Sexual Selection & Mating systems

I. Sexual selection overviewA. HypothesisB. MechanismsC. Natural Selection v. Sexual SelectionD. Males more affected

II. Mating systems & sexual selectionA. Scramble competitionB. Mate Guarding BehaviorC. Leks & ChorusesD. Satellites & InterferersE. Sperm Competition

I. Sexual Selection overview

Defined as: Directional selection that acts on genetically variable phenotypic traits that affect the reproductive success of the individuals of a particular sex

Sexual selection can explain the persistence of conspicuous differences between females and males

A. Hypotheses: Why do females prefer certain phenotypic traits?

1) Direct Benefit: Certain male characters provide a direct benefit to females in terms of increased fecundity.

2) Good Genes

Male characters are "indicators" of "good genes",

3) Sensory bias or sensory drive

some aspect of the sensory world biases

females to "prefer" or notice ascertain

characters – • eg, if the species feeds on red prey, their

visual system may be tuned to red wavelengths, therefore, females prefer males with red ornaments

B. POSSIBLE MECHANISMS:

1) Pre-mating sexual selection:• selection for species recognition (females mating w/own

species) could possibly lead to elaborate secondary sexual characteristics

• females could detect differences in the number of deleterious alleles in an male

2) Post-mating sexual selection:• sperm competition • polyandrous species have larger sperm, faster swimming

sperm, more aggressive sperm

C. Natural Selection v. Sexual Selection

D. Males more affected by sexual selection than females

Females –

Males – (sperm is cheap) spend energy on mate selection ability, therefore under greater sexual selection pressure

II. Mating systems & sexual selection Polygamy - single individual mates with more

than one individual of the opposite sex (resource use)

Polyandry – (male defense, resource defense) Polygyny – (female defense, resource defense, Lek,

Scramble competition) Monogamy – 1 male mates with 1 female (mate

guarding/assistance)

Dendrobates vanzoliniiDendrobates ventrimaculatus

Larval habitat affects mating strategies

A. Scramble competition

Explosive breeders = Female available for short periods, spatially aggregated

1) Scramble competition = males compete to locate females as fast as possible, mate as fast as possible & with as many females as possible

Rana sylvatica

Bufo bufo, Scaphiopus

B. Mate Guarding Behavior If searching for mate is costly, then it

may be worthwhile to invest energy in guarding her from other males

Monopolize the female strategy

C. Leks & Choruses Lek =

Chorus = anuran males calling from particular perches… females approach the males

D. Resource Defense Monopolizing resources

which are attractive to females gives males an advantage

Plethodon cinereus

E. Satellite frogs & Sexually interfering salamanders, lizards & snakes

Satellite males – don’t vocalize, but rather wait near a calling male to intercept females that are attracted to the calling male

Interfering males – prevent female from mating with another male

Rana clamitans

High numbers of male garter snakes die soon after emerging from hibernation because they are attacked by crows. She-males at the center of a mating ball, however, are less exposed to predators – also remain warmer…

Thamnophis sirtalis parietalis – males swamp the few females that emerge…

F. Sperm Competition

Multiple paternity Genetically

superior sperm more likely to survive (better to mate with several males)

Chiromantis xeramplina – African gray treefrog