Transcript Document

• Female reproductive success is largely
determined by parental effort
• Male reproductive success is largely determined
by mating effort
• Because females invest much time and energy
in offspring, a wasted mating is costly
• Therefore, females are predicted to be more
choosy in selecting mates than males
Female choice of mates:
•
•
What criteria do females use in choosing
mates (e.g. resources, ‘good genes’)?
How important has female choice been
in the evolution of male traits that
are:
- not used in male-male competition?
- costly in terms of the male’s survival?
(e.g. the peacock’s tail, bright color in
many male birds)
How do females benefit from choosing
particular males?
•
•
Direct (non-genetic) benefits
- Better resources
- Increased parental investment
Indirect (genetic) benefits
- Fitter offspring
How do females benefit from choosing
ornamented males?
•
•
Are benefits direct?
- Better resources
- Increased parental investment
Are benefits indirect?
- Does the trait indicate good genes?
- Is it an honest reflection of male quality
(e.g. survival, resistance to
parasites)
- Is it costly (a handicap)?
- Is it also used in male-male competition?
Mean no. of nests per male
2
Before
1
0
Mean no. of nests per male
2
After
1
Widowbirds
0
Shortened Control 1 Control 2 Elongated
Tail treatment
Responses of female house finches to males of
different plumage color
Mean rank of male
4
3
2
1
0
Yellow
Orange
Orange-Red
Red
3
Feeding rate
2.5
2
1.5
Male fitness in
house finches
1
0.5
Son's plumage brightness score
0
100
150
120
140
Plumage brightness score
160
146
142
138
140
144
148
152
156
Father's plumage brightness score
160
Stickleback fish
• Male brightness is correlated with condition.
• Females prefer males with brighter bellies.
• Males infected with parasites are duller, not
chosen by females
Reliable cues that cannot be faked
• They seem arbitrary (bright colors,
elaborate song) but what they
indicate is not
• They are costly to acquire
• “Condition-dependent” traits
Stripe size of male offspring (mm)
Stripe size of male offspring (mm)
70
65
60
55
50
45
40
45
70
50
55
60
65
Stripe size of putative father (mm)
65
60
55
Heritability of
stripe in great tits
50
45
40
45
50
55
60
Stripe size of foster father (mm)
65
Mate choice and tail length in barn swallows
No of days to pairing
25
20
15
10
5
0
Shortened
Control 1
Control 2
Tail treatment
Elongated
Male fitness in barn swallows
100
80
No. of mites
on offspring 60
in other nests
40
20
0
90
100
110
120
Male tail length (mm)
130
High-ranking males
% of nests containing
extra-pair young
0.6
0.4
0.2
0
Test:
Aggressive
Playback
Control:
Submissive
Playback
Control:
No Playback
Low-ranking males
% of nests containing
extra-pair young
0.6
0.4
0.2
0
Test:
Submissive
Playback
Control:
Aggressive
Playback
Control:
No Playback
Proportion of male followings
1
Proportion < 5m
0.8
0.6
0.4
0.2
0
1994
1995
1994
1995
1
0.8
0.6
0.4
0.2
0
1994
1995
1994
1995
1
Song rate
0.8
Effects of
bands on
male bluethroats
0.6
0.4
0.2
0
1994
1995
BO males
1994
1995
Non-BO males
Can female choice lead to the evolution of traits
that are costly in terms of natural selection?
YES:
1. Females can use an elaborate trait to assess
a male’s resources, parental ability, and/or
genetic quality.
2. If the trait is costly, it is an honest signal of male
quality, because males in poor condition
can’t cheat.
3. Although the trait may be arbitrary, what it
signals is not.
** The cost component is crucial **
Daughter's preference for red
Stickleback fish
18
8
-2
-12
-22
0.72
0.74
0.76
Son's intensity of red coloration
0.78
1. If the male trait and the female’s preference
are heritable, the ‘trait’ gene and the
‘preference’ gene may become linked.
2. Female choice may drive the male trait
beyond the optimum point in terms of
natural selection.
3. High quality males should be able to
overcome at least some of this cost
No. of individuals
Female
preference
Costs (e.g predation
pressure)
Dull
Male coloration
Optimum, natural
selection
Bright
Optimum,
female choice
Runaway (Fisherian) sexual selection:
1. There is a genetic correlation between female
preference and the male trait.
