What We Thought

Sexual size dimorphism of a male (left) and female (right) California sea lion
(image copyright Chris Grossman)


California sea lions have a classic polygynous mating system, in which a few large territory-holding males do almost all of the mating.  Males are approximately three to four times the size of females (see photo at right), an extreme example of sexual size dimorphism. (more on polygyny and sexual dimorphism)

Sexual Maturation

Both sexes of California sea lions reach sexual maturity at around four to five years of age. Although females begin to reproduce at this time, males typically do not because they cannot gain access to females. Males continue to grow throughout their lives, and thus large males are also older (see graph below, left). Small, young males are unable to hold a territory, and were thought to either not reproduce or to do so only occasionally by sneaking into another male’s territory or harassing females on the edge of a harem. Subadult males often clump on the periphery of colonies in so-called “bachelor” areas (Reidman 1990).


Overall, only about ten percent of males hold territories, while the other ninety percent remain in bachelor areas or on the periphery of harems. Males fast while holding territories, and do not leave the colony to hunt (Wolf et al. 2005). Good territories are determined by access to shade or shoreline, easily defined and defended boundaries, and other aspects sea lions seem to care about but that researchers have yet to deduce (Robertson et al. 2008).  Bachelor areas are often the worst real estate in the colony, with little access to shade or shoreline. Such features help seal lions regulate their body temperature and are important in warm climates, such as the Gulf of California (Wolf et al. 2005).


Territorial males interact aggressively with one another, barking, chasing, and directly fighting. Male-female aggression is rare. Those with larger territories and close male neighbors display more aggression (Young et al. 2008). Non-territorial males display little aggression, and seldom bark comparted to territorial males (see graph below, right) (Kunc and Wolf 2007).

Correlation of size and age of male Galápagos sea lions
(Meise et al. 2014)
Vocalization (barking) of male Galápagos sea lions at begining and end of breeding season (Kunc and Wolf 2007). Territorial males are the dark bars, non-territorial males are the white bars.

However, this is not the whole story.

Go to What We Think Now for new information about the puzzles and complexities of the system.

Go to How It Evolved for theories on how the system arose.