Sexual Selection in Animals

Learning Objectives

  1. Differentiate between internal and external fertilization
  2. Define biological fitness and parental investment, and explain why females are more likely than males to be “choosy” about mates
  3. Define sexual selection, and give examples of secondary sexual characteristics
  4. Describe different ways that males compete for mates

Sexual reproduction starts with the combination of a sperm and an egg in a process called fertilization. Fertilization can occur either inside (internal fertilization) or outside (external fertilization) the body of the female.

External Fertilization

The information below is adapted from OpenStax Biology 43.2

External fertilization occurs in aquatic environments where both eggs and sperm are released into the water, a process called spawning. Water protects the eggs from drying out during development. Getting the sperm and egg together requires that the gametes be released at the same time and in the same location to increase the likelihood of fertilization (otherwise all those gametes are wasted!)  How does that happen?

  • In some species, including some fish, crustaceans, mollusks, and other invertebrates, there are environmental (water temperature, length of daylight) or biological (pheromones) cues that cause males and females to release gametes at the same time.  In this situation, males and females are often not interacting with each other as individuals, but massed together so that all sperm and all eggs are in the same location.
  • In other species, including many amphibians, individual males court individual females to induce the female to release the eggs, at which point the male releases the sperm to fertilize that individual female’s eggs.

Cauliflower coral broadcast spawning. Image credit: Lindsey Kramer/U.S. Fish and Wildlife Service,

During sexual reproduction in toads, the male grasps the female from behind and externally fertilizes the eggs as they are deposited. (credit: “OakleyOriginals”/Flickr)

Internal Fertilization

The information below is adapted from OpenStax Biology 43.2

Internal fertilization occurs most often in land-based animals, although some aquatic animals also use this method. There are three ways that offspring are produced following internal fertilization:

  • Fertilized eggs are laid outside the female’s body and develop there, and the embryo receives nourishment from the yolk that is a part of the egg. This occurs in most bony fish, many reptiles, some cartilaginous fish, most amphibians, two mammals, and all birds.
  • Fertilized eggs are retained inside the female’s body, but the embryo receives nourishment from the egg’s yolk and the young are fully developed when they are hatched. This occurs in some bony fish, some sharks, some lizards, some snakes, some vipers, and some invertebrate animals.
  • Fertilized eggs are retained inside the female, and the embryo receives nourishment from the mother’s blood through a placenta. The offspring develops in the female and is born alive. This occurs in most mammals, some cartilaginous fish, and a few reptiles.

Internal fertilization has the advantage of protecting the fertilized egg from dehydration on land. In many instances, the embryo is isolated within the female, which limits predation on the young. Internal fertilization also increases the likelihood of fertilization by a specific male. Fewer offspring are produced through this method, but their survival rate is higher than that for external fertilization.

Females are “choosy;” males are not

“Females are choosy, while males are not” is a gross generalization, but biologically it tends to be true across most species that reproduce sexually, whether they reproduce with internal or external fertilization. Why do you think this is? Generally females invest more energy, care, and time in the offspring than a male does, and she has a limited number of eggs compared to practically limitless sperm produced by males of most species. Because of her higher investment, if a female mates with a male of poor genetic quality, and her offspring don’t survive as a result, she has wasted a lot of energy and resources and ended up with nothing. But a male can mate with practically unlimited numbers of females with little loss of energy or resources, regardless of how successful those offspring may be. In other words, eggs are “expensive” and sperm are “cheap.” Thus, generally a female maximizes her reproductive success by mating with the “best” male she can, while generally a male maximizes his reproductive success by mating with as many females as possible.

Any situation where one gender (or both genders) selects specific individuals to mate with will result in a phenomenon called sexual selection. Sexual selection is a type of natural selection where one sex has a preference for certain characteristics in individuals of the other sex. As with any type of selection, this preference increases the reproductive success of individuals who have the preferred characteristic.

Because females of most sexually reproducing species are “choosy,” females are often the gender that sexually selects traits in males.  As a result, males compete with each other for access to females or induce a specific female to mate with them. As a result of this competition, sexual selection often leads to sexual dimorphism, distinct differences in size or appearance between males and females. These differences in size or appearance,  called secondary sexual characteristics, are exaggerated or showy traits that are associated with mating behaviors and reproductive success. Examples include breasts, showy tails and headpieces, and crazier traits like the length of the eye-stalks in stalk-eyed flies (see image below).

Stalk-eyed flies have eyes at the end of long stalks, and they compete for mates by measuring the distance between their eyes. Evidence shows that wider eye placement wins in these bouts of male competition. (Image credit: Jojo Cruzado – stalk eyed fly, CC BY 2.0,

Sexual dimorphism can lead to specific behaviors in males that increase their reproductive success. Significant energy is spent in the process of locating, attracting, and mating with the sex partner.

Competition among males occurs whether species mate via internal or external fertilization. In species with internal fertilization, only one male can mate with a female at a time, so males compete with each other for mating with a particular female. In species with external fertilization, the female controls how and when the eggs are released, and males compete for access to her eggs outside of her body.

