Animal Behavior Unit 9 ReviewMating Systems & Sexual Selection

Key Concepts

• Mating systems describe how males and females pair up for reproduction in a population
• Sexual selection is a type of natural selection that favors traits that increase mating success rather than survival
• Anisogamy, the difference in gamete size between males and females, is a fundamental driver of sexual selection
  • Males typically produce many small, motile sperm while females produce fewer, larger, nutrient-rich eggs
  • This leads to different optimal reproductive strategies for each sex
• Operational sex ratio (OSR) is the ratio of sexually active males to sexually receptive females in a population at a given time
  • OSR influences the intensity of competition for mates and the direction of sexual selection
• Bateman's principle states that the sex with the higher potential reproductive rate (usually males) will compete more intensely for access to the sex with the lower potential reproductive rate (usually females)
• Sexual dimorphism, differences in size, coloration, or other traits between males and females of a species, often results from sexual selection
  • Peacocks (Pavo cristatus) exhibit striking sexual dimorphism, with males possessing elaborate tail feathers to attract females

Types of Mating Systems

• Monogamy involves the formation of exclusive pair bonds between one male and one female for at least one breeding season
  • Monogamy is common in birds, such as bald eagles (Haliaeetus leucocephalus), where both parents contribute to raising offspring
• Polygyny occurs when one male mates with multiple females, each of which only mates with that one male
  • Polygyny is often associated with male defense of resources or territories that attract females
  • Examples include red deer (Cervus elaphus) and gorillas (Gorilla gorilla)
• Polyandry is a rare mating system in which one female mates with multiple males, each of which only mates with that one female
  • Polyandry can occur when males provide essential resources for offspring survival, such as in the jacana bird (Jacana jacana)
• Polygynandry, also known as promiscuity, is a mating system where both males and females mate with multiple partners
  • Polygynandry is common in species with little parental care, such as many insects and some mammals like chimpanzees (Pan troglodytes)
• Lekking is a polygynous mating system where males aggregate in a specific area (the lek) to display and compete for female attention
  • Females visit the lek to assess males and choose a mate based on their displays
  • Lekking occurs in species such as the greater sage-grouse (Centrocercus urophasianus) and the Ugandan kob (Kobus kob thomasi)

Sexual Selection Basics

• Sexual selection operates through two main mechanisms: male competition and female choice
• Male competition involves males directly competing with each other for access to females or resources that attract females
  • Competition can take the form of physical fights, displays, or sperm competition
• Female choice occurs when females preferentially mate with males possessing certain traits or behaviors
  • Females may assess male quality based on indicators such as size, coloration, or courtship displays
• Fisherian runaway selection is a positive feedback loop that can lead to the evolution of exaggerated male traits
  • If a male trait is attractive to females, the trait will spread in the population, leading females to prefer even more extreme versions of the trait over time
• The handicap principle suggests that costly male traits, such as the elaborate tail of a peacock, serve as honest signals of male quality
  • Only high-quality males can afford to bear the costs of developing and maintaining such traits
• Sensory bias can influence the evolution of male traits, as males may exploit pre-existing female preferences for certain sensory stimuli
  • For example, the bioluminescent courtship displays of male fireflies may have evolved to exploit a female preference for light signals

Male Competition

• Male competition can take the form of contests, where males directly fight for access to females or resources
  • In red deer (Cervus elaphus), males engage in roaring contests and physical fights to establish dominance and gain mating opportunities
• Scramble competition occurs when males race to find and mate with females before their competitors
  • In many insect species, such as the European earwig (Forficula auricularia), males scramble to locate and mate with females first
• Endurance rivalry involves males continuously displaying or defending resources to outlast their competitors
  • Male fiddler crabs (Uca sp.) wave their enlarged claw to attract females and deter rival males, with the most persistent males securing the most matings
• Sperm competition occurs when the sperm of multiple males compete to fertilize a female's eggs
  • In many species, males have evolved larger testes, more sperm production, and sperm with morphologies that enhance their competitive ability
  • Damselflies (Calopteryx sp.) engage in sperm removal, where males use specialized structures to remove rival sperm from a female's reproductive tract before mating

