In his 1871 book The Descent of Man, after discussing human evolution, Darwin sweeps through the animal kingdom looking for secondary sexual characters. These are features of an animal that are different between males and females, but not directly involved in the act of sex. Thus, for example, a lion’s penis is a primary sexual character, whereas his mane is a secondary sexual character. Everyone is familiar with such characters when it comes to mammals and especially birds – who doesn’t like a good paradise bird courtship ritual – but what about the fishes?
Though there are many kinds of secondary sexual characters, Darwin writes (in good Victorian fashion) that ‘all structures and instincts by which the male conquers other males, and by which he allures or excites the female, will be fully discussed, as these are in many ways the most interesting.’ Darwin does not find many of these in such marine animals as worms or sea slugs, but when he turns to the fishes we are treated to an orgy of males fighting to the death.
During the mating season, male salmons develop a rather wicked-looking hook on the lower jaw (figure 2), which they use to tear at each other, often leading to serious injuries and occasionally death. Most vivid is the description of sticklebacks. With little subtlety, Darwin only briefly mentions the nest-building behaviour of male sticklebacks and then jumps straight to the fighting. Males of these tiny fish are well known to develop bright colours and use their armour and spines in rather fierce combat. In a scene in true Game of Thrones style, Darwin quotes a colleague who writes that he saw a stickleback “during a battle absolutely rip his opponent quite open, so that he sank to the bottom and died.”
Darwin also gives examples of more benign sexual characters. This includes such fishes where the males have bright colours and impressive ornaments (figure 3), essentially making them the marine equivalent of the paradise bird (figure 1). Rather intriguing is the fact that, contrary to mammals and birds, in fishes it is often the males who take exclusive care of the young. This includes the ferocious stickleback, who nurses the little ones and chases away other fishes from the nest (including female sticklebacks) but also the seahorse, in which the male hatches the eggs in a pouch.
Darwin’s real strength, as so often, lies in his unifying treatment of the subject. The problem, though, is that the ocean was a rather underexplored area of the planet in his time (which to a large extent it still is today). He stresses this when discussing the fishes by writing that “little is known about their courtship, and not much about their battles.” On one occasion Darwin shows a picture of a male fish sporting an impressive beard with unknown function (figure 4), cautiously suggesting it might have something to do with sexual behaviour, otherwise why would only the male have it? After all, perhaps the females just find bearded men sexy. Darwin would no doubt have liked to believe that.