Lecture 17. Mating Systems Notes

Mating Systems

• Based on Alcock's Animal Behaviour, 10th Edition, Chapter 8.

Group Structure

• How are groups structured according to their sexual behavior?
• What drives this structure?

Types of Mating Systems

• Monogamy
• Polygamy
  • Polygyny
  • Polyandry
• Polygynandry
• Promiscuity

Monogamy

• James Wittenberger suggested monogamy is adopted when no other system works.
• Males producing more gametes typically benefit from multiple partners.
• Circumstances leading to male monogamy:
  • Ensuring paternity (mate guarding hypothesis)
  • Ensuring offspring survival (mate assistance hypothesis)

Theories Behind Monogamy

• Mate guarding hypothesis: Prevents female from mating with others if males are scarce, ensuring paternity.
• Mate assistance hypothesis: Paternal care increases reproductive success more than seeking additional mates.
• Example: Male yellow-breasted chats are more aggressive to same-sex rivals. Simulated female extraterritorial intrusions lead to extrapair courtship and copulation.

Mate Guarding Hypothesis

• Females might seek other mates if the male leaves.
• If new females are hard to find, guarding the current mate ensures paternity.

Mate Assistance Hypothesis

• Paternal care may boost reproductive success more than seeking additional mates, favoring monogamy.
• Example: Male guarding in crickets ($Grillus campestris$) reduces predator attacks on females and offspring, increasing survival.

Female Enforced Monogamy

• Females may force males to stay and care for young when bi-parental care is advantageous.
• This involves preventing males from attracting other females.
• Example: Mated burying beetle females emit sexual pheromones longer to keep the male's attention.

Monogamy in Mammals

• Most males tend towards polygyny; monogamy is less common.
• Male parental care contributes to offspring survival.
• Occurs where females live apart in small territories.
• Example: California mouse offspring survival is better with male parental care.

Monogamy in Birds

• Most birds are socially monogamous.
• Male parental help increases breeding success.
• Hormones like testosterone may relate to parental care; lower testosterone may improve offspring survival.
• Two parents maintain warmer, more constant egg temperatures.
• Example: Spotless starlings show that both parents' involvement increases the number of fledged young.

Extra-Pair Copulations (EPCs)

• Social monogamy ≠ sexual monogamy.
• Evidence of EPCs in socially monogamous birds.
• Males benefit by fathering more offspring but risk adultery if not guarding their mate.
• Females' gains from EPCs:
  • Genetic benefits: Fertility insurance, good genes, genetic compatibility.
  • Material benefits: More resources, better protection, infanticide reduction.

Advantages of Polyandry for Females

• Genetic benefits:
  • Fertility insurance hypothesis: Reduces the risk of unfertilized eggs due to an infertile male.
  • Good genes hypothesis: Improves offspring viability or sexual attractiveness.
  • Genetic compatibility hypothesis: Increases the chance of genetic compatibility with her eggs.
• Material benefits:
  • More resources hypothesis: More mates provide more resources or parental care.
  • Better protection hypothesis: More mates offer more protection from harassment.
  • Infanticide reduction hypothesis: Greater confusion about paternity reduces infanticide.

Polyandry

• Generally disadvantageous for males due to reduced reproductive probability and paternity uncertainty.
• Occurs when females' ability to produce eggs defines the limiting resource.
• May result when there are few females and resources are limited, often in variable climates.
• Cooperative paternal care arises from uncertain paternity.
• Example: Galapagos hawks have limited breeding space and high mortality rates.

Polygyny

• Generally, not adaptive for females as it reduces their reproductive chances and resource/parental care for offspring.

Male-Female Conflict

• Male success increases with female group size, but female success may decrease.
• Example: Yellow-bellied marmots ($Marmota flaviventris$) show this conflict.

Theories of Polygyny

• Polygyny threshold model
• Female defense polygyny
• Resource defense polygyny
• Scramble competition polygyny
• Lek polygyny

Polygyny Threshold Model

• Females may be better off as a secondary mate on a good territory than a primary mate on a poor territory.
• Example: Red-winged blackbirds prefer males with central territories.
• Problem: Deceitful males (e.g., pied flycatchers) trick females.

Female Defence Polygyny

• Female distribution theory: Males monopolize females in defendable clusters.
• Example: Montezuma oropendola females cluster on single trees, easily defended by a male.

Resource Defence Polygyny

• Males monopolize resources used by several females when resources are patchily distributed.
• Example: Male wrens build and display nests to attract females.

Scramble Competition Polygyny

• Plenty of non-choosy females compete for males.
• Neither females nor resources can be monopolized.
• Some males are successful at finding receptive females.
• Example: Horseshoe crabs in explosive breeding assemblages.

Lek Polygyny

• Lek is an aggregation of males in competitive displays to attract females.

Lek Polygyny - Female Choice

• 'Winning' males secure most copulations.
• Possible explanations:
  • Hot-spot hypothesis: Location matters.
  • Hot-shot hypothesis: Especially appealing males.
  • Female preference hypothesis: Females prefer large groups to compare male quality.
• Examples: Uganda Kob, White-bearded Manakin, Sage grouse.

Lek Paradox

• If females choose males with the best genes, genetic variation should disappear quickly.
• Lekking species still show large genetic variation.
• DNA analysis shows more males fertilizing females than observed at leks.
• Females may mate with 'beta' males outside leks to ensure genetic variation.

Hermaphroditism

• Many species have both functional sexes or can change sex.
• Simultaneous hermaphroditism is common when finding mates is difficult.
• Examples: snails, echinoderms, and worms

Changing Sex

• Some fish (clownfish, wrasses, etc.) and some lizards can change sex.
• Clownfish hierarchy: the dominant male changes sex when the female dies.

No-Males

• Parthenogenesis: development from unfertilized egg cell.
• Common in inverts (aphids, stick insects, bees).
• In vertebrates, only found in reptiles (lizards and snakes).

Same-Sex Sexual Behaviour

• Common in many species (mallards, giraffes, social birds, and mammals).
• No species (except asexual ones) has been proven to lack it.

Adaptive Explanations

• Same-sex sexual behavior can include pair-bonding, parenting, and affection.
• Benefits:
  • Gaining experience
  • Social status
  • Bonding
  • Relieving tension
  • Experiencing pleasure
  • Shared care of the young

Other Forms of Non-Reproductive Sexual Behaviour

• Masturbation
• Oral sex
• Interspecies sex
• Sex involving juveniles
• Necrophilia

Take-Home Message

• Sexual behaviour in nature is varied.
• The environment selects for mating systems.
• Alternative reproduction modes exist.
• Same-sex sexual behaviour is widespread and has potential adaptive benefits.