Sexual Dimorphism

Sexual Size Dimorphism iscommonMale BiasedFemale Biased

Sexual Size Dimorphism• What are the selective pressuresresponsible for the evolution andmaintenance of SSD?• To what extent is the evolution of SSDconstrained by the shared genomes ofmales and females?

Optimal body size differsDisruptive Selection

Why?• Sexual selection• Resource division• Dimorphic niches

Resource division• Food competition between sexes

Dimorphic niches• Fecundity selection– For large females size = fecundity• Protandry– Early male maturity at size = mating

Sexual selection• Male preference for larger females• Small male size for display agility• Large male size confers advantage– Male-male competition– Nest defense against predators

Other Hypotheses• Sex conflict – harassment, cannibalism• Physiological tolerance– differential growth rates• Gravity hypothesis– Male male compOthers?

Why?• Sexual selection• Resource division• Dimorphic nichesHow would you test amongThese 3 hypotheses?Paper & predictions

Allometry of Sexual SizeDimorphism• Degree of SSD is often afunction of body size

AllometryPower Law:y ∝ x blog(y) ~ blog(x)€€

Allometryb > 1:Hyper-allometryLog (Male Size)b = 1: Isometryb < 1:Hypo-allometryLog (Female Size)

Allometry of SSDMain large scale pattern:• Rensch’s Rule

Rensch’s Rule: male-biased SSDAs body size increases SSD decreasesB > 1Log (Male Size)Log (Female Size)

Rensch’s Rule: female-biased SSDAs body size increases SSD decreasesB > 1Log (Male Size)Log (Female Size)

Non-Rensch’s AllometryFemale-biased SSDAs body size increases SSD increasesB < 1Log (Male Size)Log (Female Size)

Hypotheses• Rensch’s Rule: b > 1– Sexual Selection for large males– Differential-Plasticity (physiologicaltolerancedimorphism)• Non - Rensch’s Allometry: b < 1– Fecundity Selection– Protandry– Sex selection for large females

SSD Allometry in Horseshoe Crabs

Female Biased SSDFemaleMale

Log Male SizeSSD Allometry is Isometric in HSCs3.23.0b = 1.03P = 0.5932.82.62.6 2.8 3.0 3.43.2Log Female Size

Dimorphism and Latitude0.400.36R 2 = 0.62F = 18.5P = 0.001Dimorphism ((F/M)-1)0.280.240.2020 25 30 4035Latitude45

SSD and Latitude• Other studies have found same pattern– due to sex differences in growth rate• Suggests females can’t grow as large innorthern populations

SSD and alternative tactics• Female mimics & Large dominant males• Linkage and pleiotropy restrict evolutionof alternative phenotypes

State of field• More studies needed on fecundityselection, display agility, resourcedivision• Mechanisms: condition dependence,• Understanding exceptions to Rensch’srule – environments? LH traits?• Not well organized conceptually - IMHO