Antarctic Fur Seals Sparring (Arctocephalus gazella)

When one sex is different to the other, this is known as "sexual dimorphism". Seals spend much of the year swimming through the seas fishing as they go, while they may spend time with other seals, they do not form strong bonds with a partner to rear their young.

When born, the young are fed only on milk by the mother and soon learn to fish for themselves. Birthing usually takes place on a beach with many other seals and the female comes into oestrous (i.e. is ready to mate again) shortly after giving birth. Because of this life-style, many seals have evolved a strong sexual dimorphism. The males arrive on the birthing beaches often before the females and fight to establish and defend their territories. The most successful male, often known as the "beachmaster" gets to mate with as many as a hundred females that are in his territory. Therefore there is a great advantage to being big and strong, and if a male is not among the biggest and strongest, he will not get to mate at all. This has led to a pronounced sexual dimorphism. (difference in size and/or appearance between the sexes).