War of the sexes, 7

The battle continues

By nature, the negotiation between the sexes is a dynamic process. The tension between males and females continues and, accordingly, the compromises struck between them in their quest for genetic supremacy are ever changing.

The seeds of change can be detected on the rocky beaches of Rona on which grey seals breed. Rona is a small island well to the west of the Orkney Islands off the north coast of Scotland. At the best of times it is a wild and windy place, bearing the brunt of the Atlantic swell. In October, when the seals give birth and then immediately mate again, it is frequently lashed by gales; the exposed cliffs and gullies shudder under the pounding waves. But the appalling conditions are apparently of no consequence. Rona hosts the densest population of grey seals in the North Atlantic—about 600 breeding females.

The grey seal is a classical polygamous species with a very marked size difference between the sexes. Whereas every cow can expect to breed, the bulls are not so fortunate. Each one lives in the hope that one day he will be big enough and sufficiently good fighter to win his own harem of cows. Sexual selection among bull grey has therefore favoured the most powerful pugilists, and the biggest warriors get their chance to mate with perhaps a dozen females each season.

However, a few of the lesser bulls, which stand no chance of succeeding in combat, turn luckily—and it is all down to the cows. Although most happily fall for the victorious bulls, a minority of females take a fancy to the males of a more gentle disposition which lounge on the sidelines. Luckily grey seals can be recognised by their individual markings. It has therefore been possible to discover that these cows tend to return in successive years to the same males, and they appear to strike monogamous ‘marriages’.

Clearly two separate mating strategies are underway, but perhaps the female grey seals are beginning to exercise a preference for less disruptive and less heavy bulls to father their pups. If so, their choice is nudging evolution towards establishing monogamy in place of the current strongly polygamous arrangement.

We know that the nature of habitats favours some breeding systems over others. Perhaps this is the case with these seals, which probably bred on sea ice during the last Ice Age. Now that the climate has improved and the ice retreated, grey seals may still be in the process of adapting to the change—and this includes establishing a new relationship between the sexes.