Animal Behavior - Sex Differences and Mating Systems

Sexual behavior differences between males and females arise from differences in parental investment.
These differences affect the rate of individual offspring production.
The sex with the potential to leave more descendants benefits from high sexual activity and preparedness to mate.
Inequality in receptive individuals (operational sex ratio) leads to competition for mates within one sex, while the other sex can be choosy.

More mates increase an individual's fitness.
Better mate quality increases an individual's fitness.
Differential reproductive variance exists between sexes.
Competition for mates is driven by a biased operational sex ratio.
Lower parental investment leads to preparedness to mate.
Higher parental investment and resource donations lead to being unprepared to mate.
Differences in gamete size, parental investment, and resource donations contribute to these dynamics.

Male Sygnathids (pipefish, seahorses) have a brood pouch providing nutrients and oxygen to fertilized eggs.
Males with free pouch space actively choose mates.
They discriminate against small, plain females in favor of large females who can provide larger clutches of eggs.

Sex differences are not always fixed.
Some female Iberian moles possess an intersex phenotype.
Mammalian sex is generally determined genetically, but genes direct tissue development for sex-specific traits.
Increased androgen synthesis in female intersex moles leads to masculinized genitalia and aggression.

Sex roles are dynamic and shift during a single breeding season in two-spotted gobies.
From May to July, male abundance declines, while female abundance increases.
This results in a steady decline of the operational sex ratio.

Adjustment in the operational sex ratio is accompanied by a change in mating competition.
Male-male and female-female agonistic behavior changes over time.
Courtship behavior by males and females also varies.
Graphs illustrate these changes from May to July.

Animal mating systems are defined by the number of sexual partners each sex has.
- Monogamy: 1 male partner, 1 female partner.
- Polygyny: More than 1 female partner, 1 male partner.
- Polyandry: 1 female partner, More than 1 male partners.
- Polygynandry and Promiscuity: More than 1 male partner, More than 1 female partner.

In primates, the number of mates accepted by females correlates with investment in WBCs.
Polyandrous species challenge the female’s immune system more.
In birds, polyandrous species have higher bacteria and yeast levels in cloacal swabs than monogamous species.

Mate limitation hypothesis: evolves when potential mates are dispersed and costly to locate.
Mate guarding hypothesis: evolves when individuals can restrict their partner's mating behavior.
Mate assistance hypothesis: evolves when resources are critical and both parents are needed to rear young.
Infanticide hypothesis: risk of male infanticide influences female tactics and monogamy evolution by forming a protective bond.

Males maintain a relationship with a single female who provides egg clutches.
Pairs greet each other in the morning and ignore other members of the opposite sex.
A male’s brood pouch can accommodate only one clutch, so there is no benefit to courting multiple females.
Some Hippocampus species males do not accept eggs from multiple females.

Evidence linking infanticide risk and monogamy is strong in primates but lacking in other mammals.
Paternal care appears to be a consequence, not a cause, of monogamy in mammals.
Monogamy evolved where breeding females are intolerant of each other and female density is low.
Consistent with the mate guarding hypothesis.

Males tend to live with females in two-adult units more often when females live apart in small territories.
It is difficult for a male to gain access to multiple females, so the optimal strategy is to stay with one.

Polyandry is associated with territorial scarcity, leading to a male-biased operational sex ratio.
Intense competition for females has favored males capable of forming cooperative defense teams.
A breeding female may acquire up to eight mates who help rear a single offspring per breeding event.

Females fight for and defend territories to accommodate several males.
A territorial female mates with all members of her harem, providing each with his own clutch of eggs.
Males care for eggs exclusively.
Males are also likely to care for offspring sired by other males; 75% of clutches laid by polyandrous females are of mixed paternity.

Good genes hypothesis: produces offspring of higher genetic quality or viability.
Genetic compatibility hypothesis: increases the odds of receiving genetically complementary sperm.
Genetic diversity hypothesis: increases heterozygosity of offspring.
Inbreeding avoidance hypothesis: avoids inbreeding with the social partner.

Female grey foam nest treefrogs' eggs may be fertilized by 1 to 12+ males.
Polyandry has fitness benefits: offspring of polyandrous females are more likely to survive.
Male cavies have very large testes due to intense sperm competition.
When female yellow-toothed cavies were restricted to a single mate, they had fewer surviving offspring compared to females allowed to mate with four males.

Additional resources hypothesis: gain access to additional resources from their partner.
Female red-winged blackbirds may forage on territories of males with whom they have engaged in extra-pair copulations.
Genetically monogamous females are chased away.
Additional care hypothesis: gain more caregivers to help rear young.

Female Neotrogla barklice have a penis-like structure called a gynosome; males lack an intromittent organ.
These barklice live in Brazilian caves and feed on scarce bat carcasses and guano.
Nutritious nuptial gifts may cause strong selection for multiple mating by females.
A polyandrous female may receive up to 11 spermatophores over her lifetime.

In dunnocks, both sexual partners of a polyandrous female help rear her brood if they have both copulated often enough with her.
Female dunnocks ensure this paternal threshold is reached by soliciting matings from whichever male (alpha or beta) has had less time in their company.