Sexual Selection & Reproductive Behaviour – Comprehensive Study Notes

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

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