Sexy Sons and Choosy Mums

By Sam Finnegan

Have a look around in nature and it doesn’t take long to find things that look, quite frankly, ridiculous. Jumping spiders have a strange, staccato dance that looks suspiciously good when put to the tune of YMCA; stalk-eyed flies, as the name suggests, carry their eyes on the ends of enormous stalks by blowing up their faces; and peacocks make it nice and easy for predators to spot them with those gigantic, gaudy tails.

Male stalk-eyed flies carry their eyes on the end of enormous stalks. Photography by Rob Knell, CC-image from

Male stalk-eyed flies carry their eyes on the end of enormous stalks.
Photography by Rob Knell, CC-image from Wikimedia commons

You’d be forgiven for asking what on Earth is the point of it all. And, looking through the traditional lens of natural selection, it certainly seems like a puzzle. Surely these animals could better spend their time and resources finding food? Or, at least not making themselves exaggerated and colourful food adverts for hungry predators? But life isn’t just about surviving; it’s also about having sex.

Suppose you’re a bird of paradise in Papua New Guinea. You might opt to look drab and boring and not bother with the elaborate dances. You could instead spend your time foraging. You could become the fittest, healthiest bird of paradise in the land, and you’d certainly be less likely to be killed by humans looking to make a hat out of your feathers. But the choosy ladies just wouldn’t give you the time of day. You might be great at surviving, but if being drab means you fail to pass on your genes, the next generation will be all pomp and pageantry.

This concept of evolution happening not by who lives and who dies, but instead by who does and doesn’t get laid, is referred to as sexual selection. Darwin himself recognised it as an important process in evolution, but it was developed further in the early part of the 20th century by the renowned biologist, statistician and eugenicist (but let’s just skip over that last bit), Ronald Fisher. Fisher considered the genetics behind it. He thought about the genes underlying the growth of male traits considered attractive to females. He then supposed that if there were also genes in females that controlled their attraction to those traits (which it seems like there is), the capacity was there for the male traits to evolve to a ridiculous, exaggerated degree. For example, if a female bird prefers to mate with males with long tails, then her female offspring are likely to inherit their mother’s tastes, and her male offspring will inherit the trait that made their dad so damn strapping in the first place – in this case a long tail.

In fact, one of the best things a female can do is to have sexy sons. To maximise the spread of her genes, which evolutionists might argue is the point of life, the female should mate with a really, really attractive male. This will ensure her sons, taking after their father, attract more than their fair share of the next generation’s females, and provide their mother with lots of grandchildren. This idea in evolutionary biology is often referred to as “the sexy son hypothesis.”

In many cases, it might not even particularly matter what the trait is that the females find so attractive. Just the mere fact that they want it, whatever it is, means that the guys either have it, or risk becoming a genetic dead-end.

Female zebra finches are inexplicably attracted to coloured leg bands.

Female zebra finches are inexplicably attracted to coloured leg bands.
Photography by Keith Gerstung, CC-image from Wikimedia commons

One example of this came in 1982, with a puzzling result from a lab studying sexual selection in Zebra finches1. Zebra finch females are attracted to the songs the males sing, and to the colour of their bills. But Nancy Burley and colleagues at the University of Illinois found that her study females had bucked the normal trend and were now incredibly interested in the coloured leg bands used by the scientists for identification. For some reason that is yet to be fully explained, the females were immensely attracted to males with red leg bands, while males with green bands were left out. Whether it’s because the red bands accentuate the males natural colouration or just plain whimsy, this strange result was found again and again in many repeated experiments. It even spawned a whole new area of research investigating how Zebra finches change their behaviour depending on the colour of their band. But that’s a conversation for another time.

Female choice of mate, then, is clearly a very important part of evolution, and what influences this choice remains a trendy topic in biology today. Whether a female’s choice for some strange trait directly benefits the spread of her genes, or is completely arbitrary, one thing seems apparent: sex makes things weird.


1. Burley, N., Krantzberg, G. & Radman, P. 1982. Influences of Colour-banding on the conspecific preferences of Zebra finches. Animal Behaviour 30, p. 444-455

Tagged with:

See more posts by

Stay informed

Click here to subscribe to our RSS newsletter by email.

Find Us

University College London is the administrative lead.

North-West Wing, UCL, Gower Street, London, WC1E 6BT

Follow us on Twitter