 300 d lactation) fur seals. Body mass was similar in neonates of both species, but <i>A. gazella</i> neonates were longer, less voluminous, and had larger foreflippers. The species were similar in rate of preweaning growth in body mass, but growth rates of linear variables were faster for <i>A. gazella</i> pups. Consequently, neonatal differences in body shape increased over lactation, and <i>A. gazella</i> pups approached adult body shape faster than did <i>A. tropicalis</i> pups. Our results indicate that preweaning growth is associated with significant changes in body shape, involving the acquisition of a longer, more slender body with larger foreflippers in <i>A. gazella</i>. These differences suggest that <i>A. gazella</i> pups are physically more mature at approximately 100 d of age (close to weaning age) than <i>A. tropicalis </i>pups of the same age<br />