Wednesday, December 26, 2012

Sexual selection: are you man enough?

OK, guys, what would you do for love?
A peacock's impressive display
photo: 10mpx cg, Wikimedia Commons

Risk your life spreading out a 5-foot long tail and strutting around with it in the open?

Carry around a heavy rack of horns (antelope, beetles) or antlers (deer) for months or years at a time, in order to fight to the death against guys you were friends with when you were young?

Dance alone in front of your target lady, showing off your moves despite attracting potentially dangerous attention (competitors & predators for birds; thieves, assailants, or possibly the neighbors, for us)?

satin bowerbird at its gazebo-like bower,
decorated with every blue item it could find
Build a blue or natural-wood gazebo for your woman of choice to show her what a good provider you are (when she hasn’t shown any interest yet)

Serve your own head to your mate as dinner like certain spiders or the preying mantis (warning: excellent but somewhat macabre video)?

I didn’t think so.

But some animals do, despite the potential harm these traits or activities do to their survival.

What’s the deal – why evolve traits that might shorten their lives?

How would this help their fitness?

Remember that fitness is not just about survival: to pass its genes on to the next generation, an organism must not only survive but also reproduce. Not that any of you could forget this bit of the equation...  

As background, natural selection theory (“the gradual, non-random process by which biological traits become either more or less common in a population as a function of differential reproduction of their bearer”) holds that the traits (e.g. tail length, wing width, number of legs or eyes) that best adapt their owner to the surrounding conditions are most likely to survive and be passed on to the next generation ("survival of the fittest").

But natural selection doesn’t explain the many examples of elaborate, and seemingly non-adaptive, traits that clearly do not aid in the survival of their owners. For example:

greater kudu with long, curly horns
heavy ornate horns
(greater kudu)
plate from Mark Brazil's Birds of East Asia
crazy ornate tails
spotted unicorn fish
a snorkel? no, the horn of a spotted unicorn fish
photo: Stefan Koeder, Flickr

Leaving offspring, rather than survival per se, is key for biological fitness! These traits exist because they provide their owner with greater reproductive success. Why is this?

Charles Darwin wrote: “... when the males and females of any animal have the same general habits ... but differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection.”

Just as natural selection favors certain traits that aid survival, sexual selection favors traits that help individual animals reproduce, even if they decrease the animals’ chances of survival.

Why sexual selection?

In order to mate, males need to gain access to females and vice versa, but not all individuals will be equally successful at this task.

Under sexual selection, one gender favors certain traits to be present in their mate, and individuals with those preferred features will have better success finding a mate and be more likely to have more kids, spreading their genes (which include the attractive trait(s)) through the population.

male elephant seal with females & young
a calm moment for a male elephant seal (arched upside down!)
with his 5 females, 1 young, and 3 newborns

Preferred features might be weapons such as horns or large body size that can help drive away the competition (usually other males), or they might be dazzling displays of color and style that attract members of the opposite sex as mates.

Female choice

One sex — usually the female— chooses among the available males (I know ladies, it doesn’t seem that way in our species!). And they don’t choose any old male, they want the best!

OK, but what is the best?

And why do females want the best?

And how to males get to be the best?

And why do females choose?

And does this apply to humans?

Great questions! These are the subjects of a few future posts.
Huli wig-makers of Papua New Guinea create these
snazzy wigs from bird-of-paradise feathers and
their own hair (grown for years!) -
they are a sign of men's health and strength
(yes, I was feeling pretty plain among this group!)

There is a mountain of biological, social, and psychological research about sexual selection -- it’s a fascinating that tries to explain much of the living world around us – so I’ll just introduce this subject today and follow up with some impressive examples in subsequent posts.

  • What is the best?  The best males (and females) are those most able to survive, provide, and produce offspring that are healthy and adept at reproducing.
    unbeaten Frankel just retired,
    breeding available for US$200,000
    photo: REUTERS/Eddie Keogh
    If larger or faster or brighter-colored males are healthier or better protectors, then they are probably better.

    People understand this innately: think of race horses and the massive stud fees paid for the chance to have one’s own horse mate with the winners.

  • Why do they want the best?  Females want their own genes get passed to next generation, so they prefer males who will help them do that.

  • How do males get to be the best?  This of course depends on the species. Traits and behaviors that attract a mate for reproduction, by advertising the male’s ability to provide benefits, and those that deter rivals, which give the male access to one or more females while avoiding injury, are most successful.

  • why reproducing is key to biological fitness
    image: Understanding Evolution
  • Why do females choose?  This is longer one. Females invest more in each reproductive cell (eggs have loads of nutrients and their development is long and potentially risky) than males do (sperm are a dime a dozen, include only genetic information, and are easily produced). So which sex will want to be more careful and choosy in picking a mate?

We’ll get to some specifics and cool examples of these later!

For now, here are just a few of some of the excellent web sites explaining various components of sexual selection:


  1. Couples have for centuries sought to influence the gender of their offspring. More than seven centuries ago the ancient Chinese developed a birth calendar said to be able to predict gender on the basis of when conception occurred. But it is best to go for gender selection natural because it is a reliable one.

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