Lecture 11 - Mating Systems and Parental Care

Learning Objectives

  • Describe the full spectrum of mating-system classifications.
  • Understand ecological drivers that link resource distribution to mating strategies.
  • Explain the diversity, costs, benefits, and evolution of parental care.

Key Concepts & Definitions

  • Anisogamy
    • Females produce few, large, energetically expensive eggs.
    • Males produce many, tiny, energetically cheap sperm.
    • Sets up male–male competition and potential for males to mate with several females ⇒ polygyny.
  • Sexual selection acts more strongly on the sex with the higher variance in reproductive success (usually males).
  • Polygyny threshold
    • Minimum territory or male-quality difference at which a female gains higher fitness joining an already-mated male than pairing with an unmated male of lower quality.
  • Philopatry – return to natal site for breeding; prominent in jacanas.
  • Iteroparity vs. Semelparity
    • Iteroparity: repeated breeding at intervals.
    • Semelparity: breed once explosively then die (e.g., coho salmon).
  • r– and K-selection
    • r ≈ intrinsic reproductive rate; K ≈ carrying capacity.

Spatial Distribution of Resources & Mating Systems

  • Fig 18.1 demonstrates that where food is patchy and predators present, large groups form; multiple mating for both sexes likely ⇒ polygyny.
  • Dense forests hinder signalling → monogamy becomes adaptive because finding multiple mates is costly.

Classification by Social Association

• Monogamy – 1 ♂ : 1 ♀
• Polygyny – 1 ♂ : ≥2 ♀♀
• Polyandry – 1 ♀ : ≥2 ♂♂
• Promiscuity – no prolonged bonds; multiple mating by ≥1 sex.
• Polygamy (generic) – umbrella term for all non-monogamous systems.

Ecological (Operational) Classification

  • Monogamy – neither sex can monopolise >1 mate.
  • Polygyny
    • Resource-defence
    • Female-defence
    • Male-dominance (lek)
  • Polyandry
    • Resource-defence polyandry
    • Female-access polyandry (♀♀ limit access to ♂♂ without defending resources)

Monogamy

  • Occurs when renewable resources are scarce & widely spaced.
  • Benefits
    • Mate assistance in rearing offspring.
    • Reduced search time; long-term pair bonds.
    • Sea-gulls: re-pairing with former mate ⇒ higher success (less aggression; synchronised courtship).
  • Stats: ≈90\% of bird species are socially monogamous.
  • Small social units reduce predation risk.

Polygyny

Resource-Defence Polygyny

  • ♂ defends critical sites (feeding / nesting).
  • Female choice depends on territory quality.
  • Example: walnut flies; variable territory quality explains polygyny threshold.

Female-Defence Polygyny

  • Females are already gregarious (e.g., seals limited to haul-out sites).
  • ♂♂ guard clusters of females; extreme male–male competition and high variance in male fitness.

Male-Dominance Polygyny (Lekking)

  • ♂ provides no resources or care.
  • ♂♂ aggregate in leks = small defended display arenas.
  • Females visit, choose, copulate, depart to rear young alone.
  • No direct benefits; indirect (good genes) or copying.
  • Hammer-head bats: 6\% of ♂♂ obtain 79\% of copulations.
  • Ungulates:
    • Topi: large ♂♂ solo territories most successful; smaller ♂♂ cluster in leks.
    • Uganda kob: opposite pattern—lek males outperform solitary ones.

Scramble Competition Polygyny

  • No territories or dominance.
  • Explosive breeding assemblages: frogs in temporary ponds; horseshoe crabs intercept females on shore.

Polyandry

  • Rarer because egg investment > sperm.
  • Conditions favouring: unpredictable food, high nest predation ⇒ females lay multiple clutches while ♂♂ incubate.
  • Birds with male-only incubation (e.g., spotted sandpipers, jacanas).

Case Study: American Jacana

  • ♂ territories small; ♀ super-territories encompass several ♂ nests.
  • ♀ 50 % larger; provide little direct care; dominance over ♂♂.
  • Functional reversal of polygyny: ♀ specialise in egg production.

Ecology & Mating: Blackbird Case Study

  • Red-winged blackbird (polygynous)
    • Stable, long-lasting food (seeds/insects).
    • ♂ returns early, defends territory for months.
    • ♂ provides no care; ♀ builds nest & feeds young.
  • Tricolored blackbird (nomadic, quasi-monogamous pairs within huge colonies)
    • Food in temporary, concentrated patches.
    • Colony relocates once food depleted; synchronous, rapid breeding (≤1 wk).
    • Both sexes invest heavily but over short period.
  • Conclusion: resource temporal stability shapes social system.

