Chapter 1-7: Sexual Selection - Key Concepts Flashcards

Recap & Warm-Up Activities

  • Brief quiz on sibling rivalry (obligate vs. facultative cainism)

    • Older chick always kills the younger → obligate siblicide

    • Both chicks survive only in food-rich years → facultative

    • Nazca booby = obligate; Blue-footed booby = facultative

  • Quick brood-parasitism quiz (cuckoo vs. host)

    • Cuckoo lays mimetic eggs & removes a host egg immediately after laying

    • Host may abandon nest if cuckoo seen; relies on visual & auditory “passwords” to reject foreign chicks

Introduction to Sexual Selection (2-Lecture Block)

  • Central evolutionary question: Why are males usually colorful, weaponized, or perform elaborate displays while females are comparatively drab and choosy?

  • Today’s focus: evolutionary origins of sex differences in reproductive behaviour; tomorrow → sex-role reversal, mating systems, sexual conflict

Clarifying Key Terms

  • Sex = biological category (chromosomes, gametes, genitalia)

    • Not always binary; e.g.

    • Intersex conditions in humans

    • Simultaneous hermaphrodites (earthworms, many reef fish)

  • Gender = human social construct (internal identity); cannot be assigned to animals

  • Sexuality = patterns of sexual attraction & mating context, in both animals and humans

  • Writing tip: When describing animals’ reproductive categories, use “sex,” not “gender.”

Foundational Concepts Driving Sex Differences

Anisogamy ("unequal gametes")

  • Females: few, large, energetically expensive, often non-motile eggs

  • Males: numerous, tiny, energetically cheap, motile sperm

  • Illustrative numbers

    • Coho salmon ♂ lifetime sperm ≈ 100\,000\,000\,000 vs. ♀ eggs ≈ 3\,500

    • Superb fairy-wren ♂ testes store ≈ 8\,000\,000\,000 sperm at once

    • Single human ejaculate ≈ 350\,000\,000 sperm; ovaries hold only a few \times 10^2 egg precursors

Consequences of Anisogamy

  • Eggs = limiting resource → females typically the choosy sex

  • Males compete for access to rare, valuable eggs

  • Internal fertilization amplifies choosiness (females control fertilisation, invest heavily in post-zygotic care)

Bateman’s Principle (1940s fruit-fly experiments)

  • Male fitness (offspring #) rises roughly linearly with mating frequency
    \text{Fitness}{♂} \propto N{\text{matings}}

  • Female fitness plateaus after the first successful mating

    • One mating can theoretically fertilise all eggs for lifetime

  • Predicts: ♂ maximise quantity of mates; ♀ maximise quality of mates

Two Mechanisms of Sexual Selection

1. Intrasexual Selection ("within-sex" competition)

  • Usually ♂–♂ but can be ♀–♀

  • Favours evolution of weapons

    • Horns, antlers, tusks, enlarged mandibles, spines, etc.

Comparative Study: Bovid Horns

  • Data from 91 species

  • Variable of interest: mean group (harem) size vs. mean horn length

  • Pattern for males

    • Positive correlation: larger harems → longer horns

    • E.g. African buffalo: harem ≈ 50 ♀, long horns

    • Red-flanked duiker: harem ≈ 1 ♀, tiny horns

  • Females: horn length unrelated to group size (natural-selection function = anti-predator defence)

Alternative Mating Tactics (Condition-Dependent, NOT genetic)

  • Occur when a few dominant males monopolise fertilisations, leaving subordinates with “making the best of a bad job.”

