(2026 Exams) Intersexual & Intrasexual

Test yourself on (2026 Exams) Intersexual & Intrasexual

Test your knowledge with free interactive questions on Seneca — used by over 10 million students.

Intersexual Selection

Intersexual selection is the selection between the sexes (i.e. the strategies that males use to select females and vice versa). Strategies of mate selection are different between the sexes.

Female mate selection

Intersexual selection is the preferred strategy of females and represents female preference for quality over quantity.
This is because a woman produces far fewer ova in her lifetime than a man produces sperm.

Female mate selection cont.

Women also use far more energy producing ova than men use producing sperm.
This explains why females need to make sure that they mate with the best males, while males are concerned with producing more (rather than higher quality) offspring.

Trivers (1972)

Trivers (1972) emphasises that females need to invest more time and resources before, during and after the birth of offspring.
So females try and choose a genetically fit partner who is able and willing to provide resources.
This leaves males to compete for the opportunity to mate with fertile females.

Trivers (1972) cont.

Both sexes are highly selective in their approach because they are both disadvantaged if they invest in substandard partners.
But the consequences of making a bad choice are more serious for females.

Inherited characteristics

The preference for a fit male determines the features that are passed onto the offspring.
- For example, if height is considered an attractive attribute, then over successive generations of females it will increase in the male population.
- This is because females would mate with taller males, producing a greater preference for taller partners.

Fisher (1930)

Fisher (1930) proposed his 'sexy sons' hypothesis to explain this phenomenon:
- Females mate with males who have desirable characteristics.
- Their sons then inherit this ‘sexy trait’. This increases the likelihood that the successive generation will mate with the females' offspring.
- This is known as the 'runaway' process.

Intrasexual Selection

Intrasexual selection refers to competition between (intra) males to mate with females.

Male mate selection

Intrasexual selection is the most common strategy of males and reflects male preference for quantity over quality.
The winner of the competition gets to pass the characteristics that contributed to his victory onto his offspring.

Consequences of intrasexual selection

Intrasexual selection has caused dimorphism.
- Dimorphism refers to the obvious differences between males and females.
  - For example, because larger males are more likely to win physical competitions with other males, being a larger male is an advantage.
But because females do not compete for reproductive rights, there is no evolutionary drive towards favouring larger females.

Further consequences of intrasexual selection

Intrasexual selection has also had behavioural and psychological consequences, although these are more controversial.
- For example, when males get fertile females, they protect them from competing males.
- This means they can benefit from behaving aggressively.

Anisogamy

Anisogamy suggests that the best male reproductive strategy is to mate with as many fertile females as possible.
This is because they need relatively low levels of energy to produce sperm and they have less responsibility in caring for any offspring.