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EEOB 400: Lecture 9 Sexual selection Sexual dimorphism Sexual dimorphism Sexual dimorphism Photo: Bill Love “Considering that colors of chameleons often reflect their “mood”, one has to wonder why mating elicits such a different color response in males and females.”

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EEOB 400: Lecture 9

Sexual selection


Sexual dimorphism

Photo: Bill Love

“Considering that colors of chameleons often reflect their “mood”, one has to wonder

why mating elicits such a different color response in males and females.”

- Lizards: Windows to the Evolution of Diversity (2003) L.J. Vitt & E.R. Pianka


Sexual dimorphism






Sexual dimorphism

Sexual size dimorphism


The peacock’s tail

Extravagant male ornaments

The peacock’s tail greatly impairs his mobility…how could such a trait evolve?


Sexual selection

Darwin’s second “major” book:

1871 On the Descent of Man, and

Selection in Relation to Sex

Why a theory of sexual selection?

Darwin needed a theory to explain the many

extravagant traits that seem to reduce survival

e.g. the peacock’s tail

What is sexual selection?

Sexual selection is distinguished from

natural selection by the following criterion:

Sexual selection arises through variance in

mating success


Sexual selection

Is sexual selection different from natural selection?

Darwin saw them as distinct - only sexual selection

could produce traits that compromise survival

The basic principles are identical – selection favors

whatever gets more genes into the next generation

In sexual selection, fitness is measured relative to

members of the same sex

Two kinds of sexual selection

Intrasexual selection– mating success determined by within-sex interactions

e.g., male-male combat

Intersexual selection – mating success determined by between-sex interactions

e.g., female choice of males

(also referred to as epigamic selection)


Mating systems

Monogamy One male mates exclusively with one female

Polygamy Individuals mate with more than one partner

Polygyny Some males mate with more than one females

Polyandry Some females mate with more than one males

Promiscuity Males mate with more than 1 female and vice versa





Mating systems

Mating systems influence sexual selection

Strength of sexual selection

Variance in mating success

Some males = 0 mates

Some males = 1 mate

Some males = 2 mates

All males have

same mating

success = 1 mate

Most males = 0 mates

One male = 8 mates

Strong Polygyny


Moderate Polygyny


Harem polygyny

Elephant seals (Mirounga)- breeding females cluster together on beaches

- allows males to defend a harem of many females at once

Bull male elephant seals engage

in violent, bloody fights over

females – large size confers an

advantage in male combat

Male elephant seals weigh up to 3x more than females!

Male reproductive success is highly variable:

8 individual males inseminated 348 females in one study!


Lekking polygyny

Lekking - males aggregate in particular areas called leks, display for females


Combination of male competition…

Males may fight for position in center

of lek

…and female choice:

Females choose a mate…often dominant male or male in the center

Lekking in Black Grouse, Fallow Deer & Stalk-Eyed Flies


Territorial defense polygyny


An area that is defended for exclusive

use of the defenderagainst rivals

Territories may be defended by males

or females and for multiple purposes

What is being defended?

Sometimes territories are defended simply for resources:

food, basking sites, dens or hiding places, etc.

This may still be important for sexual selection, e.g. if females “choose” male territories based on the resources within the territory

In many species, males set up territories around females (or vice versa) – in this case it becomes similar to a harem defense polygyny


% females % males

# offspring surviving to 1 yr.

Reproductive success is more

variable in males than females

Many stags never reproduce, some

may sire up to 24 offspring!

Male combat

Red Deer (Cervus elaphus)

Why are these

males fighting?


Alternative reproductive tactics

Remember…there may not be a “best” way to be a male (or female)


Sex roles

Macho males and choosy females?

Whydo males and females fall into these “typical” sex roles?

In part, it reflects a bias in the species typically studied…

…but is there also some inherent biological reason?

Bateman’s principle:

The sex which invests the most in offspring will become a limiting resource

over which the other sex competes ( = sexual selection)

Anisogamy: difference in the size of male and female gametes

Gametic investment: sperm are consequently cheaper than eggs

This predisposes females to a greater level of parental investment

Males can easily produce enough sperm to fertilize all of a female’s eggs,

but the reverse is not true:

mating opportunity limits male reproductive success

fecundity limits female reproductive success




Most jacana species exhibit harem polyandry

and “sex role reversal”

Males maintain small territories

Males perform all parental care

Females mate with multiple males

and then leave eggs with males

The number of males a female mates

determines her reproductive success

because she doesn’t care for eggs

Sexual dimorphism

Females larger than males by 60% in mass (unusual for birds)

Females aggressively fight other females and also kill their chicks (infanticide)



When is polyandry favored?

