Sexual Selection in the Sea

Darwin’s postulates & evolution

• IF– Variation: phenotypic variation among

individuals within population – Inheritance: some variation passed

genetically to offspring– Reprod. Excess: not all offspring survive– Selection = differential fitness!

• Survival & reprod. NOT random—some phenotypes have greater fitness than others

• THEN •EVOLUTION = composition of population changes generation to generation

Sexual Selection

Sexual selection = selection for traits concerned solely with reproduction

– Traits selected for via:

1) Intrasexual selection = competition for access to mates or resources mates attracted to

2) Intersexual selection = mate choice by one sex for members of the other sex

(i.e., males advertise, females choose best displays)

Sexual selection vs. natural selection: why the distinction?

Although the two forms of selection operate the same way, they can conflict with each other!!!

Traits increasing reproductive success may be detrimental to survival

Asymmetry of the Sexes

Anisogamy = “gametes of size”Females: few large gametes

(eggs = expensive)Males: many small gametes

(sperm = cheap)

Conflict between sexes!

Reproductive Fitness: Females: low variation Males: high

variation

Bateman’s Principle

•RS depends on access to mates

•RS depends on ability to produce eggs & rear young

= choosy = competitive

If this pattern holds…

What determines strength of sexual selection?

1. Operational Sex Ratio (OSR) = the ratio of receptive females to receptive males

2. Parental Care• Greater the difference in PI between

sexes = strong SS

3. Mating system

OSR = receptive females/receptive males

OSR skewed toward one sex or the other (biased sex ratio) strong SS

Males >> Females males compete

Females >> Males females compete

Parental InvestmentPI = costly parental activities that increase the

chance of survival for offspring, but decrease the chances of producing additional offspring

Male PI = Female PI SS minimized

Greater asymmetries in PI between parents/sexes stronger SS

Males >> Females Females >> Males

Mating Systems

• Monogamy = males pair with single female for breeding season

• Polygamy: more than one mate– Polygyny = males mate with multiple

females– Polyandry = females mate with multiple

males– Polygynandry = males and females

mate multiply

Sexual Selection in Elephant Seals

Sexual Dimorphism

Body Size: males = HUGE!!!

Snouts

Elephant Seals: Mating System

Polygynous mating system

Males battle for territories

Largest males win

battle scars

Elephant Seals: Mating System

Females choose largest males

Males defend harems

Alternative strategy: sneaker males

http://www.biosbcc.net/ocean.htm

Inter-sexual selection: Inter-sexual selection: which sex chooses?which sex chooses?

• Sex with greater parental investmentSex with greater parental investment– Individuals with more invested have more to Individuals with more invested have more to

loselose– Choose mates that will “maximize return” Choose mates that will “maximize return”

on that investmenton that investment

• Females generally invest more into Females generally invest more into offspring so they are most often offspring so they are most often choosierchoosier

Dr. Pfennig, UNC Biol 63 lecture

Why choose?

• Direct benefits– Nuptial gifts– Parental care– Fertilization success– Avoidance of parasites

• Indirect benefits– “Good genes” (Zahavi 1975; Hamilton & Zuk

1981)– “Sexy sons” (Fisher)

Sex Role Reversal• Sex role reversal

– Females compete– Males choose

• Ubiquitous in the Family Syngnathidae)– Seahorses– Pipefishes– Sea dragons

Sea dragon

Pipefish

Seahorseswww.seaworld.org/.../ fish/bony/sea-horse.htm

www.abc.net.au/.../ img/water/sealeafy.jpg

Adam Jones

Male Seahorses

www.mbayaq.org/.../ content/images/ssh_sea_02.jpg

Adam Jones

brood pouch

eggs

When would you expect sex role reversal to occur?