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Charles Darwin Science

War of the sexes, 2

Warriors and wimps

The recurring theme of this book is that the opportunities created by sex differ from males and females. The reason of this asymmetry lies in the nature of their respective sex cells—sperm and eggs. Sperm are minuscule, biologically ‘cheap’ to manufacture, and are produced by the testes in astronomical numbers. Eggs, on the other hand, are comparatively large—small humming-birds, for instance, make eggs equivalent to 25 per cent of their body weight and packed with nutrients. Being ‘expensive’ to make, they are produced in much smaller numbers that sperm.

The consequences for the two sexes are profound. With a more or less fixed output of eggs, females cannot usually generate more offspring by taking on extra mating partners. The best option is to be careful in their choice of who fathers the young. Males have quite a different agenda. With almost unlimited supply of sperm at their disposal, the best reproductive strategy is to mate with as many females as possible, each of which will provide them with offspring.

From a male’s perspective, there are never enough females to go around and so, motivated by lust and sheer greed, each of them comes into serious competition with other philanderers. To be successful in the mating stakes, a male needs to win and win well. This rivalry manifests itself as raw aggression among the sturdy males of those species for which ‘biggest is best’. To be triumphant in battle, a male has to look like a warrior, act like a warrior—and mean it!

Competition for sex is the overriding evolutionary pressure responsible for fashioning the appearance of mature males, whether they be chest-thumping gorillas or heavily veiled fighting fish. This is because, in the struggle for supremacy, weapons and large body size have been overwhelmingly advantageous, enabling hefty, well-armed males to win more mates than feeble and less bold ones.

Over countless generations, macho males driven by their gonads have been willing to risk life and limb in order to rank among the most bountiful breeders of their kind. Such a valuable prize is always worth fighting for, and only the most pugilistic individuals stand a chance of winning—which is why the males of many species are larger and more irascible than their mates. To help them in their battle against rivals, warrior males throughout the animal kingdom have often become heavyweights, equipped with weapons enabling them to stab, ram, kick or wrestle. For those who compete for harems, the reward for being a successful male is proportionately high, and so the conflicts become that much more serious.

When a pair of bull elephant seals clash on the breeding beaches, no quarter is given. Each is a warrior fighting for the survival of his line. By far the largest of the seals, each bellowing bull is a quivering mound of flesh and blubber 6 metres (20 ft) long and weighting 3000 kilograms (6600 pounds)—five times the weight of a mature female… The combatants often tear their noses and gouge out chunks of their opponents’ skin. There is a lot at stake, and well-matched rivals do not give up easily. But inevitably, one of them backs off and awaits a further opportunity to challenge the beachmaster…

The odds are heavily stacked against the males. Fewer than one in ten become successful warriors commandeering their own stretches of the beach favoured by the females; the rest will die without issue or resort to sneaking a furtive mating here and there. Competition between the lusty males is therefore intense, and success will favour only the heaviest and most belligerent of them…
Horns and antlers

The most spectacular horns and antlers adorn the heads of the hoofed mammals. They come in an amazing array of shapes and sizes, resembling corkscrews, rapiers, daggers and meat hooks; some are tightly spiralled, others extravagantly branched. In many cases, the females are hornless…

Ibex, big-horn sheep, goats and musk oxen perform serious battering contests in which the opponents gallop towards each other and meet head on; it is a wonder that any participant survives such head-shattering impacts. The secret of their survival lies in the construction of their skulls…

Males of all kind have become embroiled in an arms race favouring those which can grow and deploy bigger weapons. The extinct Irish elk was one such species: the older stags sported a might spread of antlers that would dwarf those of modern deer. Like those of today’s warriors, such weapons are costly to grow—especially those of deer, which have to be regrown every year—and the individual has to be a very competent forager to find enough food to be able to ‘afford’ and replace them annually.

