Sexual Selection & Mate Choice in Animals

Instructor: Angela Hartsock

Angela has taught college microbiology and anatomy & physiology, has a doctoral degree in microbiology, and has worked as a post-doctoral research scholar for Pittsburgh’s National Energy Technology Laboratory.

In this lesson we describe sexual selection and its role in evolution. Sexual selection, both within sexes (intrasexual selection) and between sexes (mate choice), is defined with examples from the animal kingdom.

Natural Selection

Have you ever heard anyone casually use the phrase 'survival of the fittest'? When most people say this they are talking about, or at least making a metaphor out of, the idea of one animal killing another animal. But rarely are they talking about sex, and reproduction is all that really matters when it comes to evolution.

Natural selection is one of the most basic ways that organisms evolve. It depends on differences (called variations) being present within a species that allow certain members to thrive and produce more offspring. The individuals that produce more offspring pass those variations on in the form of genes. Over time, those beneficial genes become more common within the population.

Charles Darwin, the father of evolution, developed the theory of natural selection but he also spent a lot of time thinking about sex. What I mean is, he was kind of obsessed with how important sex and choosing a mate are to reproduction. He came up with another term, sexual selection, as a subcategory of natural selection to specifically address the importance of competing for and choosing mates.

Sexual Selection

Sexual selection refers to an organism's ability to successfully compete for a mate or the ability to choose a mate. This selection allows certain members of the population to reproduce more than others, therefore passing on their traits more than others. There are two types of sexual selection, one of which occurs within a sex (intrasexual selection) and one which occurs between the sexes (mate choice).

The traits that evolve in response to sexual selection are commonly called secondary sexual characteristics. These are traits that may have nothing to do with the actual act of reproduction but contribute to opportunities for mating. Darwin first dubbed these secondary characteristics as being either weapons or ornaments. Weapons, which are things like antlers or horns, provide an advantage in combat. Ornaments, which are things like elaborate feathers or absurdly large genitalia, are put on display to attract mates.

A male moose displaying large antlers.
Image of moose antlers

Male peacock displaying his tail.
image of peacock tail

The display of both weapons and ornaments requires an extra investment of energy on the part of the organism. But these displays can also attract extra attention from predators (think elaborate, bright feathers). So, in the end an animal can really only invest in this extra cost if it ensures their ability to reproduce and pass that trait on to their offspring.

Intrasexual Selection

Intrasexual selection occurs when members of the same sex physically compete for access to a mate. Whoever wins (whoever has the best weapons) gains greater access to mates which means more opportunities to pass their genes on to offspring. Intrasexual selection can occur between males, which is what most of us are familiar with, but it can also occur between females.

  • Dominant Males: This is what most people picture when they think of intrasexual selection. Think of big powerful males engaging in fierce battles over their potential female mates. For example, male elephant seals, which can weigh up to 2,000 kilograms, collide violently and repeatedly in a battle over females. Meanwhile, the comparatively smaller female elephant seals bask on the sand, calmly waiting for the battle to end. The stronger or more vicious male eventually overpowers the weaker male, who then lumbers away to nurse his wounds, leaving the victor to mate copiously with numerous females. Winning this battle ensures his genes have a greater chance of being passed on to the next generation of elephant seals.

Male elephant seals battling over female mates.
image of elephant seal

  • Dominant Females: Female to female competition also exists in the animal world. A fascinating example is the spotted hyena. The female spotted hyenas are larger and more muscular than the males, and they compete fiercely to be the alpha female. As the alpha female they have the greatest access to mates and have more opportunities to produce offspring than the subordinate females. These alpha moms also seem to pass on their aggressiveness and physical stature to their pups through exposing their pups to hormones in the womb.

Spotted hyena
image of hyena

In both male to male and female to female combat, the process typically favors the larger, more aggressive, or stronger members of each sex. This frequently results in a dramatic difference in size between the sex that engages in combat and the sex that is being fought over for mating privilege.

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