Sex at Sea – How to Avoid the Beach Master

The southern elephant seal, Mirounga leonina, is a textbook example of an extremely polygynous mammal. As the breeding season begins dominant males fight for control of dozens to hundreds of females on his section of beach real estate (Figure 1). Females within the harem, are monopolized by the dominant male (called a beach master). The beach master is believed to father the vast majority of offspring from his harem females. This system evolved because terrestrial birthing sites (beaches) are rare and females are forced to congregate on the few available beaches. In addition, females show high site fidelity, returning to the same breeding/birthing beaches each season. Intense male-male competition leads to a few beach masters controlling access to most breeding opportunities (polygyny).

Figure 1. Two adult male elephant seals battle for the rights to a harem of females. (From Flickr/Mike Baird)

Because so few males get to breed, theory predicts that subordinate males should seek alternative mating strategies. After all, there is pressure on these males to seek any copulations they can get. Indeed, subordinate bachelor males often patrol the surf zone waiting for an opportunity to steal copulations when the beach master is preoccupied. In contrast, females are not thought to seek alternative mating strategies because they benefit genetically by mating with the dominant male. However, new research indicates that female elephant seals may have a few tricks up their sleeves.

Female elephant seals were thought to breed annually, but de Bruyn and colleagues (
2011) used 25 years of mark-recapture data to show that females may skip a breeding season and haul out to give birth every other year instead (Figure 2). Since mating was believed to occur only on the breeding beaches (after pupping), the scientists wanted to know where were these females mating?

Figure 2. Relative percentages of 1032 breeding female southern elephant seals from cohorts 1983 to 1997. Females that bred annually, from first breeding to disappearance, are represented by solid black bars. Females that breed annually at least twice before skipping a breeding season are the hatch bars and females that showed a more random interrupted breeding pattern are the white bars. (From de Bruyn et al. 2011)

By using satellite-tracking, the scientists were able to show that two females skipped the annual haul out but returned to give birth the following season (Figure 3). Fortuitously, one of the two tracked females was a 2-year-old virgin female. This implies that mating must have taken place somewhere other than the breeding/birthing beach. Tracking data showed that it was unlikely that either female mated while hauled out on a remote beach or on ice flows. Thus, it is likely that both females mated at sea.

Figure 3. The satellite tracked movements of female BB193 (2004) and GG335 (2008) at sea. The breeding/birthing beach is located on Marion Island. The tracking data shows that both females were at sea at the time they would have become pregnant prior to returning to Marion Island to pup. (From de Bruyn et al. 2011)

The alternative “at sea” mating strategy has important implications for our understanding of polygyny in elephant seals. As the authors state, “ If polygyny does not preclude females from adopting alternative mating strategies, the term ‘polygyny’ may be misleading.”

de Bruyn, P., Tosh, C., Bester, M., Cameron, E., McIntyre, T., & Wilkinson, I. (2011). Sex at sea: alternative mating system in an extremely polygynous mammal Animal Behaviour, 82 (3), 445-451 DOI: 10.1016/j.anbehav.2011.06.006