When observing the Northern elephant seal colonies found dotted along the Californian coast from December to March each year, one feature stands out above all others: size. Female elephant seals can weigh up to 750 kg, which is fairly impressive, until you see one next to a 2,300 kg male – that’s a male:female size ratio of about 3:1, among the largest of all vertebrates. By comparison, the same ratio in humans is a mere 1.15:1.
Why is there such a huge size difference between male and female elephant seals? Like most things in life, it all comes down to sex – or in this case, sexual selection. Elephant seal females aggregate in huge harems numbering in the hundreds, and each harem is defended by one very busy (and very jealous) male.
Smaller males might try to sneak matings under a harem holder’s impressive nose, but they’re rarely successful. In total, just 4% of males are responsible for 85% of matings in a given season, a situation referred to as reproductive skew. Reproductive skew just means that not all individuals have an equal chance of attaining mates, because some individuals are stronger or sexier.
Male elephant seals defend their harems primarily through strength, fighting bloody battles against would-be usurpers. This competition between members of the same sex is called intrasexual selection. High reproductive skew means that the rewards for being the largest male around are huge, and so males have been evolutionarily selected to attain large size.
By contrast, females experience low reproductive skew: whether you’re the strongest or the weakest female in a harem, you’re unlikely to have trouble securing a mate. The result is that there’s no reason for females to invest massively in size the way that males do, which is why male elephant seals are so much larger than females.
Reproductive skew is interesting because it can be the cause of all kinds of interesting evolutionary phenomena, including the remarkable sex size differences of elephant seals. It also has important implications for population genetics. Because only a tiny proportion of male elephant seals mate each season, the genetic diversity of populations is really low, which might make conservation efforts more difficult for this and other species with high reproductive skew. It turns out that equality between the sexes isn’t just a moral good– it’s also beneficial for genetic diversity.