• southern sea lion;
  • Otaria byronia;
  • sexual dimorphism;
  • Peninsula Valdes;
  • body mass;
  • growth rate;
  • maternal behavior


Male southern sea lion, Otaria byronia, pups born at Peninsula Valdés, Argentina, were significantly heavier (13.7 ± 1.6 kg, n = 37 vs 12.3 ± 1.4 kg, n= 38) and longer (0.82 ± 0.04 m, n= 29 vs. 0.79 ± 0.03 m, n= 29) than females. The rate of mass and length gain during the first month of life did not differ between the sexes (males: 0.21 kg/d, 0.003 m/d; females: 0.20 kg/d, 0.004 m/d). Sex differences in pup body size were not reflected in female nursing behavior, attendance patterns, and duration of feeding trips. These results suggest that southern sea lion mothers invest more energy in sons than in daughters during gestation, a difference consistent with the importance of large body size to the reproductive success of males in a polygynous species.