Skull growth
During growth the male elephant seal skull not only becames bigger, but changes its shape. The main drive of this change is probably related to the elephant seal mating system. Males need to fight to get access to females, and, therefore, they need a skull well adapted to bite the other males and to fight.
Skull of a two year old male
Even when considering just potentially breeding males, aged 7 years or more, the variation in skull size and shape is definitely impressive.
Skulls of seven males aged 7 to 12 years, killed by orcas at Sea Lion island
The following image show the big difference between a 7 year old subadult (Smi) and a 12 year old adult (Gilbe). While Smi had a rather average body size for its age class, Gilbe was definitely a small adult male, so the comparison somehow underestimate the average difference between subadult ad adult males.
Comparison of skulls, Smi (7 year) vs Gilbe (12 year), Smi's skull in red
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