Mating Systems

What is monogamy?

A mating system in which one male mates with one female, either sequentially or for life. Can be sexual, social, or genetic monogamy.

Examples of monogamous mammals

Prairie voles, owl monkeys, California mice, sengi.

Why are males ever voluntarily monogamous?

Because monogamy sometimes increases their reproductive success when: (1) offspring require paternal assistance, (2) mate guarding is critical, or (3) ecological constraints make finding additional females unlikely.

What is polygyny?

One male mates with multiple females. This is the most common mating system in mammals.

Why do males use different polygyny tactics?

Because the distribution of females determines which strategy (scramble, female defense, resource defense, lek) yields the highest reproductive success.

What is polyandry?

One female mates with several males. An example is tamarins.

Why are females ever polyandrous?

To obtain genetic benefits ("good genes," fertility insurance, compatibility) and/or material benefits like extra resources, parental care, protection, and reduced infanticide.

What is polygynandry?

Multiple males mate with multiple females, with exclusive relationships between some individuals. Seen in lions, chimps, bonobos.

What is promiscuity?

Free, non-exclusive mating among any male and female in a group. Seen in hares, rabbits, bobcats.

How common is monogamy in mammals?

Rare — only ~3-5% of mammalian species.

Why is monogamy uncommon in male mammals?

Males cannot become pregnant or lactate, so sexual selection typically favors polygyny unless specific conditions shift the cost-benefit balance.

Mate assistance hypothesis

Males help raise offspring because paternal care dramatically improves offspring survival. Examples: California mice, Djungarian hamsters, beavers.

Mate guarding hypothesis

Monogamy evolves when females remain sexually receptive after mating and males must guard them to prevent re-mating. Example: prairie voles.

Protection from infanticide

Males protect infants from rival males; monogamy reduces infanticide risk. Example: white-handed gibbons.

Ecological constraints hypothesis

When females are widely dispersed or occupy small defensible territories, a male cannot secure multiple females — leading to monogamy.

Example of ecological constraint leading to monogamy

Sengi — males have small ranges overlapping one female; OSR even; sexual monomorphism.

Dik-diks monogamy — What hypotheses are supported?

Mate guarding (males conceal estrus scent marks, females wander if unguarded) and no ecological constraint (territories with more resources do not predict polygyny). Male care not critical for offspring survival.

Genetic benefits of polyandry

Fertility insurance, good genes, genetic compatibility, increased offspring viability. Examples: Gunnison's prairie dogs, yellow-toothed cavy, brown antechinus.

Material benefits of polyandry

(1) More resources, (2) More paternal care (golden lion tamarins), (3) More protection + reduced infanticide via paternity confusion (langurs, bank voles)

Polygyny tactics depend on

Female distribution (in space & time).

Scramble competition polygyny

Males roam widely to find scarce, dispersed, brief-window receptive females. Example: 13-lined ground squirrels.

Female defense polygyny

One male defends a cluster of receptive females. Seen in elk, gorillas, lions, zebras, and spear-nosed bats.

Resource defense polygyny

Males defend territories containing resources that females visit: watering sites, food-rich patches, predator-protected zones. Seen in antelopes, impalas, marmots.

Lek polygyny

Males defend small display arenas; females visit only to choose mates. A few males monopolize most mating. Examples: topi, hammer-headed bats.

Polygyny threshold model

A female will choose a polygynous relationship over a monogamous male if the resources or benefits gained exceed the cost of sharing the male.

Example supporting the polygyny threshold

Thirteen-lined ground squirrels — females receptive only 4-5 hours/year, so they choose males with greater mating opportunities even if he has mated previously.

How do females benefit from extra-pair mating (in monogamous species)?

Maintains genetic diversity in offspring and reduces risk of infertility from a single mate.

Why do tamarins show extreme male care in polyandry?

Females produce twins and require heavy parental investment; males evolved to carry and care for offspring extensively.

Social vs genetic monogamy

Animals may form long-term pair bonds socially while still mating outside the pair (extra-pair copulations).

Why is genetic monogamy rare?

Even in "monogamous" species, DNA evidence shows extra-pair paternity is common.

Example where males gain weight for paternal duties

Golden lion tamarin males gain muscle mass while female is pregnant to better carry twin offspring.

Why do females mate with multiple males in infanticidal species?

To obscure paternity, reducing likelihood a male will kill the infant.

Which mating systems increase paternal care?

Monogamy and polyandry — because males have higher paternity certainty or fewer mating opportunities.

Why are most mammals polygynous?

Because female reproductive rate is slow (gestation + lactation), male potential reproduction is limited by access to females, not resources.