Why are males (usually) showy and females (usually) choosy?

The answer to the question above is sexual selection.

Sexual selection is a type of natural selection in which individuals with certain inherited characteristics are more likely than others to find mates and pass on their genes to the next generation. Thus, these beneficial characteristics become more common in the population over time.

There are two major forms of sexual selection. One is intrasexual selection where individuals of one sex compete directly for mates of the opposite sex. In many species, this usually occurs among males. For example, the male deer with the largest antlers may discourage his competitors or the male bird with the most dramatic plumage may have a psychological advantage over his rivals.

Peacock (bent)
In the case of the peacocks, while their long and beautiful tails may make them more visible to potential predators, such tails also help them to attract mates. If this benefit outweighs the risk of predation, then the bright plumage may be reinforced as it increases the chances of overall reproductive success. However, this may also explain why the tails of peacocks do not grow past a certain length, as doing so may lower their chances of escaping from their predators, and thus surviving to reproductive age.

The other form of sexual selection is intersexual selection (or mate choice), where individuals of one sex (usually the females) are choosy in selecting mates from the opposite sex. Often, the female’s choice depends on the showiness of the male’s appearance or behaviour. For instance, female deer tend to prefer males that display elaborate antlers. Size of these antlers may indicate combat effectiveness, proper nutrition and good health, all of which are favourable traits in a potential mate. Alternatively, in crickets and many other insect species, a courting males offers a gift of food to a prospective mate. The larger or the higher quality the food offering, the better the chances the male will be accepted. In contrast, male songbirds use their singing prowess to convince females of their fitness. Here, the ornaments and the behaviours displayed by the males serve as an indication of the male’s health and suitability as a mate.  This may also explain why so many of nature’s ornaments such as brighter colours, larger sizes, and antlers are only seen on males.

So what gave rise to female preferences for certain characteristics in males in the first place? One theory is that the traits females tend to prefer in males are often ones correlated with “good genes”. For instance, if the trait preferred by females is a good indicator of the male’s overall genetic quality or health, both the male trait and the female preference for it should increase.

One fascinating example that ties both together is that of the enlarged claw of the male fiddler crab. The male fiddler crab uses his enlarged claw to intimidate rival males and to attract females. An instructor of mine likened this approach to human males having flashy sports cars. A male with such a car may intimidate rival males from pursuing the target of his choice, whereas for the female, that the male can afford to have such a car in this day and age (with the COE prices the way they are!) may serve as an indication of the male’s ability to support himself and thus, may indicate “good genes”. Same same but different?

Fiddler crab
The male fiddler crab has one enlarged claw and one regular sized claw.

Now that we are aware of such behind-the-scene selection processes, perhaps we can think about how we can use these to our advantage (e.g., bring food along the next time you’re courting someone!)

Disclaimer: The author is not responsible for any failed attempts at courting.

P.S. All photos were taken by me in Singapore.

Sources

  1. Campbell, Neil A., and Jane B. Reece. Biology. Eighth ed. San Francisco, California: Pearson/Cummings, 2008, pp. 481-83.
  2. Solomon, Eldra Pearl, Charles Martin, Diana W. Martin, and Linda R. Berg. Biology. Australia: Cengage Learning, 2015, pp. 1137-1146.

 

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