What We Think Now

New molecular techniques allow paternity tests to be conducted alongside behavioral observations. This has led to the discovery that California sea lions are only weakly polygynous and that territorial males do not father nearly as many pups as was inferred from their behavior. Although the majority of males do not father pups, those that do father only a few, usually less than three (see graph below, left) (Porschmann et al. 2010). This is a far lower number than the number of females in a typical territory. The disparity is probably due to a combination of female choice and aquatic copulations that take place away from the colony and its structure, although researchers are not entirely sure.

This page details some of the newly discovered disconnects between the traditional model of the California sea lion mating system (described on the What We Thought page) and what actually happens in sea lion colonies.

Number of paternities assigned to males Galápagos sea lions at one colony
(Porschmann et al. 2010)

Sexual Maturation: Or is it alternate mating strategies?

While territorial males do father more pups on average than non-territorial males, the overwhelming numbers of non-territorial males means than the majority of pups are in fact fathered by non-territorial males. It may be that non-territoriality is not just a characteristic of younger smaller males incapable of successfully competing, but rather an alternate mating strategy (more on alternate mating strategy). Being non-territorial is far more successful than would be guessed from observing an onshore colony because many or most copulations take place in the water, away from the main colony (Flatz et al. 2012).

Territories: More like leks?

Females move around considerably between different territories, and the males do little to stop them. Neighboring males actually exhibit little aggression toward each other and territory boundaries are generally stable. Some territories donít even have femalesómales just defend land (Robertson et al. 2008). A substantial portion of barking is directed at females, possibly in an effort to appeal to them (Kunc and Wolf 2007). All these facts point towards the idea that female choice plays more of a role in the mating system than previously thought. Territories may be more like leks than closely guarded harems. In a leck system, males display in designated areas in order to impress choosy females, but do not fight one another (more on leks).

Aggression: Female-female

Female-female aggression is actually more common than male-male aggression. Pups are small and vulnerable to the crowd of other sea lions, and mothers aggressively defend their offspring from encroachment by other females. An overall high density in the colony and a greater female-to-male ratio increase female-female aggression (Young et al. 2008). This important component to the social and mating system has been previously ignored in favor of the showier and louder male-male aggression.

Aggression: Or is attendance the important thing?

Reproductive success (paternities) vs proportion of time at colony (attendence) of male Galápagos sea lions  (Porschmann et al. 2010). Black and white circles represent data from two different years. Trendlines for those years are also show.

Male aggression may have very little to do with reproductive success. The greatest correlate with reproductive success is not size, dominance, aggression, or territory size, but rather attendance, the percentage of time a male is present at the colony (see graph at right). A territory-holding male fasts during the breeding season and relinquishes his territory if he leaves to hunt. Larger males capable of longer fasts hold territories longer, and thus have greater attendance and greater reproductive success. With size factored out, attendance increases reproductive success of both territorial males and non-territorial males (Porschmann et al. 2010, Meise et al. 2014). In this new model of attendance as the most important factor, the benefit of holding a territory becomes less clear. Discovering the answer is a current line of research.

 

For both old and new theories of the origin of the mating system, go to How It Evolved.