Alternative Reproductive Tactics (ARTs)

Coho Salmon

  • Two irreversible life-history tactics chosen at ~6 mo:
    • Hooknose: large, red, weaponised; tactic = fight.
    • Jack: small, early-maturing; tactic = sneak.
  • Reversible behavioural tactics while on spawning ground: fight or sneak (matrix in Fig 18.8).

Marine Isopod (Paracerceis sculpta)

  • Three male morphs with equal lifetime success:
    • Alpha: large, defend sponge harems.
    • Beta: medium, female mimics; gain coaxial matings.
    • Gamma: tiny, mobile sneakers; infiltrate harems.

Parental Care Spectrum

  • Increases with organismal complexity.
  • Extremes:
    • Broadcast spawning fish/invertebrates ⇒ no care.
    • Primates: up to 25\% of offspring lifespan invested.
  • Birds: diverse strategies
    • Megapodes bury eggs in compost heaps for heat.
    • Penguins fast & incubate without feeding.

Ecological Determinants of Parental Effort

  • Reproductive effort = energy + risk now that reduces future reproduction.
  • Stable environments (K-selected)
    • Large body, slow development, low fecundity, high care.
    • Home ranges; competition intense; strategy — quality over quantity.
  • Fluctuating environments (r-selected)
    • Rapid dev., high fecundity, low care.
    • Populations shaped by abiotic mortality.
    • Coho salmon: huge pre-spawning costs ⇒ semelparity; death after breeding.
  • Scarce/difficult food promotes prolonged dependency for learning.

Which Sex Should Provide Care?

Confidence of Parentage (Trivers 1972)

  • Internal fertilisation ⟹ male uncertainty; may desert.
  • Female certain ⇒ higher likelihood to stay.

Association with Embryos (Williams 1975)

  • Care evolves in sex physiologically linked to embryos.
    • Internal fert. & gestation: favours female care.
    • External fert. in male territory: favours male care.
  • Teleost data (Table 18.1):
    • Internal fertilisation families – mostly female care (14 vs 2 ♂).
    • External fertilisation families – male care dominates (245 families).

Taxon-Specific Rules

  • Birds: bi-parental care common; males incubate, feed, even secrete crop milk (pigeons).
  • Mammals: gestation/lactation restrict direct male care; hence polygyny common, especially in precocial species. Altricial young require more biparental input.
  • Insects show extremes from none to male egg-brooding (water bugs, Fig 18.13).

Differential Investment Curve (Fig 18.10)

  • Early: male territory defence costly > female.
  • Mid: egg production swings cost to female.
  • Later: male incubation > female.
  • Implication: dynamic cost‐benefit influences mate desertion decisions.

Example: Elephant Seals

  • Highly polygynous colonies.
  • Sexual dimorphism: ♂ ~3× size; enlarged proboscis & chest shield.
  • Dominance hierarchy; <\tfrac13 males copulate.
  • Zero paternal care; pups can be trampled inadvertently during copulations.
  • Graph (Fig 18.12) shows positive relationship between dominance rank & copulatory success.
    • Prediction: males at lower left (low rank) likely young/subordinate; upper right are older, larger, high-status.

Biparental & Male-Biased Examples in Mammals

  • Silver-backed jackals: both sexes hunt & defend territory cooperatively.
  • Tamarins/marmosets: males carry neonates immediately after birth ⇒ maternal feeding efficiency.

Parent–Offspring Recognition

  • Adaptive to avoid misdirected care.
  • Modal cues vary:
    • Birds: vocal signatures; cliff swallows have complex individual calls vs barn swallows (Fig 18.15) due to colony size differences.
    • Dolphins: individual whistle signatures used for mother–offspring & group recognition.

Parent–Offspring Conflict (Trivers 1974)

  • Ratio of cost to mother / benefit to offspring increases with offspring age (Fig 18.16).
    • <1 ⇒ mutual benefit.
    • 1–2 ⇒ conflict zone; offspring still benefits, mother fitness declining.
    • >2 ⇒ both lose; offspring weans.

Ethical & Evolutionary Implications

  • Understanding mating systems informs conservation (lek species vulnerable to over-harvest of top males).
  • ARTs illustrate that multiple strategies can be evolutionarily stable (isopod morphs equalised fitness).
  • Human parallels: resource distribution & parental investment theories contribute to anthropology and social policy debates.