Reproductive behaviours and sexual selection
Why reproductive behaviour is intraspecific
- Reproductive behaviours are responses to other members of the same species — rivals and potential mates.
- They exist because reproduction is where differences in reproductive success are largest, so selection on them is exceptionally strong.
The origin of the asymmetry between the sexes
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Almost everything in this topic follows from one fact: gametes differ in cost.
- An egg is large and expensive, so a female can produce relatively few.
- Sperm are tiny and cheap, so a male can produce enormous numbers.
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The consequence is a difference in what limits each sex's reproductive success:
- A female's output is limited by the resources she can convert into eggs and offspring. Mating with more males does not increase it.
- A male's output is limited by the number of females he can fertilise. Mating with more females increases it directly.
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This produces two predictable outcomes:
- Females are usually choosy, because each reproductive attempt is a large investment and a poor choice is costly.
- Males usually compete, because access to females is the limiting factor for them.
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Where the investment is reversed — as in species where males incubate and rear the young — the roles reverse too, which is strong evidence that investment, not sex itself, is what drives the pattern.
Sexual selection
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Sexual selection is selection arising from differences in mating success rather than survival. It takes two forms:
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Intrasexual selection — competition between members of the same sex, usually males.
- Favours weapons and size: antlers, horns, large body mass, fighting ability.
- Also favours displays that settle contests without fighting, since both rivals gain by avoiding injury.
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Intersexual selection — mate choice, usually by females.
- Favours ornaments and displays: bright colour, elaborate song, complex courtship, gifts of food.
Why females are choosy
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Choice must yield a benefit, and there are two kinds:
- Direct benefits — the male provides something material: a territory with food, parental care, a nuptial gift, or freedom from parasites.
- Indirect (genetic) benefits — the male provides only alleles, so choice must be selecting good alleles.
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For indirect benefits, the display must be an honest signal — one that a low-quality male cannot fake.
- Honesty is maintained by cost: an elaborate display is expensive to produce and often makes the male more conspicuous to predators.
- Only a male in good condition — well fed, disease-free, efficient at foraging — can afford that cost and survive.
- So the display is a reliable indicator of quality, and a female choosing the best display obtains alleles for efficient foraging, disease resistance and good condition for her offspring.
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This explains the apparent paradox of ornaments that reduce survival: a trait that lowers survival can still spread if it raises mating success by more.
Mating systems
| System | Description | Typically where |
|---|---|---|
| Monogamy | One male, one female, often for a season or life | Both parents needed to rear young |
| Polygyny | One male, several females | Male provides no care; resources are clumped |
| Polyandry | One female, several males | Male provides most parental care |
| Lek | Males gather to display; females visit only to mate | Male provides only alleles |
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The mating system follows from who provides care and how resources are distributed.
- Where young cannot be raised by one parent, monogamy is favoured, because a deserting male gains little if his offspring then die.
- Where one parent suffices and resources are clumped, a male can monopolise several females — polygyny.
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Lek breeding in New Zealand: the kākāpō is the only flightless, lek-breeding parrot. Males clear a bowl on a ridge, and boom — a low-frequency call that carries several kilometres — for weeks. Females travel to the display arena, choose a male, mate, and raise the chick entirely alone.
- Because the male contributes only alleles, female choice is under exceptionally strong selection: the display is the only information available about his quality.
- Kākāpō also breed only in rimu mast years, when fruit is abundant enough to support chick rearing — linking reproductive behaviour to the annual rhythms covered earlier in this topic.
Courtship behaviour and its functions
- Courtship is not decoration. It performs several distinct jobs, and questions often ask for these:
- Species recognition — ensuring the partner is the right species, preventing wasted gametes in hybrid matings that produce infertile or inviable offspring.
- Assessment of quality — allowing the female to judge condition and genetic quality.
- Synchronisation — bringing both partners into reproductive readiness at the same time, so gametes are released together.
- Suppression of aggression — allowing close approach in animals that would otherwise attack an intruder.
- Pair-bond formation — establishing cooperation needed for shared parental care.
Selective advantage
- Male competition. A male that wins contests gains access to more females and fathers more offspring. Since he provides no care in such systems, offspring number is limited only by matings achieved.
- Female choice. A choosy female obtains either resources and care that raise offspring survival, or alleles that make her offspring better able to survive and reproduce. Either way more of her offspring survive to breed.
- Courtship. Correct species recognition avoids wasting an entire reproductive attempt on a hybrid mating; synchronisation raises the proportion of eggs fertilised.
- Parental care. Care raises the survival of each offspring, and is favoured where the increase in survival outweighs the cost of the additional offspring the parent could otherwise have produced.
Worked Example
Worked Example
In a lek-breeding native bird, males display at a communal arena and provide no parental care. Researchers record:
- Males with the longest display bouts obtain most of the matings — the top three males achieve 72% of all matings.
- Long-display males carry fewer blood parasites than short-display males.
- Long-display males lose about 12% of body mass over the display season.
- Chicks fathered by long-display males have higher survival to fledging than those fathered by short-display males, although all chicks are raised only by females.
Explain why female choice for long display bouts provides a selective advantage.
Answer:
Establishing what the female can be gaining. The male provides no parental care, no territory and no food — the females raise the chicks alone. So the female receives nothing material from her choice. The only thing the male contributes is his alleles, which means any advantage must be a genetic (indirect) benefit.
Why the display is an honest signal. For choice to be worth making, the display must carry reliable information that a poor-quality male cannot fake. The data show it is costly in two ways:
- Displaying males lose about 12% of body mass over the season, because displaying takes time that could be spent feeding and consumes energy directly.
- Displaying at a communal arena for long periods makes a male conspicuous to predators.
Only a male in good condition — one that forages efficiently and is not weakened by disease — can sustain that cost and survive it. A low-quality male attempting a long display would exhaust his reserves or be killed. The display therefore cannot be faked, and its length is a reliable indicator of quality. This is why cost is what makes the signal informative.
What quality the signal indicates. Long-display males carry fewer blood parasites. Resisting parasites requires an effective immune response, which is partly heritable, and a parasitised male has less energy available for displaying. So display length indicates parasite resistance specifically, as well as general condition and foraging efficiency.
Why this benefits the female's offspring. A female choosing a long-display male gives her chicks alleles for parasite resistance and efficient foraging. The chick survival data confirm the benefit is real: chicks of long-display males survive to fledging better even though all chicks are raised by females alone.
That last point is the decisive control. Because males contribute no care, the difference in chick survival cannot be explained by better paternal provisioning — the only thing that differs between the chicks is their paternal alleles. This is what establishes that the female is genuinely selecting for genetic quality rather than for a resource.
The selective advantage.
A female that chooses a long-display male produces chicks that are more likely to survive to fledging, and, inheriting the same resistance alleles, more likely to survive to breed themselves. She therefore leaves more surviving descendants than a female that mated at random.
Because the preference itself is heritable, her daughters inherit the preference and her sons inherit the display alleles. Both are favoured, so the preference and the trait increase in frequency together — which is why lek systems produce such extreme skew, with the top three males taking 72% of matings.
A necessary qualification. The long display reduces the male's own survival — 12% of body mass and greater exposure to predators. It spreads anyway because the gain in mating success outweighs the loss in survival. Sexual selection and natural selection are acting in opposite directions on this trait, and the observed display length is the balance point between them.