Sexual selection, mate choice and parental investment
Parental investment can affect how quickly each sex returns to mating and how available partners are, influencing competition and choice. These are conditional tendencies. A heritable trait that lowers survival can still be favoured if its mating benefits raise overall reproductive success.
What a learner can do afterwards
- Predicts tendencies in mate competition and choice from parental investment and the availability of ready partners.
- Explains how a costly ornament can be favoured despite lowering survival.
- Uses a role-reversed case to test a conditional investment prediction without treating it as proof of a universal rule.
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Sexual selection arises from differences in mating success. Parental investment is effort spent producing or caring for offspring at a cost to other reproductive opportunities. Greater investment can delay returning to mating, making ready partners scarcer. This often favours more choice by that sex and more competition by the other. These are tendencies: partner availability and ecology also matter, and both sexes can compete and choose.
A heritable ornament may lower survival yet be favoured if its mating benefits increase overall expected reproductive contribution. Count both effects. For example, after survival costs are included, ornament carriers might average 8 surviving offspring rather than 6. Cost alone does not make an ornament an honest signal of condition. Honesty requires evidence that the signal reliably reflects condition.
Competition among individuals of the same sex for mating opportunities is called intrasexual selection. Mate choice between sexes is called intersexual selection. If males fight and females choose among them, the fights show competition and the choices show mate preference. Both processes can affect mating success.
Suppose males invest more overall, brooding delays their next mating, and ready males are scarce relative to females. The investment argument predicts greater female competition and male choosiness. Male egg carrying alone does not establish these conditions. A matching role-reversed case supports the prediction but does not prove a universal rule. Measure total investment, readiness and mating behaviour.
As in foraging, time and energy used for one action cannot also fund another. Hours spent displaying instead of feeding chicks have an opportunity cost: chick-feeding effort forgone. That does not tell us how many chicks will die. For reproductive decisions, compare expected reproductive contribution, including offspring survival and future opportunities, rather than energy gain alone.
Investment and partner availability influence competition and choice. Heritable ornaments can be favoured despite survival costs, but cost alone does not establish signal honesty.
Lesson, questions and study cards written for this dataset.
Readings and study cards written for this dataset. The linked video is credited to Khan Academy. Publisher licence terms have not been verified.
More to read
Separating competition from mate choice
Separate two processes in a breeding group. Individuals may compete with others of the same sex for mating opportunities. They may also select a partner from the other sex. These processes are called intrasexual competition and intersexual mate choice. Both can produce differences in mating success.
An individual can compete for one partner and reject another potential partner. Competition and choice are behaviours to measure, rather than permanent labels for each sex.
An inherited display can increase mating success while reducing survival. To judge whether it can be favoured overall, combine those effects in expected reproductive contribution. Extra matings alone do not establish an advantage if survival or offspring outcomes offset their benefit.
What evidence would establish signal honesty?
A preference, a cost and an honest signal are different claims. Researchers might find that mates prefer a song and that singing attracts predators. Those findings establish attraction and a survival cost. They do not establish that the song identifies singers in better condition.
To call the song an honest signal of condition, researchers need evidence that its variation reliably reflects variation in condition. Measurements or experiments can test that relationship. A cost may help explain the relationship, but its presence alone does not demonstrate honesty.
The signal's overall evolutionary advantage is a further question. An inherited ornament can be favoured despite a survival disadvantage when its benefits increase expected reproductive contribution after the disadvantage is included.
Predicting mating patterns from measured conditions
Begin with the whole investment in offspring. Producing eggs, gestation and parental care may each use resources or delay another reproductive attempt. Seeing males carry eggs therefore does not establish which sex invests more overall.
Then ask how many individuals of each sex are ready to mate. Suppose greater male investment delays their return to mating, leaving ready males scarce relative to females. The investment argument predicts greater female competition and male choosiness under those conditions.
A matching observation supports the prediction. It does not show that investment alone explains every species. Partner availability and ecology may affect the pattern, and both sexes can show competition and choice. Measure the degree of each behaviour instead of forcing every population into two fixed roles.
From time budgets to reproductive outcomes
A parent cannot spend the same hour displaying and feeding young. Choosing the display gives up the feeding effort that hour could have supplied. That is an opportunity cost, not a known number of offspring deaths.
Estimating the reproductive outcome takes more evidence. Ask how each action affects offspring survival, the parent's survival and future breeding opportunities. Care may benefit the current brood while reducing resources available for another attempt. Display may attract mates while leaving less time for care.
This extends the accounting used in foraging but changes the outcome being compared. Net energy gained per minute does not by itself describe reproductive success. Compare expected reproductive contribution across the relevant opportunities and state the assumptions.
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