**2. Female preference for the trait pushes the
trait beyond the point where it is adaptive,
even for ‘high quality’ males.
**3. There are no longer any fitness benefits to
mating with males with exaggerated traits.
**4. Females gain only by producing ‘sexy sons’.
**5. When the decrease in survival counterbalances sexual attractiveness, the
runaway process will halt.
Indicator (‘Good genes’/Handicap) models
1. Female choice may drive the male trait beyond the
optimum point in terms of natural selection
2. High quality males should be able to overcome
at least some of this cost
3. Costly male traits are preferred by females
because they indicate high heritable fitness
• If the trait is expressed in proportion to
condition, a well-developed trait should
signal the carrier’s condition
• Females who mate with such males produce
both fitter sons and fitter daughters
• Good evidence for condition-dependence of
secondary sex traits
• Some evidence that females who choose
males who have an extreme version of
a trait gain direct benefits (resources)
• Some evidence that male condition has some
genetic basis.
• Do females ever choose males on the basis of
genes only, independent of the resources
they provide?
Number of mates
8
6
4
2
0
140
150
Eye-spot number
Mating success in
peacocks
160
Offspring growth in peacocks
Mean weight on day 84 (g)
1,480
1,440
1,400
1,360
1,320
1,280
1,240
1,200
120
125
130
135
140
145
150
Mean area of father's eye spot (mm2)
155
Sons
Daughters
What criteria do females use in
choosing mates?
 Good resources?
 Good condition?
 Good genes independent of resources?
How important has female choice been
in the evolution of male traits that
are:
 not used in male-male competition?
 costly in terms of the male’s survival?
The degree of sexual dimorphism is a
good indication of the intensity of
sexual selection.
Monomorphic: Often monogamous.
Males invest in parental effort.
Dimorphic: Often polygynous.
Males invest in mating effort.
Why does sexual selection seem to occur
more in the form of intra-sexual selection
in mammals and inter-sexual selection
in birds?
What is the relation between female choice
and male-male competition?
Does male redwing blackbird song function to
deter other males, to attract females, or
both?
• Males sing louder, switch song types more
for females than for males.
BUT
• Territories broadcasting song are invaded less
rapidly by other males
What is the relation between sexual
selection and natural selection?
No. of individuals
Female
preference
Costs (e.g predation
pressure)
Dull
Male coloration
Optimum, natural
selection
Bright
Optimum,
female choice
Guppies
Predation
12
High
Medium
Low
Spots per fish
11
10
9
8
7
Streams
Guppies
Rivulus (predator on
juveniles)
Control
Spots per fish
12
Crenicichla (predator
on adults)
Start of
experiment
10
8
0
2
4
6
8
10
Months
12
14
16
18
20
Guppies in Trinidad
• Male ornamentation is important for female
choice, but not for male-male competition.
• There may be a genetic correlation between
female choice and male ornamentation.
• Ornamentation may reflect male quality:
*Diet
*Parasite load
*Predation risk
Male size/ Female size
5
Singing behavior
4
3
2
1
0
M=F
M >> F
M only
M >>> F
M>F
M=F
Prairie voles: Monogamous
• Males and females have same-sized territories.
• They perform equally well in mazes
• The hippocampus is the same size in both sexes.
Meadow voles: Polygynous
• One male’s territory encompasses several females’
• Males out-perform females in mazes
• Males have a bigger hippocampus
Sexual selection: Summary
Intra-sexual selection generally concerns
male-male competition.
• It can affect size, weapons (e.g. antlers,
canines), and brain structures
Inter-sexual selection generally concerns
female choice
• Females may base their choice on:
– direct examination of resources
– male traits that are correlated with
resources
– male genetic quality independent of
resources
• Male ornaments are not arbitrary, but have
probably evolved because females
use them as reliable indicators of male
resources and/or genetic quality
• Inter-sexual selection (female choice) does
not simply reaffirm the results
of intra-sexual selection (male-male
competition) or natural selection.
• It can lead to the evolution of male traits
that are costly in terms of fighting and/or
survival.