Abstract
A bstract Blood samples were collected from 156 free‐ranging southern elephant seals ( Mirounga leonina ) from Peninsula Valdes to study the variation in blood parameters related to ontogeny and the annual cycle. Samples ranged from newborn pups to adults. Interactions between age and sex showed different trends in ontogenetic changes of blood parameters (MANOVA, Wilkis' Lamb‐da 12,243 = 0.75, P vs . 20.3 g/dl) to adulthood (19.8 vs . 22.3 g/dl). Red blood cell counts (RBC) did not change significantly from pups to juveniles (2.8–3.2 10 6 /μl), but varied for adults at different times of the annual cycle. Breeding females had higher RBC than molting females (3.1 vs. 2.4 10 6 /μl). Changes in blood parameters are related to the development of diving capabilities from pups to juveniles. Changes in adults were associated with different stages of the annual cycle, and these may be the result of the requirements imposed by pregnancy and fasting duration.