Sexual Selection differences among indivs in mating opportunity

Sexual Selection differences among indivs in mating opportunity • NS: who SURVIVES • SS: who REPRODUCES • SS can explain some deleterious/costly characteristics that can’t be explained by NS Ornaments/Courtship Displays/Fights Sexual Dimorphism

Types of Sexual Selection Competition (usually M) • Sperm is cheap • RS with # of matings Choice (usually F) • Eggs are expensive • No Δ in RS with # of matings e. g. Newts # offspring # mates

Sexual Selection depends on Investment • Higher investment = limited resource • Usually Indiscriminate M & Choosy F But, Sex role reversal/ biparental care/nuptial gifts

Male-male competition 1) COMBAT • Competition for access to F or territory • Correlation b/w dominance rank & # of matings • Size often important in competition (Sexual Size Dimorphism ~ Degree of Polygyny) e. g. 1: Elephant Seals e. g. 2: Male marine iguanas larger than optimal for survival

2) Sperm competition Fertilization more important than mating 1) Large ejaculate/lots of sperm 2) Mate guarding 3) Copulatory plugs 4) Sperm removal Comparative method: relative testis size often correlates to mating system e. g. Primates 45 kg: 110 g 70 kg: 40 g 200 kg: 30 g

3) Infanticide • Males increase repro opportunities • Not good for female • Often female has no choice • e. g. Lions

Female Choice – Benefits? On Basis of Resources vs. In Absence of Resources e. g. Scorpionflies “making males pay” Nuptial gift: arthropod – salivary – forced copulation

male territory quality ranked according to shade Polygyny Threshold e. g. Lark Bunting • choose on the basis of M territory • Fitness benefits day of female arrival

Direct Benefit F may choose on the basis of : gift, paternal care, territory If no M investment, how do F choose?

Non-resource based Female Choice • Mutant F with no preference for showy M • non-showy M offspring survive better • M offspring not preferred as mates survival benefit must > repro benefit

Runaway Sexual Selection • Innate female preference for showy M

The ‘Runaway’ Process • SS trait associated with higher survival • Mutant F (preference) = RS b/c high quality sons • 2 advantage: M with trait preferred by high proportion of pop’n as mutation spreads • Reinforcement • Offspring carry genes for trait & preference (linkage disequilibrium) Advantage: Survival + Mating

Honest Advertisement • F preference for traits which demonstrate good quality • Zahavi – Handicap Principle – Characters that reduce survival – “I must be good to pull this off” (good genes) – Indicator must be costly so M can’t cheat – Often cost of trait is less for high quality males

Runaway vs. Good Genes • Open-ended • Heritable variation in male character • SS b/c sons have mating advantage • Open-ended • Heritable variation in male fitness • SS b/c sons have survival advantage

Indicators of Health • e. g. guppies – orange: diet quality • Special case of “good genes”: – Hamilton & Zuk : parasite load & plumage brightness – Species (not individuals) with brighter plumage gen’lly higher parasite load able to use colour as indicator of risk of parasitism