6.4: Sexual Selection and Mate Choice

Sexual Dimorphism

Males and females of certain species are often quite different from one another in ways beyond the reproductive organs (Figure \(\PageIndex{1}\)). Males are often larger, for example, and display many elaborate colors and adornments, such as the peacock’s tail, while females tend to be smaller and duller in decoration. These differences are called sexual dimorphisms and arise from the variation in male reproductive success.

Definition: Sexual Dimorphism

Morphological and behavioral differences between males and females of the same species is known as sexual dimorphism.

Figure \(\PageIndex{1}\): Morphological differences between males and females observed in (a) peacocks and peahens, (b) Argiope appensa spiders (the female spider is the large one), and (c) wood ducks.. (Copyright; author via source)

Sexual dimorphism results from the different selective pressures on males and female to secure mates. At any given time some females are developing eggs, gestating, or carrying for young and are thus, not available to mate. This creates a bias in the sex-ratio with more males available to mate at any one time then females. As a result females almost always mate, while mating is not guaranteed for males. Furthermore, females often can maximize reproductive success with a single partner, while male fitness increases with the number of female partners he obtains (Figure \(\PageIndex{2}\)).To maximize fitness males should invest more effort in mating effort (securing a mate) while females should be more choosy about which mate they select.

Figure \(\PageIndex{2}\): Males maximize their fitness by having as many mates as possible. In contrast, females maximize their fitness by having fewer, high quality mates. This difference in how males and females maximize their reproductive success can lead to conflict between the sexes.. (CC BY 4.0; Sehoya Cotner and Deena Wassenberg via The Evolution and Biology of Sex)

For most species, the bigger, stronger, or more decorated males usually obtain the vast majority of the total matings, while other males receive none. This can occur because the males are better at fighting off other males, or because females will choose to mate with the bigger or more decorated males. In either case, this variation in reproductive success generates a strong selection pressure among males to obtain those matings, resulting in the evolution of bigger body size and elaborate ornaments in order to increase their chances of mating. Females, on the other hand, tend to get a handful of selected matings; therefore, they are more likely to select more desirable males.

Sexual Selection

The selection pressures on males and females to obtain matings is known as sexual selection. Sexual selection takes two major forms: intersexual selection (also known as ‘mate choice’ or ‘female choice’) in which males compete with each other to be chosen by females; and intrasexual selection (also known as ‘male–male competition’) in which members of the less limited sex (typically males) compete aggressively among themselves for access to the limiting sex. The limiting sex is the sex which has the higher parental investment (typically females), which therefore faces the most pressure to make a good mate decision.

Figure \(\PageIndex{3}\): This male elk has large antlers to compete with rival males for available females (intrasexual competition). In addition, the many points on his antlers represent health and longevity, and therefore he may be more desirable to females (intersexual selection). (Copyright; author via source)

It is important to note that both types result in selection of male traits. Female mate choice results in ornaments. Traits such as elaborate displays, fancy feather, bright colors, etc. Male-male competition results in traits that make males better at fighting for or defending females. Horns and antlers are an excellent example (Figure \(\PageIndex{3}\)). Large body size is another common result of male-male competition. It is also important to note that sexual selection selects for traits that increase reproductive success but often at a adaptive cost. These traits also make it more difficult to survive.

Video: Sexual Selection Explained

Watch the video below for an overview of sexual selection including:

• how Sexual Selection differs from Natural Selection. Then, we'll cover the 2 most important aspects of Sexual Selection:
• An overview of male competition and female choice.
• Some hypotheses regarding the benefits females recieve by being choosy.

What Benefits to Females Receive for being Choosy?

Direct Benefits

Direct benefits increase the fitness of the choosy sex through direct material advantages or resources. These benefits include but are not limited to increased territory quality, increased parental care, and protection from predators. There is much support for maintenance of mate choice by direct benefits and this approach offers the least controversial model to explain discriminate mating.

One example of a sexually selected trait with direct benefits is the bright plumage of the northern cardinal, a common backyard bird in the eastern United States. Male northern cardinals have conspicuous red feathers, while the females have a more cryptic coloration (Figure \(\PageIndex{4}\)). In this example, the females are the choosy sex and will use male plumage brightness as a signal when picking a mate — research suggests that males with brighter plumage feed their young more frequently than males with duller plumage. This increased help in caring for the young lifts some of the burden from the mother so that she can raise more offspring than she could without help.

Figure \(\PageIndex{4}\): Northern cardinals have a short, stout bill, adapted for cracking seeds. This species is the only bright red bird with a crest. Both males and females have rosy red to orangish red beaks. Females are also crested, but their plumage is dominated by yellowish gray coloration. (CC BY 2.0; James St. John via flickr)

Indirect Benefits

Indirect benefits increase genetic fitness for the offspring, and thereby increase the parents' fitness indirectly. When it appears that the choosy sex does not receive direct benefits from his or her mate, indirect benefits may be the payoff for being selective. These indirect benefits may include high-quality genes for their offspring (known as the good genes hypothesis) or genes that make their offspring more attractive (known as the sexy sons hypothesis).

The Handicap Principle

Sexual selection can be so strong that it selects for traits that are actually detrimental to the individual’s survival, even though they maximize its reproductive success. For example, while the male peacock’s tail is beautiful and the male with the largest, most colorful tail will more probably win the female, it is not a practical appendage. In addition to being more visible to predators, it makes the males slower in their attempted escapes. There is some evidence that this risk, in fact, is why females like the big tails in the first place. Because large tails carry risk, only the best males survive that risk and therefore the bigger the tail, the more fit the male. This idea is known as the handicap principle.

Figure \(\PageIndex{1}\): A male bird of paradise: This male bird of paradise carries an extremely long tail as the result of sexual selection.The tail is flamboyant and detrimental to the bird’s own survival, but it increases his reproductive success.This may be an example of the handicap principle. (CC BY-SA 2.0; Markaharper1 via Wikipedia)

The Good Genes Hypothesis

The good genes hypothesis states that males develop these impressive ornaments to show off their efficient metabolism or their ability to fight disease. Females then choose males with the most impressive traits because it signals their genetic superiority, which they will then pass on to their offspring. Though it might be argued that females should not be so selective because it will likely reduce their number of offspring, if better males father more fit offspring, it may be beneficial. Fewer, healthier offspring may increase the chances of survival more than many, weaker offspring.

Video: Bird of Paradise Mating Dance

Watch the video below for an extradorinary courtship display from these weird and wonderful creatures. From episode 1 “Pole to Pole”. This is an example of the extreme behaviors that arise from intense sexual selection pressure.

Attribution:

• 4.4.2: Sexual Selection by boundless license CC BY-SA 4.0
• Mate Choice by Wikipedia. License CC BY-SA 3.0

References

• Moller, A.; Jennions, M. (2001). "How important are direct benefits of sexual selection?". Naturwissenschaften. 88 (10): 401–415.
• Kokko, H.; Brooks, R.; Jennions, M.; Morley, J. (2003). "The evolution of mate choice and mating biases". Proceedings of the Royal Society B: Biological Sciences. 270 (1515): 653–664.
• Linville, Susan U.; Breitwisch, Randall; Schilling, Amy J. (1998). "Plumage brightness as an indicator of parental care in northern cardinals". Animal Behaviour. 55 (1): 119–127.