Source: BBC

By David Robson

Once upon a time, animal courtship was thought to run something like a Barbara Cartland novel. The rakish males battle it out for a chaste female, who sits around choosing the prince charming to father her young. While her mate may sow his wild oats far and wide, she patiently tends her brood.

Notwithstanding a few counterexamples, these roles were thought to be largely the same across the animal kingdom: males were thought to be promiscuous, dominant and aggressive and the females chaste and passive. For many people, it was just the natural order of the world.

But have we been blinkered by our own cultural prejudices, casting animals in the kinds of roles we saw in the society around us? That is the view of a small but growing number of biologists. “It’s almost like they are using this locker-room logic – counting which males ‘score’ the most,” says Joan Roughgarden at the Hawai’i Institute of Marine Biology.

The dividing line between male and female is frequently blurred or easily crossed

Researchers such as Roughgarden argue that it was a classic case of “confirmation bias”. Many biologists were seeing what they wanted to believe, and then using the results to justify prevailing cultural norms. “You get this back-and-forth: science is reinforcing societal mores, and the mores are reinforcing what the science is saying,” says Zuleyma Tang-Martinez at the University of Missouri – St Louis.

The result, Tang-Martinez and Roughgarden believe, is that scientists have often failed to recognise astonishingly diverse sexual behaviours across the animal kingdom. There are now myriad examples of animals that break the rules entirely – from intersex kangaroo to a fish with four separate “genders”.

If they are right, we should rethink many of our assumptions about sex differences. As with humans, the dividing line between male and female is frequently blurred or easily crossed.

A peacock (Pavo cristatus) with his tail (Credit: Axel Gornille/naturepl.com)

Much of our modern understanding of sex differences came from Charles Darwin’s struggles to explain the peacock’s tale. How could such a cumbersome and extravagant display ever contribute to the animal’s survival?  “The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!” he wrote in an 1860 letter to his colleague Asa Gray.

Darwin saw the same patterns – males being “passionate”, females “coy” – across the animal kingdom

Darwin’s solution was “sexual selection”: a form of evolution that comes directly from the challenges of reproduction.

When many males compete for a single female, each male has to show off his worth in some way; either through direct combat, or in a showy display that proves he would be the healthiest father for her young. The resulting arms race led to the evolution of ever more excessive traits in the males of certain species: hence the peacock’s tale, which helps it to advertise its good health to the peahen.

Darwin saw the same patterns – males being “passionate”, females “coy” – across the animal kingdom. Later, the evolutionary biologist Angus John Bateman argued that this could be explained through basic economics.

A Somali ostrich (Struthio molybdophanes) hatching. Its egg represented a huge investment for its mother (Credit: Denis-Huot/naturepl.com)

Eggs, Bateman said, are huge and packed full of nutrients, making them costly to produce. By contrast, sperm are so small they can be produced in their millions.

The bottom line is that males have evolved to be promiscuous and females have evolved to be choosy

This means the stakes of the mating game are much higher for a female, and so she needs to choose her gamble carefully. Meanwhile, the male has sperm to spare, letting him take a gamble wherever he chooses.

The female’s investment is even greater if she has to spend time gestating and rearing the young, so she needs to make sure she chooses a mate who will give her young the best genes and the best chances of survival.

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