17. Mating Systems

A mating system describes the complex behaviors associated with acquiring a mate and involves multiple aspects of reproductive strategies. These include:
- The nature of social bonds between the sexes, which can vary significantly among species, influencing the dynamics of pair formations.
- Contributions of both sexes to parental care, which are critical in determining the success of offspring and can affect the reproductive strategies employed.

Key behaviors defining mating systems:
- Display behaviors: Features such as elaborate courtship rituals or physical adornments that attract potential mates, often seen in species like peacocks.
- Intersexual competition for mates: This includes behaviors where one sex (commonly males) competes for the attention of the opposite sex through displays of strength or supremacy.
- Sexual division of investment: Refers to how different sexes allocate resources toward producing gametes or parental care. For instance, females often invest more in fewer, larger eggs, while males may produce many smaller sperm.
Calculations can be made based on:
- Frequency of copulations: The number of mating occurrences can provide insight into reproductive success and strategies.
- Timing of copulations: Seasonal variations can influence mating success and competition.
- Social pair bonds: Understanding the strength and stability of bonds can elucidate the reproductive success rates among various pairs.
Juvenile involvement: In many species, juveniles may assist parents in raising siblings instead of breeding themselves in their first breeding season, which can enhance survival rates of the offspring.

Mating systems can be categorized based on various criteria, including:
- Social pair bonds: The strength and type of relationships established between mated individuals.
- Gene flow: The exchange of genetic material between populations, which can be influenced by mating practices.
- Ecological factors: Habitat characteristics that can influence mate availability and competition.

Approximately 90% of bird species exhibit monogamous behavior (socially monogamous), indicating close social bonds for the raising of offspring. This high prevalence highlights the adaptive advantages of shared parental care in birds.
This leaves about 10% of bird species that exhibit polygamous systems, where mating involves more than two partners.

Social monogamy: Involves a temporary bond between male and female where they share parental responsibilities but may engage in extra-pair copulations with others, influencing genetic diversity.

Polygamy: This broad term encompasses various mating strategies where an individual mates with multiple partners. Specific types include:
- Polygyny: One male mates with multiple females, often seen in species where females are distributed across territories.
- Polyandry: One female mates with multiple males, which may occur in environments where male care increases offspring survival.
- Polygynandry: Multiple males and females form social bonds, complicating traditional pair bonding models.
- Promiscuity: Individuals engage in copulations without forming strong, lasting bonds, often seen in species dining in large social groups.

In mammals and fish, the level of monogamy is significantly lower (around 3% observed in fish species) due to less parental investment shown by males, as they often do not engage in activities such as incubation or post-natal care, affecting reproductive strategies.

Parental investment: An increase in egg investment by females often leads to polygynous systems as females prioritize superior male genes over quantity.
Paternity certainty: Variable distribution of resources and female territories impacts mating systems, influencing whether males can reliably ensure paternity.

Resource Defense Polygyny:
- Example: Red-winged blackbirds exhibit this behavior, where females may choose to become secondary females on a superordinate male's territory, optimizing their reproductive success.
- Polygyny Threshold Model: Suggests that females will opt to mate with a lower-quality male if it means securing a superior territory for their young.
Male Defense Polygyny: Males protect groups of females, dividing natural clumps where females congregate, increasing mating opportunities for the defending males.
Sequential Polyandry: Females may mate with multiple males in succession, particularly advantageous when males provide nesting sites or care for the offspring.

Promiscuity: In many species, such as those displaying in lek systems, females select mates based on displays or performances, which often occur in non-monogamous settings, reflecting a lack of strong social bonds.
Leks: Defined areas where males aggregate to showcase their qualities in order to attract females, an example of how complex mating behaviors can manifest in evolutionary contexts.
Variability in Success in Lek Mating: Not all males in leks have equal access to mating opportunities, often leading to a few males gaining the majority of the reproductive success while others remain unsuccessful.

Environmental conditions play a critical role in shaping the variability of mating systems, influencing resource availability, and territorial dynamics that dictate interactions between the sexes.
Extra-Pair Copulations (EPCs): Common in socially monogamous species, EPCs can increase genetic diversity and often challenge traditional views of monogamous relationships and parental investment, verified by genetic studies.

Mating systems are diverse and complex, determined by ecological, social, and investment factors. These interactions lead to various reproductive strategies across species. Understanding these influences is crucial for comprehending the evolutionary pressures shaping mating behaviors and strategies in different ecological contexts.