Sexual Selection

1. 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

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

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

4. 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

5. 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 2) Sperm competition 45 kg: 110g 70 kg:40g 200 kg: 30 g

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

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

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

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

10. 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

11. Runaway Sexual Selection • Innate female preference for showy M

12. 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  Survival + Mating  Mating

13. 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

14. 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 Runaway vs. Good Genes

15. 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