9.3 How does sexual selection work?
Sexual selection, the process through which individuals compete for mates, primarily takes two forms: intersexual selection and intrasexual selection. Intersexual selection, often referred to as mate choice, involves individuals of one sex choosing among members of the opposite sex based on the attractiveness of certain traits that those individuals possess. Intrasexual selection, also called mate competition, involves one sex competing with members of the same sex for access to mates.
Typically, the sex that is selective and “choosy” about deciding who to mate with is the sex that invests more in gamete production prior to mating. Conversely, the sex that is chosen is also the sex that fights with members of the same sex for access to mates, and is traditionally the one that invests less in gamete production. As you’ll remember from Chapter 4 Section 8.4, the definition of “female” is the sex that produces larger gametes, so by definition females invest more in gametes and males invest less (see the size differences between human eggs and sperm in Figure 9.3). In many species, females produce just a few large and costly eggs, while males produce many small and less expensive sperm. Because of this difference in gamete production and investment, known as anisogamy, females are typically the choosy sex and males typically compete with other males for access to females.
Generally, it is unusual for males to be choosy about their mates. There are many reasons for this. Gamete production and investment is one reason. Another reason why females are typically the choosy sex has to do with the level of investment in offspring care, known as parental care. For example, following sexual reproduction and fertilization, most mammals develop within the body of their mothers. The developing offspring of most mammals then get their food and oxygen from the blood of their mothers through a spongy organ called the placenta. Even marsupial offspring, who are not fully developed when born, are usually carried by their mothers in a pouch until they are able to walk on their own. We suspect that you have noticed a pattern here—females typically invest more than males when it comes to caring for developing offspring. This is another major reason why females are choosy about their mates.
However, there are interesting exceptions. In some animals, males provide a great deal of parental care to their offspring. For example, in emperor penguins each female produces a single egg. She then transfers the egg to her male mate and leaves to spend the winter in the open ocean in search of food and other resources. During the Antarctic winter, which lasts about four months, male emperor penguins huddle in groups, guarding their eggs and keeping them warm. An extreme example lies in seahorses, among whom males get pregnant and carry their offspring during development, after which they give birth to baby seahorses.
As a result of males investing a great amount of time and energy into caring for their offspring during development, some species of animals show a reversal in which sex is the choosy sex. For example, in many poison-dart frogs, males are the sole providers of parental care to developing offspring. As such, female poison-dart frogs will fight amongst each other in the presence of calling males, and some have been observed to court a single singing male in the field.
Historically, much of the research on sexual selection has focused on what happens between males and females prior to mating. For example, a lot of work has been dedicated to understanding how males signal to attract females, and what females look for in potential mates. However, it is important to note that sexual selection can occur before and after a female and male mate. Before mating, individuals will signal their quality to potential mates. After mating, individuals can bias paternity in their favor through various processes including cryptic female choice and sperm competition, which we will discuss later in this chapter.
Check Yourself