Stags sometimes sustain smashed antlers or broken legs, or are blinded in one eye. In one population, battles over rutting supremacy accounted for 20 per cent of all adult male mortality and in Germany 5 per cent of stags are killed every year through fighting. Some 10 per cent of bull musk oxen die from fractured skulls, despite the reinforced nature of their foreheads, and no less that 60 per cent of narwhals sport broken tusks or have pieces or twisted ivory buried into their flesh—doubtless all wounds uncured through fighting.
Sneaky males

The problem for most males is that they must often wait on the side-lines, sometimes for years, until they are in a position to challenge the dominant breeders—and then most will fail. In the interim, they resort to sneaky tactics. In southern fur seals, the beachmaster are typical warriors and each stakes out a territory which it defends violently from other males, creating the most vicious fights in the animal world.

The bulls aim for the vulnerable soft skin around the fore flippers, ripping huge gashes in them with their teeth. The combatants sometimes end up with horrific injuries, such as torn muzzles, dislocated jaws, missing eyes and great chunks bitten out of their pelts. At this time, the bulls appear to be immune from pain; those which have commandeered prime positions on the beach rarely stand down and they valiantly stave off challenges from neighbouring males. Many pups are crushed in the resulting mayhem on the crowded rookeries…

Several major lakes nestle in Africa’s Great Rift Valley. There are algal scrapers, leaf choppers, scale eaters, shell crushers, diggers, hunters and plankton filterers; there is even one species that survives by biting out the eyes of other fish. Many are colourful and have remarkable breeding arrangements; in Lake Tanganyika, fifteen kinds employ empty water-snail shells as receptacles for their eggs, although one, called Lamprolugus callipterus, is especially interesting. This shell-brooding cichlid holds the record of proportionately the largest males in the animal kingdom. The fully grown ones are giants, up to thirty times the size of their mates; in human terms, this is equivalent to the difference between a 80 kilogram (180-pound) man and the average newborn baby. There is a good reason for this disparity between the sexes…
Gender jumpers

So the warriors and dandies of the natural world may gain mates through brute force or low cunning. But so relentless is the drive to carry on their genetic line that the males of some species have evolved other quite astonishing ploys to maximize their breeding potential. One surpassing technique is gender jumping…

Aggression plays a key role in the life of a gender-jumping wrasse. Each territory contains a tyrannical male which firmly dominates his harem of six or more mates. Only by continually demonstrating his command over them can he prevent one of them from changing sex and usurping his position of power. When young, the small wrasse join the harems as spawning females at the bottom of the packing order and, bearing the brunt of everyone’s hostility, their masculine tendencies are suppressed. But as they grow, each has the potential to be a male. The chance to switch sex and status comes with the death of the despotic male. Within and hour or two of his disappearance, the largest and most dominant female becomes aggressive and starts to behave like the departed ‘master’, chivvying the rest of the females and defending the area against neighbouring males. Should one of them beat her into submission, her transformation will be halted. If not, within about ten days or so, ‘she’ will be irrevocably changed to a fully functioning ‘he’ and produce active sperm.

chapter-a Big and brawny, that’s the female anemone fish (left). The wimpish male just supplies sperm. When she dies, he grows, jumps gender, and lays eggs.


Small is sexy

In the vertebrates and the insects, extreme sexual dimorphism—huge differences between the two sexes—has come about because the males have evolved into weapon-bearing warriors designed for acquiring harems. However, in species in which males have opted for dedicated monogamy, the females are usually the larger sex; in some cases, the males are miniaturized. ‘Dwarf’ males are found in a variety of flatworms, nematodes, crustaceans and molluscs. In the oyster Ostrea pulchrana, the large females host the small males on their shells and may even retard their growth through some chemical influence.

Charles Darwin was aware of degenerate males when he studied barnacles… Some barnacles are parasites, bearing little resemblance to crustaceans, and with separate sexes. The vanishingly small males enter their mates as free-swimming larvae and settle inside their partners’ tissues, resembling alien parasites themselves!

In some species, once the tiny male has made contact with his mate, he bonds with her for life. His body merges with hers, even sharing her blood supply, because once the male is in situ he depends utterly upon his ogreish mate for nourishment. In the end, the male is reduced to a fleshy appendage, a blob of testis under the complete control of the gravid female.

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