Female Choice

• Females often prefer males with exaggerated or conspicuous traits, which may indicate male quality or genetic compatibility
  • In guppies (Poecilia reticulata), females prefer males with brighter and more colorful spots, which may signal better health and foraging ability
• Courtship displays allow females to assess male quality and make mate choices
  • Male birds of paradise (Paradisaeidae) perform elaborate dances and display colorful plumage to attract females
• Acoustic signals, such as songs or calls, can also be used by females to evaluate male quality
  • In túngara frogs (Engystomops pustulosus), females prefer males with more complex calls, which may indicate better condition and developmental stability
• Females may also assess male parental quality, favoring males that demonstrate good parenting skills or the ability to provide resources
  • In the three-spined stickleback fish (Gasterosteus aculeatus), females prefer males that build higher quality nests, as this indicates better parental care
• Cryptic female choice occurs when females influence the outcome of sperm competition after mating with multiple males
  • Females may store sperm from different males in separate compartments or selectively release sperm to favor certain males over others

Alternative Mating Strategies

• Alternative mating strategies are tactics employed by individuals to increase their reproductive success when they cannot compete effectively using the primary strategy
• Sneaker males are smaller, less competitive males that attempt to steal fertilizations from dominant males
  • In the bluegill sunfish (Lepomis macrochirus), sneaker males dart in and release sperm while a dominant male is spawning with a female
• Satellite males do not defend territories but instead associate with dominant males to intercept females
  • In the ruff (Philomachus pugnax), satellite males have different plumage and courtship behaviors than territorial males and attempt to mate with visiting females
• Female mimicry involves males mimicking female appearance or behavior to avoid aggression from other males and gain access to females
  • In the bluehead wrasse (Thalassoma bifasciatum), some males mimic female coloration and behavior to sneak matings with dominant males' harems
• Context-dependent strategies occur when individuals switch between alternative tactics based on their relative competitive ability or environmental conditions
  • In the horseshoe crab (Limulus polyphemus), males may switch between satellite and dominant behaviors depending on their size and the intensity of competition

Evolutionary Implications

• Sexual selection can lead to the rapid evolution of exaggerated traits and behaviors that may not directly enhance survival
  • The elaborate antlers of the Irish elk (Megaloceros giganteus) likely evolved through sexual selection but may have contributed to the species' extinction
• Sexual conflict arises when the evolutionary interests of males and females diverge, leading to an evolutionary arms race between the sexes
  • In the water strider (Gerris sp.), males have evolved grasping structures to hold onto females during mating, while females have evolved anti-grasping structures to resist unwanted mating attempts
• Speciation can occur as a result of sexual selection, particularly when populations diverge in their mating preferences
  • In the Hawaiian Drosophila fruit flies, sexual selection on male courtship displays has contributed to the rapid diversification of the group
• Sexual selection can also influence the evolution of parental care strategies, as the costs and benefits of care may differ between the sexes
  • In many bird species, such as the great tit (Parus major), both males and females provide parental care because the benefits of biparental care outweigh the costs of lost mating opportunities
• The intensity of sexual selection can vary depending on environmental factors, such as resource availability or population density
  • In the guppy (Poecilia reticulata), the strength of female preferences for colorful males is influenced by the level of predation risk in the environment

Real-World Examples

• In the lion (Panthera leo), males compete intensely for access to female prides, with larger, more dominant males securing the most mating opportunities
  • Male lions also exhibit conspicuous manes, which may serve as signals of quality to females and rivals
• The peacock spider (Maratus volans) performs elaborate courtship dances involving vibrant, iridescent displays to attract female attention
  • Males also engage in ritualized combat, with the winner gaining access to the female
• The sage grouse (Centrocercus urophasianus) mating system involves leks, where males gather to perform elaborate courtship displays
  • Females visit the leks to assess males and choose the most attractive individuals as mates
• In the pipefish (Syngnathus typhle), males provide all the parental care by carrying developing embryos in a specialized brood pouch
  • Females compete for access to males, and males are choosy about which females they mate with based on their size and quality
• The hermaphroditic sea slug (Chromodoris reticulata) engages in "penis fencing" during mating, where individuals attempt to stab their partner with their penis to inject sperm and avoid the costs of female parental investment
  • This unique mating behavior is an example of the extreme measures some species undergo due to sexual conflict and the evolutionary arms race between the sexes