Mating Systems 17

Mating Systems

• Mating systems can be defined by the number of copulatory partners per individual per breeding season.
• They explain patterns of aggregation of males and females in the environment.

Types of Mating Systems

• Monogamy: Individuals mate with one partner.
• Polygamy: Individuals mate with multiple partners.
  • Polygyny: 1 male + multiple females.
  • Polyandry: 1 female + multiple males.

Monogamy

• 1 female + 1 male.
• May occur when offspring require both parents, indicating a high degree of parental care.
• Seen in > 90% of birds and < 10% of mammals.
  • Male assistance hypothesis: males help provision young (examples: canids, marmosets).
  • Mate guarding hypothesis: females are solitary, and males can defend only 1 female (examples: dik-dik, many rodents).
• Very rare in herps, fish, and insects (example: carrion beetles).

Polygyny

• > 1 female per male.
• > 90% of mammals.
  • Males defend females when they can.
  • Males disperse further, presumably to find a female group.
• Some birds:
  • Males defend food or nesting sites (examples: oropendolas, grouse).

Types of Polygyny

1. Female defense polygyny
   • Occurs when females are grouped, or when females have smaller territories than males.
     • Example: Sandperch
   • Common in mammals – elk, gorillas.
   • Some birds – Red-winged Blackbirds.
   • Some arthropods – marine amphipods.
2. Resource defense polygyny
   • Occurs when males defend a resource needed by females.
   • The resource is exchanged for matings (resource = direct benefit to females).
     • Examples:
       • Food – Streamertail hummingbird
       • Nest sites – Weavers (bird family: Ploceidae)
3. Scramble competition polygyny
   • Occurs when females are widely dispersed, and the breeding season is short (males can’t monopolize females).
     • Examples:
       • 13-lined ground squirrel – explosive breeders (large numbers become reproductive simultaneously in a SHORT breeding season).
       • Many amphibians.
       • Horseshoe crabs.
   • Males with the greatest reproductive success are those that move around the most.
   • Males of the thirteen-lined ground squirrel cover much more ground during the 2-week mating period when receptive females can be encountered.
   • Males of this species do not attempt to defend mating territories.
4. Lek polygyny
   • Occurs when males defend a small breeding territory (no resources), which only includes a display area (lek).
   • Purest form of sexual selection – all male contributes to offspring are his genes (excellent system to test female choice).
     • Examples: (a few in several diverse groups)
       • Some birds (bowerbirds, manakins).
       • Some mammals (hammer-headed bat, some antelope).
       • Some insects (some butterflies).

Hypotheses for lek formation:

1. Hotspot hypothesis – males congregate where females are likely to travel and compete for the best sites (example: many insects that lek).
2. Hotshot hypothesis – males congregate around a few very attractive males in an attempt to gain matings by association/proximity (example: sage grouse).
3. Female preference hypothesis – males congregate because females are attracted to male groups (example: kob antelope).

Polyandry

• > 1 male per female.
• Occurs where resources are patchy and briefly available, or predation is high.
  • Sequential polyandry – female clutch size is limited (examples: phalaropes).
  • Simultaneous polyandry – females defend males from other females, clutch size is less restrictive (example: Jacana).
• Rare in birds.
• Very rare in mammals.
• Common in fish, insects.

Promiscuity

• Multiple males and females.
  • Some birds
  • Some mammals
  • Some fish
  • Common in insects
• Genetically, many birds are promiscuous!
  • ~ 75% of bird species have extra-pair offspring - Brouwer and Griffith 2019.

Conflicts Between the Sexes

• Conflicts between the sexes may lead to multiple mating systems in the same species!
  • Example: Dunnock (Prunella modularis)

Group Discussion Questions

1. For each of the following groups of organisms, what is the MOST common strategy?
   • a) Birds
   • b) Mammals
   • c) Fish
2. In a mammal, what kind of strategy would you predict would be favored if:
   • a) Females are not solitary and males can guard multiple mates?
   • b) Resources are scarce and require multiple males to defend them?
   • c) A high parental investment is required per offspring?