Lecture 31: why females and males behave differently

Parental Investment and Mate Choice: Why Males and Females Behave Differently

Introduction and Caveats

• This lecture primarily focuses on binary sexes (males and females) for clarity and example illustration.

• It's important to acknowledge significant exceptions to these generalizations, including non-binary categories and diverse forms of attraction (same-sex, different-sex) observed throughout the animal kingdom. Additional information on these topics is often explored in other contexts (e.g., "rainbow happy hours").

• The core topics for today are Parental Investment and Mate Choice.

Parental Investment

Definition and Costs

• Parental investment refers to any investment by the parent in an individual offspring that increases the offspring's chances of surviving to adulthood, at a cost to the parent's ability to invest in other offspring.

• Example: Sparrow Feeding Offspring

  • Feeding Costs: The parent expends significant energy finding food, regurgitating it, and repeatedly traveling to and from the nest. This means less time and energy for the parent to feed itself.

  • Predator Exposure: The act of feeding often exposes the parent and offspring to predators. If the offspring is loud (as in the video example), it further increases this risk by attracting attention.

  • Time and Energy: Parenting involves substantial allocation of time and energy to ensure offspring survival.

Certainty of Paternity and Parental Care

• A key factor influencing parental investment is the certainty of paternity (or maternity, though typically paternity is more variable).

• Principle: As the certainty of paternity increases, the probability of a male engaging in parental care also tends to increase. This is because parents are investing in ensuring the survival and reproductive success of their own genetic material.

Examples of Parental Investment Strategies

• Zebra Finches:

  • Typically monogamous.

  • Exhibit internal fertilization.

  • Both males and females provide high parental investment.

• Sage Grouse:

  • Exhibit internal fertilization.

  • Males perform elaborate courtship dances to attract females.

  • Males are polygynous and provide virtually no parental investment; females bear the entire cost of raising offspring.

• Male Jawfish:

  • Exhibit external fertilization (females lay eggs, males fertilize them externally).

  • Males then collect and protect the fertilized eggs in their mouths for extended periods.

  • This represents a huge cost for the male, as eating becomes difficult or impossible, but ensures high paternal certainty and safeguards his genetic material.

Sexual Selection

Darwin's Contribution

• Introduced in Charles Darwin's The Descent of Man.

• Definition: A form of natural selection concerning the advantage certain individuals have over others of the same sex and species, in exclusive relation to reproduction.

• Darwin suggested individuals might survive equally well but not be equally "fit" due to differential access to mating opportunities.

Types of Sexual Selection

1. Intrasexual Selection:

   • Competition within one sex (typically males) for direct access to members of the opposite sex.

   • Often involves physical combat, displays of strength, or territorial defense.

2. Intersexual Selection (Mate Choice):

   • Choice by individuals of one sex (typically females) for particular members of the opposite sex based on certain traits or qualities.

   • Can lead to the evolution of elaborate phenotypes and behaviors.

• General Pattern: Males are more likely to compete for access to females, while females are more likely to be discriminating in their choice of partners.

Initial Investment and Reproductive Success

• The difference in initial investment between sexes explains this general pattern.

• Drosophila (Fruit Flies) Example:

  • Females: Mating once, twice, or thrice generally yields the same average number of offspring. A single mating event maximizes their reproductive output and fitness.

    • The egg is metabolically expensive to produce and limits the total number a female can make.

  • Males: Reproductive output (number of offspring) increases linearly with the number of females they mate with.

    • Sperm is relatively inexpensive to produce, allowing a male to fertilize many females.

• Implications:

  • Females: Tend to have greater consistency in reproductive success; most females get mated.

  • Males: Show much greater variation in reproductive success; some males fertilize many females, while many others are left without mating opportunities.

• Conclusion: The inexpensive nature of the initial male parental investment drives strong competition among males for mating opportunities, while the high cost of egg production makes females selective.

Parental Investment Theory (Revisited)

• Any investment by a parent in an individual offspring that increases the offspring's chances of surviving to the cost of the parent's ability to invest in other offspring.

• Because the costs of reproduction (especially initial investment in gametes and subsequent care) are often higher for females, females are generally predicted to be more choosy in their selection of partners.

Mate Choice and Mating Systems

Classification of Mating Systems

• Simplified into three main types based on the number of partners each sex may have:

1. Polygyny:

   • Lit. "many females" ($ext{poly}$ = many, $ext{gyny}$ = female).

   • A single male mates with multiple females.

   • Male Fitness Benefit: From an evolutionary and fitness perspective, it is generally in a male's best interest to mate with as many females as possible.

     • Yellow-bellied Marmots Example: As the number of females per group increases, the number of yearlings produced per male also increases, demonstrating a direct fitness benefit for males in polygynous systems.

   • Female Costs: For females, being part of larger polygynous groups often incurs costs:

     • Sharing of resources (food, nesting sites).

