Feature

Why are Males so Bizarre?

There is a logical reason for the oddness of males...

Dr Simon D. Pollard

The "Victorian Room" in the Canterbury Museum is a space filled with cultural and natural history artefacts from last century. Towering over a mummy, and overshadowering a timid looking rhinoceros, is the skeleton of a male Irish elk, whose empty eye sockets and sardonic grin, greet visitors as they come in. Erupting from the elk's skull are antlers so large that it is hard to believe they could be supported by the head beneath them.

In a glass cabinet, beside the elk, is the skeleton of a female elephant seal with the huge skull of a male beside it. If the rest of the male's skeleton was attached to his skull, the cabinet would have to be four to five times its present size.

Perched above the bones is a stuffed male peacock, its head cocked to the side, as if it is staring sadly out the window into the Botanic Gardens. A huge plumage of tail feathers flares out behind its body, like an awkward fan, give the impression that the bird is standing in front of a feather curtain.

How on earth have the male Irish elk and peacock evolved such strange and seemingly maladaptive structures? Wouldn't supporting these aberrant appendages conflict with adaptations that have evolved through natural selection to help the animals survive? And why are the male elephant seal and male Irish elk so much larger than the females of these species?

A good place to start unravelling the evolutionary enigma of bizarre male traits are sperm and eggs. Because most animals, including ourselves, reproduce sexually, half the genetic makeup of offspring come from a female's egg and half from a male's sperm. This is how our selfish genes jump ship to the next generation. However, females usually invest a lot more time and energy in the care and development of offspring. Consequently, females are a valuable resource to males. Males compete with one another to have their genes inside a female egg, because the female will do most of the work in helping those genes survive. After all, mammal offspring are dependent on the female to act as an incubator until they are born, and then to breastfeed them once they are outside her body.

But the female's genes may have their chances of survival in future generations compromised, if they are combined with those of an inferior male. After all, the condition of a male will tell the female something about the characteristics her offspring are likely to inherit from him. And, once the egg is fertilised there is little a female can do to reverse a bad mate choice.

Imagine, a female blue whale that mates with a genetically inferior male. While he may spend a short time mating, she will be pregnant for 21 months with a baby carrying his weedy whale genes. So, females should evolve characteristics that make them choosey about who they mate with, and males should evolve characteristics to convince females they are worthy suitors.

Strange Characteristics

Charles Darwin was troubled by strange male characteristics when he was explaining natural selection as an important mechanism of evolution. He suggested that sex can create selection pressures that seem in opposition to adaptations that have evolved through natural selection. He called this process sexual selection and described two types of sexual characteristics; primary and secondary. Primary sexual characteristics were "directly connected with the act of reproduction"-- sperm and eggs and reproductive organs.

But, as the exhibits in the Victorian Room show, males and females can also differ by having characteristics with no mechanical role in insemnation. Through sexual selection, males have evolved secondary sexual characteristics for competing with rival males and impressing females. Darwin considered "the greater size, strength, and pugnacity of the male, his weapons of offense or means of defence against rivals, his gaudy colouring and various ornaments, his power of song and other such characters" as examples of male secondary sexual characteristics.

But, there is no divine watchdog keeping an eye on sexual selection. In the case of the Irish elk, climatic changes led to forests growing in elk habitats. Males could not adapt rapidly enough to these changes, and their previously successful antlers led to their extinction about 11,000 years ago.

That male elephant seals can be five times heavier than females is testimony to how competition between males drives the evolution of characters for winning fights and the attention of females. Bigger males are more likely to be able to defend a territory and harem of females against rival males. The costs of being bigger pay off for the male in terms of reproductive success; he has more offspring than a smaller male.

When males fight over territories or females, the fights are often contests of strength, rather than fights to inflict injury or death. Male Irish elk probably banged antlers together to sort out disputes, rather than escalating the fight to a level where injury was likely. Similarly, I could usually assess the strength of a rival by arm wrestling.

However, a big male with huge antlers may just have to show a rival the size of his antlers to sort out a dispute. If my rival had arms like Popeye, I would be foolish to even attempt arm wrestling. I may break a nail or strain a muscle and this could disadvantage me in another dispute or in courtship. If you look at the Irish elk's antlers face on, they do look more impressive for their size, rather than as weapons, and I'm sure a lot of disputes were settled by opponents sizing each other up.