  • Traits flexible; shift with body size, age, social context; unequal average fitness

Example: New Zealand Giraffe Weevil (Pepeke Nīkau)

  • Extreme body-size range: 12–90\,\text{mm} (both sexes)

  • ♂ rostrum = weapon; length scales steeply with body size

  • Large ♂ fight (head-to-head grappling; goal: flip rival off tree)

  • Small ♂ adopt sneak tactic

    • Slip between guarding male & ♀, achieve copulation unnoticed

    • Tactic reversible with growth; condition-dependent

Alternative Mating Strategies (Genetically Fixed Polymorphisms)

  • Discrete morphs; equal average fitness via negative frequency-dependent selection

  • Classic rock–paper–scissors dynamics

Example: Side-blotched Lizards (Uta stansburiana)

  • Three genetically encoded throat-colour morphs

    1. Orange ♂ (“rock”): ultra-dominant, large territories, many ♀

    2. Blue ♂ (“paper”): monogamous, small territory, guard intensely

    3. Yellow ♂ (“scissors”): sneaker, female mimic, no territory

  • Six-year population cycles: when one morph becomes common, the morph that beats it increases, etc.

  • Key distinction: morph = lifelong; strategy ≠ condition-dependent

2. Intersexual Selection (Mate Choice)

  • Often ♀ choosing ♂ ornaments/displays, but roles can reverse

Why Be Choosy? Benefit Categories

  1. Direct Benefits to female

    • Nuptial gifts (scorpion flies: prey item consumed during mating)

    • Enhanced parental care (♂ helps rear young)

    • Sexual cannibalism / male self-sacrifice

      • Red-backed spiders: ♂ somersaults into ♀ jaws after plugging genital tract → ♀ fertilises higher % of eggs, ♂ paternity assured

  2. Indirect (Genetic) Benefits to offspring

    a. Good-Genes Model

    • Ornament indicates overall viability or disease resistance

    • Example: Pronghorn antelope

      • ♀ assess males’ endurance displays

      • Offspring of preferred ♂ grow faster & survive predation better (survival curve: attractive-male fawns live longer across first 50 days)

    b. Sexy-Sons / Fisherian Runaway

    • Trait is heritable & attractive per se, not necessarily linked to viability

    • Choosing extreme ornament → sons inherit trait + preference remains in population → daughters prefer that trait → positive feedback loop

    • Hypothetical illustration with stalk-eyed flies

      • Slightly longer eye-stalk ♂ gets more mates → produces equally viable but more attractive sons → grandmother’s inclusive fitness rises via extra grand-offspring

Tactics vs. Strategies: Quick Checklist

Feature

Alt. Mating Tactic

Alt. Mating Strategy

Genetic basis

No (condition-dependent)

Yes (polymorphism)

Flexibility

Switchable within lifetime

Fixed for life

Fitness Equality

Usually lower than dominant tactic

Equal on average via freq-dependent selection

Examples

Sneak giraffe-weevil; satellite toad

Side-blotched lizard throat morphs

Key Takeaways

  • Sexual selection = special case of natural selection acting on traits that improve mating & fertilisation success, not survival.

  • Root cause of typical sex differences = anisogamy → Bateman’s Principle

  • Intrasexual competition drives evolution of weapons; intersexual choice drives ornaments & displays.

  • Extreme male-male competition can spawn alternative mating tactics; genetic morph systems create alternative mating strategies with cyclical dynamics.

  • Female choice persists because it provides

    • Direct material gains (food, care, protection)

    • Indirect genetic gains (Good Genes) or increased mating success of sons (Sexy Sons).

Connections & Real-World Relevance

  • Conservation biology: Understanding sexual selection helps predict which traits might be at risk if population densities drop (e.g., lekking birds of paradise).

  • Behavioural ecology methodology: Comparative datasets (bovid study) vs. field manipulations (pronghorn, stalk-eyed fly selection experiments).

  • Ethical/philosophical note: Avoid conflating human gender constructs with animal reproductive biology; appreciate diversity beyond binary frameworks.

Suggested Reading

  • Rubinstein, D. & Alcock, J. Animal Behavior (11th or 12th ed.)

    • Chapters on Sexual Selection & Parental Investment

What’s Next?

  • Lecture 2: sex-role reversal, mating systems (monogamy, polygyny, polyandry, promiscuity), and sexual conflict

  • Lab session tomorrow: refine research questions; assistance from Julie & demonstrators