1. EEOB 400: Lecture 9
Sexual selection

2. Sexual dimorphism

3. Sexual dimorphism

4. 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

5. Sexual dimorphism
Weapons

6. Sexual dimorphism
Weapons
female
male
male

7. Sexual dimorphism
Ornaments

8. Sexual dimorphism

9. Sexual dimorphism
Sexual size dimorphism

10. Bizarre sexual dimorphism

11. The peacock’s tail Extravagant male ornaments
The peacock’s tail greatly impairs his mobility…how could such a trait evolve?

12. 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

13. 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)

14. 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
Monogamy
Polygyny
Promiscuity

15. Mating systems Mating systems influence sexual selection
Strength of sexual selection
Variance in mating success
All males have
same mating
success = 1 mate
Some males = 0 mates
Some males = 1 mate
Some males = 2 mates
Most males = 0 mates
One male = 8 mates
Monogamy
Moderate Polygyny
Strong Polygyny

16. Harem polygyny
Elephant seals (Mirounga) - breeding females cluster together on beaches
- allows males to defend a harem of many females at once
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!
Bull male elephant seals engage
in violent, bloody fights over
females – large size confers an
advantage in male combat

17. Lekking polygyny
Lekking - males aggregate in particular areas called leks, display for females
Lek
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

18. Territorial defense polygyny
Territory
An area that is defended for exclusive
use of the defender against 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

19. Male combat Red Deer (Cervus elaphus) Why are these males fighting?
Reproductive success is more
variable in males than females
Many stags never reproduce, some
may sire up to 24 offspring!
% females % males
# offspring surviving to 1 yr.

20. Alternative reproductive tactics
Remember…there may not be a “best” way to be a male (or female)

21. Sex roles Macho males and choosy females? Bateman’s principle:
Why do 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

22. Polyandry Jacanas Sexual dimorphism
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)

23. Polyandry When is polyandry favored? What is the consequence?
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
species

24. 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
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

25. The peacock’s tail Extravagant male ornaments
The peacock’s tail greatly impairs his mobility…how could such a trait evolve?

26. 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

27. 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)

28. 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
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

29. Fisher’s runaway model
Fitness
Tail length
Total male fitness
(survival + mating)
Fitness due to survival
Survival Selection
Sexual Selection
Female choice
adaptive for survival

30. 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

31. Sexual vs natural selection
Guppies (Poecilia) - sexual selection can favor traits that reduce survival
- laboratory selection studies by John Endler
Natural selection (predation) favors
spot patterns that match background
Sexual selection (mate attraction) favors
male patterns that contrast background

32. 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
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
Martin Wikelski’s website:

33. Sexual dimorphism and 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

34. 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