Bacular size, growth, and allometry in the largest extant otariid, the Steller sea lion (Eumetopias jubatus)

Bacula rue relatively small in terrestrially mating species of pinnipeds (o tariids and elephant seals, Mirounga), perhaps reflecting adaptive size red uction to minimize bacular fracture. Fur seals and sea lions (Otariidae) ar e a good group with which to investigate this question, because most specie s copulate solely on land and body size varies interspecifically. We studie d bacular size and relative growth in the largest extant otariid, the Stell er sea lion (Eumetopias jubatus). Bacula roughly tripled in length and incr eased 30-fold in mass between 1 and 8 years of age. Allometric relationship s changed over development; bacular length and mass changed from being init ially positively allometric to body length to negatively allometric and iso metric, respectively; bacular mass and thickness were positively allometric to body length throughout life, and apical growth was isometric then was p ositively allometric to bacular length. in adults (>7 years of age), bacula averaged 18.1 cm length (6.2% of body length), 36.7 g mass, and 2.02 g/cm density (mass:length). The baculum of Eumetopias is about the same length r elative to body length as in other adult male otariids but is about twice t he density, presumably to increase strength. Information on small or aquati cally mating species of otariid are needed to extend our findings and inter pretations.