Adaptations increase reproductive success

Recall from learning about evolution by natural selection that adaptations, the traits that increase an individual’s reproductive success, occur without conscious thought or intention on the part of the individual. Selection results in the evolution of different strategies for maximizing biological fitness, or reproductive success relative to others in the population. An individual who has, for example, 10 surviving offspring (who then go on to reproduce as well) has higher fitness than an individual who has 7 surviving offspring.

Some of the adaptations around sexual reproduction include parental investmentdirect male competition, and indirect male competition:

Parental investment is any energy, effort, or resource that a parent provides to increase the offspring’s chances of survival, but at the cost of the parent’s ability to invest in other offspring. The benefit paired with cost is an example of an evolutionary trade-off. Parental investment can include parental care, energy resources deposited in the egg, and nutrition provided to the developing embryo. Parental investment can occurs both in species with internal or external fertilization. Some examples are shown in the images below:

The male of the common midwife toad, Alytes obstetricans, carries the fertilized eggs on his hind legs until they are ready to hatch.

Image credit: Christian Fischer, CC BY-SA 3.0,

Like many bird species, hummingbirds provide food to their hatchling until the young birds are ready to leave the nest.

Image credit: Wolfgang Wander, Papa Lima Whiskey (edit) – self-made /, CC BY-SA 3.0,

Males often engage in direct male competition over potential mating partners. This type of competition occurs when females mate only with a single male, typically the “winner” of the competition. Direct male competition often includes aggression (fighting) between males, but there are other forms as well:

  • Male-male aggression: males fight with each other for access to females
  • Courtship rituals: males engage in “dances” or other displays to attract females
  • Lekking: a specialized form of courtship ritual where many males gather together in one place and “display” at the same time, allowing females to easily choose among them

Male-male aggression in Mallard ducks.

Image credit: Ken Clifton/Flickr

Competition between males is half of the story: females choose which males to mate with based on observing the male competition. Selection of the “best” male by females is called female choice or intersexual selection. Female choice and direct male competition  usually lead to selection for extremely “showy” traits that do not appear to provide any benefit to the individual’s survival, and might even make it more likely for the animal to be eaten by a predator (think of the peacock’s tail – see image below). But if the trait improves the male’s ability to produce successful offspring because more females choose to mate with him, then these traits do in fact improve an individual’s biological fitness, even at the cost of decreasing its survival! One question is why females should “care” about these showy male traits. A leading hypothesis to answer this question is the good genes hypothesis, which is the idea that these sexually-selected, showy male traits are “honest indicators” of good genetic quality. In other words, it takes good genes to make a big flashy tail (and to avoid being eaten by a predator, since that big tail slows him down), so the bigger and showier the tail, the “better” the male.

The peacock’s tail is used on courtship displays to attract females. Females prefer males with larger, more colorful tails.

Image credit: Özgür Mülazımoğlu/Flickr


This video gives a brief overview of the implications of the good genes hypothesis and sexual selection in humans:

Instead of (or in addition to) competing directly with each other to have the opportunity to mate with a female, males can also compete for fertilization of a female’s eggs after mating has already occurred! Competing after mating is also called indirect male competition, or sperm competition, and it results in one male being more successful than another at fertilizing a female’s eggs. This type of competition occurs in species where the female is likely to mate with multiple males, so instead of males directly competing with each other, they are competing via their sperm. In other words, if a female mates with more than one male, then any male whose sperm end up fertilizing more eggs is going to have more offspring, on average, than other males. So if there is a trait that makes this male’s sperm more successful than other male’s sperm, then that trait is going to end up increasing in the population over generations.

Traits that increase success during sperm competition include:

  • Mate guarding: a male remains close by the female after mating, preventing other males from mating with her until there has been time for his sperm to fertilize the eggs
  • Copulatory plugs: the male’s ejaculate includes a sticky residue which temporarily blocks entry to the female’s reproductive tract, making it difficult for other males to mate with her until after there has been time for the first male’s sperm to fertilize the eggs
  • Elaborate penis morphology: elaborate structures on the penis help remove the sperm of previous males from the female’s reproductive tract by essentially scraping out the previous ejaculate
  • Large ejaculate volume and large testes: a large volume of ejaculate helps to flush out the sperm deposited in the female’s reproductive tract by the previous male; a large ejaculate volume means the testes must also be large, in order to have enough space to hold all the sperm

The genitalia of the male Callosobruchus analis beetle is covered in spines from base to tip; the spines facilitate removal of sperm deposited in the female’s reproductive tract by previous males.

Referenced in Rönn, J., Katvala, M. & Arnqvist, G. 2007. Coevolution between harmful male genitalia and female resistance in seed beetles. Proceedings of the National Academy of Sciences 104, 10921-1092 and Hotzy, C. & Arnqvist, G. 2009. Sperm competition favors harmful males in seed beetles. Current Biology 19, 404-407.

Female anatomy can also influence the success of sperm from specific males in a process called cryptic female choice, where a female is capable of preferentially using sperm from a specific male even if she has mated with multiple males. This process is poorly understood but suggests that males competition alone does not dictate success of that male sperm in fertilizing an egg.

This video provides a great overview of sperm competition, but be aware that it erroneously refers to bonobos as having a polygynous mating system (they are promiscuous) and gorillas as being monogamous (they are polygynous). More on those terms in our next reading.