Making an Impression

But the male peacock's huge fan of feathers are not potential weapons to beat up rivals. They are to impress peahens with a display as shiny as silk and sincere as polyester. Like a neon suit with flashing lights, the male bombards the female with stimuli to rival the gaudiest and "loudest" advertisements seen on television. And peahens, like people, seem to live by the rule, that "brighter is better" and "bigger is better".

The male shimmers his long tail feathers, covered in little peacock icons, at the female hoping all this visual stimulation will convince her to mate with him. While the female may respond to his salesman-like tactics, she is also looking beyond the gloss to the condition of the salesman. If his "suit" looks badly pressed, torn or is missing a few details, he is unlikely to sell his genes to the female.

While few species can rival the peacock's loud attire, males are often more colourful than females. In many species of Old World monkeys, males have a blue scrotum and red penis which they use in threat displays to other males. Possibly a red penis against a blue background stands out more. The Mandrill not only threatens rivals with its colourful genitalia, but with a blue face and red nose as well. While many male humans have red noses, like their genitalia they are rarely used in threat displays.

Red is also a popular colour with some male lizards, who develop a red patch on their throats to attract females onto their piece of turf during the mating season. But, they "see red" when another male comes into their territory, and attack the intruder. Apparently, one male lizard was so annoyed at being put next to a bowl of tomato soup, that he attacked it by leaping into it! It's not that the lizard was so stupid that he thought the bowl of tomato soup was another lizard, it's just all that red made him see red.

After all, if lizards studied our behaviour they would wonder why some males become sexually aroused by pictures of naked females. Surely, it's not because males cannot tell the difference between a picture of a naked female and a real naked female. Although visual signals in people and lizards evolved before magazines and tomato soup, the products of cultural evolution can often have enough in common with the real thing to elicit a response.

Size is Important

The huge size of some secondary sexual characteristics, relative to the male supporting them, may not only be because of the advantage they give the male in competing with rival males, and/or displaying genetic fitness to females. What can drive their continued exaggeration to seemingly absurd levels is females' preference for the characteristic, rather than the characteristic itself.

Imagine a scenario where females that mated with males with slightly longer noses had male offspring more likely to survive than those that mated with small-nosed males. Possibly bigger noses were better at sniffing predators and prey. Consequently, the relative size of male noses will increase as big nose genes become more common. The daughters of females mating with these males inherit a preference from their mothers for mating with males with long noses. The male's nose can assume "Pinocchio proportions" way beyond its original function as a sniffing organ because of female preference for the character.

Since it is the relative size of male noses in the population that is important, what was considered a big nose a thousand generations ago may now be a dwarf, with the new "super nose" being preferred by females. Noses will continue to grow until an evolutionary equilibrium is reached where the costs to the male of having a huge honker balance the reproductive benefits.

Female preference for certain male characteristics can also originate because males exploit pre-existing biases in the female's brain to respond to particular stimuli. For example, there is a tiny predatory water mite that is sensitive to the vibrations of its prey as they swim by. When prey swim close enough, the mite reaches out with its front legs and grabs them. Male water mites expoit the female's bias to respond to particular vibrations by trembling like prey in front of the female. When she reaches out to grab the male, he gives her a packet of sperm instead of a meal.

Female bias does not guarantee that males will evolve structures or behaviours to exploit it. For example, swordtails are small fish that get their name from the many species that have males with long orange swordtail-like fins that extend behind their bodies. In a closely related species, males do not have swordtails. Remarkably, females of this species prefer males with little plastic swords attached to their fins over their regular swordless males. Possibly orange swordtails remind females of something they like to eat.

From mites to mandrills, sexual selection has produced a huge diversity of flamboyant structures and behaviours that allow males to compete with rivals and impress females. While these often bizarre structures appear incongrous from a survival perspective, the reproductive benefits of having them outweigh their costs. And, presumably, that is why so many males continue to wrestle and advertise with ungainly appendages.

While some people may consider human male primary sexual characters as ungainly appendages, our main secondary sexual characters -- being bigger and hairier than most females -- are fairly mild in comparison to the noted residents of the Victorian Room. When I look at the antlers of the Irish elk, and imagine supporting a similar structure on my own head, I am very pleased our primate ancestors' aboreal lifestyle stifled the evolution of anything sprouting from the skull. Imagine the sarcophagus in the Victorian Room having to accomodate a mummy with antlers.

Dr Simon Pollard works in the Department of Zoology at the University of Canterbury.