Mate Choice and Sexual Selection

Anisogamy

Definition: Sexes produce different-sized gametes, creating divergent reproductive strategies.
Example: Male mammals produce billions of sperm while females produce few eggs (e.g., pigs: males 20–60 billion sperm vs females <20 eggs per 18–24 days).

Differential recovery

Definition: Males recover rapidly from mating whereas females recover slowly due to gestation, lactation, and care.
Example: Killer whales: post-reproductive females boost offspring survival, while males remain dependent on maternal support.

Sexual conflict arms race

Definition: Conflict over reproduction can escalate traits that maximize each sex’s reproductive success.
Example: Scarcity of females increases male competition and increases female choice

Male reproductive strategy

Definition: Male reproductive success is limited by access to mates, selecting for quantity of partners.
Example: Lions mate thousands of times for each offspring.

Female reproductive strategy

Definition: Female reproductive success is limited by quality of mates and resources.
Example: Females prefer high-quality males and high-quality resource access.

Monogamy

Definition: Each sex mates with only one partner (rare).
Example: Djungarian hamsters: male aids thermoregulation of female and pups, increasing survival.

Mate guarding (monogamy driver)

Definition: Males guard mates to ensure paternity when extra-pair mating risk is high.
Example: House sparrows show higher extra-pair young when food is scarce due to reduced mate guarding.

Low encounter rate (monogamy driver)

Definition: Low density or wide dispersal makes finding multiple mates difficult.
Example: Harlequin shrimp males stay with females for weeks; angler fish males fuse to females as hermaphroditic pair.

Intersexual selection

Definition: Mate choice where one sex chooses individuals of the other sex.
Example: Females selecting males based on traits.

Intrasexual selection

Definition: Competition within one sex for access to mates, often producing exaggerated traits.
Example: Male–male competition for females.

Polygyny

Definition: One male mates with multiple females, while females mate with one male.
Example: Common when males can control resources or females.

Male–male resource competition

Definition: Males fight for territories, resources, or dominance to access females.
Example: Savannah baboons, deer stags, and dung beetles compete for dominance.

Resource-controlled polygyny

Definition: Males defend resources needed by females.
Example: Male cichlids defend shell middens that females require.

Female-controlled polygyny

Definition: Males control groups of females.
Example: Lion males directly control female groups.

Skewed male mating success

Definition: Few males achieve most matings while others get none.
Example: Killer whale males show disproportionate reproductive success.

Female choice under polygyny

Definition: Females choose among multiple males, driving sexual selection for male traits.
Example: Many birds exhibit female choice for bright colours, displays, and decorations.

Polyandry

Definition: One female mates with multiple males, males mate with one female (rare).
Example: Spotted sandpipers: females defend territories and attract multiple males who incubate eggs.

Female resource strategy (polyandry)

Definition: Females obtain resources or escape parental care via multiple mates.
Example: Sandpipers shift parental care to males and secure insect-rich territories.

Polygynandry

Definition: Both males and females mate with multiple partners.
Example: Barbary macaques: multiple matings confuse paternity and increase paternal care.

Polygyny threshold model

Definition: Females may choose polygyny if a high-quality male’s territory offers greater reproductive success.
Example: Better territory supports more offspring, making shared mating worthwhile.

Costs of mate choice

Definition: Female choice incurs energetic and predation costs.
Example: Sage grouse hens travel long distances and encounter predators more often when visiting leks.

Direct benefits of choice

Definition: Females gain benefits that increase survival or reproduction.
Example: Benefits include territory, paternal care, nuptial food, or reduced harassment.

Indirect benefits of choice

Definition: Offspring gain genetic advantages that enhance survival or attractiveness.
Example: Attractive fathers produce “sexy sons.”

Trait signals of direct benefits

Definition: Male traits indicate resource or care quality.
Example: Large song repertoire signals paternal care ability.

Trait signals of indirect benefits

Definition: Male traits indicate genetic quality.
Example: Peacocks: number of eyespots correlates with chick survival.

Honest signals

Definition: Traits that cannot easily be faked due to high cost or performance constraints.
Example: Females prefer exaggerated signals that indicate true quality.

Runaway selection

Definition: Female preference and male trait coevolve, amplifying traits across generations.
Example: Daughters inherit preference, sons inherit trait → trait becomes fixed when all females prefer it.