Animal Mating Systems – Comprehensive Study Notes

Key Definitions & Classification of Mating Systems

• Mating system = complete suite of behaviours/strategies governing:
  • How mates locate one another
  • Frequency/number of matings
  • Nature of pair‐bonds
  • Extent/type of parental care
• Principal axis of classification: number of mates per sex
  • Monogamy – each sex mates with only one partner
  • Often involves long-term pair bonds & biparental care
  • Polygamy (umbrella term = ≥1 sex mates with >1 partner)
  • Polygyny – 1 male, \ge 2 females
  • Polyandry – 1 female, \ge 2 males
  • Polygynandry – both sexes mate multiply but retain semi-stable pairings/coalitions (e.g.
    chimps, bonobos)
  • Promiscuity – both sexes mate multiply, no stable bonds or territories

Why Is Monogamy Paradoxical for Males?

• Classic sexual-selection theory predicts males boost fitness by maximising number of matings
• Yet monogamy is common (esp. birds)
• Key ecological variables:
  • Spatial/temporal distribution of females ⇒ “economic defendability”
  • Dispersed/isolated females → hard to guard >1 ⇒ monogamy favoured
  • Clumped females/resources → polygyny easier (male defends clump)
  • Requirement for biparental care (offspring survival ↑ when both stay)

Three Main Hypotheses for the Evolution of Monogamy

• Mate-Guarding Hypothesis

  • Male remains with single female to prevent rival insemination
  • Favoured when
  • Female receptive period short but allows re-mating (high sperm-competition risk)
  • Females scarce/hard to locate
  • Example: Hulagin shrimp – dispersed females & brief receptivity

• Mate-Assistance (Biparental Care) Hypothesis

  • Male gains greater fitness by aiding offspring than by seeking extra matings
  • Empirical test: California mice (Peromyscus californicus)
  • Warm conditions: father presence = no significant litter‐survival gain
  • Cold conditions: litter survival jumps from \approx30\% \to 80\% when father present
  • Mechanisms: thermoregulation, joint foraging, protection

• Female-Enforced Monogamy

  • Female behaviour prevents male from attracting/accepting extra mates
  • Burying beetles (Nicrophorus spp.)
  • Males emit pheromone atop carcass to lure more females
  • Resident female mounts & physically blocks male; experiment with tethered female ⇒ males signalled >4× longer when unrestrained

Monogamy Across Taxa

• Birds: \approx90\% form social pairs
  • BUT genetic studies show social ≠ genetic monogamy
  • DNA fingerprinting (1980s-90s) ➔ widespread extra-pair copulations (EPCs) & extra-pair paternity (EPP)
  • Meta-analysis n=150 spp: >90\% show some EPP; strict genetic monogamy in <25\%
  • Extremes:
    • Reed bunting – >50\% offspring, 86\% broods contain EPP
    • Superb fairywren – 72\% offspring, 95\% broods with ≥1 EPP chick
  • Rare true monogamy: North Island saddleback/Tieke
• Mammals: only 3\text{–}5\% monogamous
  • Constraints: internal gestation, lactation, often precocial young → limited male role

Costs & Benefits of EPCs / Polyandry within “Monogamy”

• Male benefits: extra offspring; costs: time away → own mate may re-mate, paternity loss
  • Example: Bank swallows – males guard mates intensively during female fertile window, then seek EPCs themselves
• Female benefits (parallel to polyandry lecture):
  • Good genes – increase genetic quality/viability of brood
  • Fertility insurance – guarantee fertilisation of all eggs (supported in blue tits)
    • Study: clutches of EPC females had significantly fewer unfertilised eggs
  • Material/resource gains – nuptial gifts, access to territories, etc.
• Female costs: desertion risk, loss of male care, disease, aggression

Case Study: New Zealand Tūī (Prosthemadera novaeseelandiae)

• Socially monogamous yet extreme EPP
  • 72\% of females mate outside pair; 57\% of offspring extra-pair
• Males \approx50\% heavier than females (unusual for socially monogamous birds)
• Data: negative correlation between male tarsus length (proxy for size) & proportion of EPP in his nest ⇒ larger males better at paternity defence/attraction
• Hypotheses: aggressive singing displays monopolise mates; large size evolved via sexual selection to reduce cuckoldry

Polyandry & Sex-Role Reversal

• Complete reversal: female competes, male provides sole care
  • Conditions: clutch transferable (eggs), high male care benefit > cost, female able to produce multiple clutches
  • Examples:
  • Spotted sandpipers (Actitis macularius)
  • Northern jacanas (Jacana spinosa)
  • Malleefowl (Leipoa ocellata)

Forms of Polygyny

• Resource-Defence Polygyny

  • Male monopolises critical resource → females visit
  • Example: Pseudoscorpions on harlequin beetles – male defends beetle’s elytra as “taxi” to rotten logs

• Female-Defence Polygyny

  • Direct guarding of female clusters (e.g., tree weaver birds, spear-nosed bats)

• Lek Polygyny

  • Males defend small display arenas devoid of resources
  • Female choice drives extreme skew
  • Sage grouse: top male ≈50\% of copulations; many males get 0
  • Other examples: cock-of-the-rock, fallow deer, mosquito swarms
  • NZ example: Lesser short-tailed bat (Pekapeka tō-roa)
  • Communal day roosts; at night males occupy “singing roosts” near colony; coat selves in urine; females visit briefly to mate

• Scramble-Competition Polygyny

  • Little/no aggression; success = speed & sensory ability
  • Two subtypes:
  • Explosive breeding assemblages – synchronised female availability (e.g.
    pond-breeding frogs)
  • Prolonged searching – females dispersed; selection for mobility
    • Cook Strait giant wētā – fastest, long-legged but light males arrive first & gain matings

Interactions with Sexual Selection & Ecology

• Economic defendability concept: costs to guard > returns = monogamy / polyandry; defendable clumps = polygyny
• Sperm competition shapes mate-guarding & EPC behaviour
• Parental investment theory: sex investing more in offspring (often female) becomes limiting resource, but role reversal possible under ecological/physiological constraints

Methodological & Ethical Notes

• DNA fingerprinting revolutionised behavioural ecology; exposed hidden genetic relationships
• Conservation implications:
  • Understanding genetic mating systems critical for managing small populations, ensuring genetic diversity
  • Example: translocations must consider true breeding structure, not assumed social pairs
• Philosophical: challenges anthropomorphic views of “faithfulness”, highlights adaptive rather than moral basis of behaviour

Quick Numerical/Statistical Summary

• Birds socially monogamous: \approx90\%; genetically monogamous <25\%
• Mammals genetically/socially monogamous: 3\text{–}5\%
• Superb fairywren EPP: 72\% offspring, 95\% broods
• Tūī EPP: 57\% offspring, 72\% females engage EPCs
• California mice: litter survival in cold ↑ from \approx30\% \to 80\% with male care
• Sage grouse lek: top ranked male ≈48\% of copulations (one study)

Key Take-Home Messages

• Mating systems are diverse, context-dependent, and often differ between social observations & genetic realities
• Distribution of females/resources + necessity of parental care jointly determine evolutionary stable strategies
• Monogamy persists via mate-guarding, cooperative parenting, or female enforcement, but is frequently violated via EPCs
• Polygyny takes multiple ecological forms; polyandry and role-reversal, though rarer, underscore flexibility of sexual selection dynamics
• Modern molecular tools are indispensable for uncovering true reproductive strategies, influencing both theory and conservation practice