Mating Systems in Animal Reproduction - Key Concepts (Lecture 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