Mating Systems in Animals

Why This Matters

Mating systems are central to understanding animal behavior and evolutionary biology—two areas heavily tested on the AP exam. When you study how animals choose mates and structure their reproductive relationships, you're really exploring the intersection of natural selection, sexual selection, parental investment theory, and resource availability. These systems don't exist in a vacuum; they emerge from ecological pressures, the costs and benefits of parental care, and the operational sex ratio in a population.

Don't just memorize which species does what. Instead, focus on why a particular mating system evolves: What are the trade-offs? How does parental investment shape male vs. female strategies? How do resources and spatial distribution of mates drive different systems? If you can explain the selective pressures behind each system, you'll be ready for any FRQ that asks you to analyze reproductive strategies.

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Pair-Bonding Systems: When Two (Usually) Work Better Than One

These systems evolve when biparental care significantly increases offspring survival, or when ecological conditions make it difficult for one sex to monopolize multiple mates. The key mechanism is that the fitness benefit of staying together outweighs the potential gain from seeking additional mates.

Monogamy

• Both parents invest in offspring care—this system is favored when young require extensive resources or protection that one parent alone cannot provide
• Social vs. genetic monogamy are distinct concepts; socially monogamous pairs may still engage in extra-pair copulations, meaning social behavior doesn't always match genetic paternity
• Common in birds (approximately 90% of species) where incubation and feeding demands make dual parental care essential for nestling survival

Serial Monogamy

• Pair bonds last only one breeding season—individuals switch partners between seasons while maintaining monogamous behavior within each reproductive cycle
• Balances genetic diversity with parental care benefits; offspring still receive biparental investment, but the population avoids inbreeding over generations
• Seasonal breeders often exhibit this pattern, as environmental cues reset mate availability each year

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Polygyny: When Males Mate with Multiple Females

Polygyny evolves when males can monopolize access to females either directly or by controlling resources females need. The underlying principle is that variance in male reproductive success increases when some males can exclude others from mating opportunities.

Resource Defense Polygyny

• Males control critical resources like food patches, nesting sites, or territories—females choose mates based on resource quality rather than male traits alone
• Female reproductive success depends on resource access; this creates selection pressure for males to defend high-quality territories
• Common in species with patchy resource distribution, such as certain dragonflies defending oviposition sites or birds defending food-rich territories

Female Defense Polygyny

• Males directly guard groups of females rather than resources, preventing rival males from mating
• Evolves when females naturally aggregate—spatial clumping of females makes it economical for males to defend them as a group
• Requires significant male investment in defense, often leading to pronounced sexual dimorphism in body size or weaponry

Harem Systems

• One dominant male maintains exclusive mating access to a group of females, often through aggressive competition with rival males
• Strong sexual dimorphism typically accompanies this system; males are often much larger or possess elaborate weapons like antlers or enlarged canines
• Reproductive skew is extreme—a small percentage of males sire most offspring, intensifying sexual selection on male competitive traits

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Polyandry and Promiscuity: When Females Mate Multiply

These systems challenge the traditional assumption that females are always the "choosy" sex. They evolve when female fitness benefits from multiple matings—through genetic diversity, paternal care from multiple males, or sperm competition.

Polyandry

• One female mates with multiple males, often transferring parental care duties primarily to males—this reverses typical sex roles
• High male parental investment is the key predictor; when males provide most offspring care, females can increase fitness by producing more clutches with different males
• Observed in jacanas, phalaropes, and some insects where males incubate eggs and raise young while females compete for mates

Polygynandry (Promiscuity)

• Both sexes mate with multiple partners without forming exclusive bonds—maximizes genetic diversity across the group
• Reduces inbreeding risk and can confuse paternity, which may decrease infanticide if males are uncertain which offspring are theirs
• Common in social species like chimpanzees and bonobos where complex group dynamics favor flexible mating arrangements

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Display-Based Systems: When Showing Off Wins Mates

In some species, males don't defend resources or females directly—instead, they compete through elaborate displays that females evaluate. Sexual selection drives the evolution of ornaments and behaviors that signal male quality.

Leks

• Males aggregate at traditional display sites where they perform courtship behaviors; females visit specifically to assess and choose mates
• No resources or parental care provided—females select based purely on male display quality, dominance rank, or position within the lek
• Intense sexual selection produces elaborate ornaments like the peacock's tail or sage grouse's air sacs; only top-ranking males achieve most matings

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Cooperative Systems: When It Takes a Village

Some species extend parental care beyond the breeding pair, with helpers contributing to offspring survival. This evolves when ecological constraints limit independent breeding or when helping relatives increases inclusive fitness.

Cooperative Breeding

• Non-breeding individuals assist with offspring care—helpers may provision young, defend territories, or provide protection from predators
• Increases offspring survival rates through shared workload; particularly advantageous in harsh environments where breeding opportunities are limited
• Often involves kin selection—helpers are frequently related to breeders, gaining indirect fitness benefits by helping raise siblings or nieces/nephews

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Quick Reference Table

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Self-Check Questions

1. Which two mating systems both result in polygyny but differ in whether males provide direct benefits to females? Explain the mechanism behind each.

2. A species shows high male parental investment, with males incubating eggs while females compete aggressively for mates. Which mating system does this describe, and what evolutionary principle explains the sex-role reversal?

3. Compare and contrast leks and resource defense polygyny in terms of what females gain from their mate choice decisions.

4. If an FRQ asks you to explain how ecological factors influence mating system evolution, which two systems would best illustrate how resource distribution shapes reproductive strategies?

5. A researcher observes that in a social mammal species, subordinate females help raise the dominant female's offspring rather than breeding independently. Using inclusive fitness theory, explain why this cooperative breeding system might be adaptive for the helpers.