     • Increased exposure to predators due to larger group size.

     • Yellow-bellied Marmots Example (Female Perspective): Optimal group size for a female is generally one or two females. As the group size increases to three or four females, the number of yearlings produced per female drops below one, indicating a reduced chance of successful reproduction for individual females.

     • Conclusion: There are often divergent selection pressures for males and females regarding group size and mating system.

   • Classic Forms of Polygyny:

     • Female Defense Polygyny: Males directly defend a group or "harem" of females (e.g., elephant seals, red-winged blackbirds).

     • Resource Defense Polygyny: Males defend resources that females need (e.g., nesting sites, food patches).

       • Cichlid Fish Example: A male cichlid builds an elaborate sand nest on the streambed, which he constantly aerates and displays. Females are attracted to the quality of the nest, lay their eggs there, and the male fertilizes them. The female then departs, possibly to lay more eggs elsewhere. The male, certain of paternity, cares for the eggs and attracts other females to lay eggs in his nest.

2. Polyandry:

   • Lit. "many males" ($ext{poly}$ = many, $ext{andry}$ = male).

   • A single female mates with multiple males.

   • Benefits for Females: While less common than polygyny, polyandry can offer specific fitness benefits to females:

     • Gifts from Males: Females may receive valuable resources or "nuptial gifts" from multiple mates. (e.g., Nursery Web Spiders: males present a wrapped prey item; if the female finishes unwrapping and eating it before mating is complete, she may eat the male. Some males even provide empty web balls as a deception).

     • Multiple Males Assisting with Young: In some systems, multiple males may help rear the offspring.

     • Ensuring Fertilization and Genetic Diversity: For species like honeybees, mating with multiple males increases the genetic diversity of the queen's sperm stores, which is crucial for the genetic health and productivity of the colony's workers.

3. Monogamy:

   • A single male and a single female mate exclusively with each other.

   • Presents a puzzle for males, given the potential fitness benefits of polygyny. Often involves shared parental care benefits outweighing the costs of foregoing additional mating opportunities.

Exaggerated Traits and Female Preference

Runaway Sexual Selection

• Sexual selection can lead to the evolution of incredibly elaborate, colorful, or seemingly impractical traits and behaviors (e.g., fancy courtship dances, oversized appendages, deep vocalizations).

• Stalk-Eyed Flies Example:

  • Males develop extremely long eye stalks, which are groomed after emergence to ensure maximum length.

  • Females strongly prefer males with the longest eye stalks.

  • Males compete by visually comparing eye stalk lengths. The male with the longest stalks typically wins mating access.

  • This is a classic illustration of female choice driving the exaggeration of a male trait to an extreme.

Other Examples of Female Preference

• Long-tailed Widowbirds: Females prefer males with long, showy tails, even if they hinder flight.

• Frogs: Females prefer males that produce deep "chucks," which serve as an honest signal of the male's body size and overall fitness.

• Zebra Finches: In experimental settings, females can develop preferences for unusual traits, such as red leg bands on males, demonstrating the plasticity of mate choice.

Lekking Behavior

• A mating system where males gather in a specific area (a "lek") that has no direct resource value, solely for the purpose of displaying to females.

• Sage Grouse Example: Males congregate on leks, fanning their distinct tail feathers and inflating chest air sacs to produce booming calls.

  • Female sage grouse visit the leks to observe the males and select mates.

  • Researchers have used artificial "female hand-boxes" equipped with cameras to study how males respond to and are chosen by females on these leks.

Development and Maintenance of Mate Preferences

• Imprinting: Mate preferences can be influenced by early developmental experiences.

  • Experiment: Female offspring raised with ornamented fathers developed a preference for ornamented males. However, females raised with unornamented fathers showed no particular preference, suggesting an experiential component to preference development.

• Honest Signals: Exaggerated traits often serve as honest indicators of male quality:

  • Health and Vigor: Brighter colors or longer, more elaborate tails can signal robust health, superior genetics, or strong immunity (e.g., low parasite burden). High parasite/virus loads are often inversely correlated with an individual's fitness.

  • Predator Avoidance: The ability to bear cumbersome ornaments while avoiding predators indicates exceptional fitness.

Male-Male Competition (Further Aspects)

• Beyond direct combat, male competition can involve complex strategies (e.g., distinct male morphs or territorial defense behaviors in certain lizards, such as Uta stansburiana, are worth further investigation as they illustrate diverse male competitive strategies).

Conclusion

Ultimately, the observed differences in male and female behaviors are largely explained by two fundamental factors:

1. Parental Investment: The relative amount of energy and resources each sex invests in offspring, and the certainty of contributing genetic material.

2. Sexual Selection: The evolutionary pressures driving competition for mates (intrasexual selection) and the preferences influencing mate choice (intersexual selection).