The testis of the Southern elephant seal demonstrates seasonal changes in weight, with highest values early in the breeding season (September-November) and lowest values in mid-winter (May-June). Weight changes originate in the seminiferous tubules, there being little variation in the interstitial tissue.

Spermatogenesis begins in August, one month before the breeding season, and stops in November. Testicular testosterone concentration is significantly elevated in the months July-September. Plasma testosterone concentration is high only in September, the first month of breeding.