When males become limiting resource for reproduction

This often occurs when low offspring survival requires

male parental care, so that males have the greater

Reproductive effort and lower reproductive rate

What is the consequence?

Sexual selection is stronger on females than males

Saddleback Tamarins,

Spotted Sandpipers,

and Red-necked

Phalaropes are

examples of polyandrous



Intersexual selection

Intersexual selection has always been controversial

We know mate choice occurs because we can observe

it directly

However, it is debated if and how some sexually

dimorphic traits evolve in response to mate choice

Intra- and inter-sexual selection

Intrasexual selection

All of the examples thus far have involved interactions within a sex

- male-male combat

- sperm competition

- female competition in polyandrous species

Darwin’s theory of intra-sexual selection was readily accepted, even in his time


The peacock’s tail

Extravagant male ornaments

The peacock’s tail greatly impairs his mobility…how could such a trait evolve?


Male ornaments

Long-tailed widowbird

Experimental manipulation of tail length

Males with unnaturally long tails attract

females away from the nests of “normal”

males or males with shortened tails

Andersson (1982) Nature 299:818


Male ornaments

  • Barn swallow
  • Experimental manipulation
  • of tail length
  • Males with elongated tails:
  • Obtain mates more
  • quickly
  • Have greater reproductive
  • success
  • Experience a cost in
  • tail size the following year
  • From Moller (1994)

Zahavi’s “handicap” hypotheses

Extravagant male traits are costly to develop and maintain

Choosing a mate with “good genes” requires an honest

signal of genetic quality

Only males in good condition (those with good genes)

will be able to fully develop and maintain an ornament

Amotz Zahavi

Hypotheses for male ornaments

Fisher’s “runaway” hypothesis

Mate choice originally evolved to facilitate adaptive

choice for traits conferring a survival advantage

Once female preference evolved, any genes that

conferred survival advantage but compromised

attractiveness would not be passed on because

surviving males would fail to mate

Ronald Fisher


Survival Selection

Sexual Selection

Female choice

adaptive for survival

Total male fitness

(survival + mating)


Fitness due to survival

Tail length

Fisher’s runaway model


Zahavi’s handicap hypothesis

Some candidate “handicaps”

Note that the handicap itself need not be heritable…it need only provide a

reliable index of fitness, and fitness must be heritable

Traits that encumber the

owner are physiologically

costly (exertion in flight)

as well as being more

expensive to develop

Asymmetry is indicative

of developmental

instability and possibly

“bad genes”. Symmetry

is chosen in some species

Bright color honestly signals

immunocompetence and

parasite/disease resistance


Natural selection (predation) favors

spot patterns that match background

Sexual selection (mate attraction) favors

male patterns that contrast background

Sexual vs natural selection

Guppies (Poecilia)- sexual selection can favor traits that reduce survival

- laboratory selection studies by John Endler


Cost of large size

  • During El Nino years, food is scarce
  • The largest iguanas are unable to meet
  • their minimum energy demands and
  • literally starve to death
  • Most of the largest animals are males,
  • so sexual and natural selection have
  • conflicting effects on male body size

Good times, bad times

Sexual vs natural selection

  • Marine iguanas (Amblyrhynchus cristatus)
  • One of the few lekking reptiles
  • Large males most successful at lekking
  • Sexual selection favors large males
  • Males 2x more massive than females

Martin Wikelski’s website:


Sexual dimorphism and sexual selection

  • Sexual selection
  • Selection arising from variance in mating success within a sex
  • Not all sexual dimorphism arises from sexual selection
  • Natural selection can also differ between sexes
  • Fecundity selection
  • Selection on female fecundity =
  • number of offspring produced
  • Fecundity is different than mating
  • success = number of mates
  • In many species where females are
  • larger than males, dimorphism is though
  • to reflect fecundity selection favoring
  • large femlae size, since larger
  • females can produce more eggs

Sexual dimorphism and sexual selection

  • Niche divergence
  • Males and females evolve to fill different ecological niches, adaptive if it
  • reduces competition for limited resources (e.g., food)
  • - Another example of natural selection causing sexual dimorphism