http://www.youtube.com/watch?v=DU4xW79ASsg
http://www.youtube.com/watch?v=VDhNutbXpFE
Mate Choice
Contests
- Armaments and weapons
- Traits that confer advantage in fighting
- Limited female choice
- Ornaments and beauty
- Traits that confer advantage in female choice
- Very active female choice
“Where one sex invests considerably more than the other, members of the latter
will compete among themselves to mate with members of the former”
- Robert Trivers (1972)
Females can only increase rep.
success by turning food into eggs
or offspring at a faster and/or more
successful rate
Females tend to be the choosier sex
So are females choosy wrt to ‘ornamental’ traits?
after Anderson (1982)
2
Before
Mean number
of nests
1
per male
Control I – unmanipulated
Control II – cut and glued back on
2
After
Mean number
of nests
per male
1
shortened
I
II elongated
controls
Tail treatment
Question: What is the significance of mate choice by females
in such species? And what is the significance of ornamental traits?
1) females choose mates at least in part based on traits that are indicators of
parental care, e.g., courtship feeding (terns) or it is territory quality rather than
the male per se.
2) Males tend to be highly ornamented with striking displays that appear to
function in the choice of mates by females.
(1) Direct Selection is more important in monogamous relationships; male
quality per se is not of interest, but rather his contribution or territory
(2) In Arbitrary Mate Choice ornate traits do not signal male quality, rather they
arise and become exaggerated through sensory bias/runaway selection
(3) In contrast, the Good Genes Hypothesis males ornaments honestly signal their
genetic quality and are used by females to choose quality males
Fluctuating Asymmetry
Handicap Principle
Parasite Load (Hamilton-Zuk Hypothesis)
Direct Selection – Females are attracted to mates for reasons having to
due with their own welfare or reproductive success.
Male provision of resources, particularly the territory, probably the
most common form of direct selection for territorial socially
monogamous species – but clearly not the case in polygynous/
promiscuous species where males contribute only gametes
Laughing Gull – courtship feeding
Male aggression and female choice may go hand in hand:
Females choose males defending high quality territories
- warm water for faster development
- sparse vegetation so the eggs ball-up
Preferred territories hotly contested for by males so that the
strongest males end up in the best site
Hanging flies (Hylobittacus)
and nuptial gifts
females benefit through the reduced
need to forage and direct energy gains
In Orthopterans, males produce their own
nourishment via a spermatophore, which
provides nutrition to the female
This clearly benefits
the female
The spermatophore may be so
valuable that sex role reversal occurs
When food is scare:
spermatophores limit female reproduction
and females should compete for males
Variance in reproductive success
males are choosy
Females
Males
Environmental quality
When food is common:
spermatophores are easily produced
and many males are available
females are choosy
Direct Selection –
Females are NOT selecting a male per se
Relative lack of exaggerated or ornamental characters
Blend of aggression among males (competition for territories) and female choice
Arbitrary Mate Choice – No underlying message of genetic quality
conveyed by traits, rather an ornament is perceived as “attractive” and the more
extravagant the more attractive
Link between an exaggerated male trait and the preference for it – i.e., such a
mating produces males with the trait and daughters with the preference for it
2 theories:
Runaway Selection
Sensory Bias
Sensory Bias (or Exploitation) may provide the proximate causation for
Runaway Selection – i.e., preferences exist before the development a preferred trait
Zebra finches choose mates
with artificial crests despite the
fact that they lack an evolutionary
history
Platyfish diverge from swordtails
prior to the evolution of the sword
tail....
Yet females show a strong preference for
males with a sword tail (after Basolo 1995)
Female swordfish will respond to computer generated images
The neat thing about swords is they
elicit the same response by females
as body length, but are relatively
easy to produce
Runaway-Selection predicts a
genetic correlation between the
male trait and female preference
due to non-random mating
Stalk-eyed flies - Cyrodiopsis
Eye span
Males
Preference
Females
50 males mate with 25 random females
10 males selected for next generation
Test for a genetic correlation by offering
females a choice between 2 males and
recording either time spent with or
number of copulations with male
females – from treatments in
which short male eye span was
selected – preferred short eye
stalk males
Arbitrary Mate Choice –
Link between male trait and female preference for it – Sensory Bias?
Ornamental traits and strong female choice are correlated
But no relationship between trait expression and males’ inherent quality
Good Genes – Females use male ornaments to gauge the genetic quality of
males partners, e.g. genes of chosen males may provide their offspring w/inherited
resistance to disease or parasites
Females can enhance the well-being of her young (e.g., increased
resistance) by choosing to mate with an individual with demonstrable health
Females should focus on male features that honestly indicate physical
condition and this promotes the evolution of those traits in males that signal
their contribution to prospective mates
Predicts: Ornaments/displays viewed as visible indicators of the underlying
genetic quality of males
3 concepts:
Handicap Principle
Hamilton-Zuk Hypothesis
Fluctuating Asymmetry
Females should focus on male features that honestly indicate physical
condition and this promotes the evolution of those traits in males that signal
their contribution to prospective mates
Handicap Principle – The size and conspicuousness of the males’s handicap
is correlated to the phenotypic quality of the male. A very effective way to produce
this is through costs that are disproportional to the males quality.
Fitness cost or benefit
Cost – low quality
Cost – high quality
Benefit
Honest y arises through
Costly signaling
EQ
low
EQ
high
Signal intensity
Handicap Principle (Zahavi 1975) - The possession of ornamental traits is a
Handicap in day-to-day-survival. Handicaps therefore are a reliable signal of
male vigor (i.e., genetic quality). http://www.arkive.org/jacksonswidowbird/euplectes-jacksoni/video00.html
Long-tailed widowbird
Honest Signal?? – Testosterone influences the development of male
secondary characters, e.g.,
song rate
comb – jungle fowl
It has to be COSTLY!!
tail length – barn swallow
intensity of plumage, bare skin color
Testosterone has a negative effect
on the immune system – maximal
elaboration of tes-driven traits
simultaneous increases the risk of
infection by disease/parasites
Advertises
“I can survive & thrive despite
suppression of the immune system”
Costly Signal Part III. – Carotenoid pigments (reds, yellows) must be consumed
in the diet – therefore they are a signal to the quality of a territory, a male’s resource
holding power, its forging aptitude, etc…
- Signal nutritional status/health
- May function in immune responses
e.g., brightness of red pigmentation of
male House Finches used by females
in mate choice, redder males are
better parents, survive better, and that
plumage color is related to dietary
intake of cartenoids.
Male Blue Tits have a UV-reflecting crown patch
Control
Sunscreen
Visible light
UV light
Mates of sunscreened male produced fewer male offspring, supporting
the notion that UV reflectance plays a role in male attractiveness
Costly Signal Part IV. – Songs and displays are costly in terms of increased
metabolic rate and missed opportunities – time allocated to singing is “lost”
In Carolina wrens (Bewick’s wren is
pictured) the BMR increases ~2-fold.
Sage grouse displays:
2-4 fold increase in BMR
Costly displays – Sage grouse
2 to 4 -fold increase in BMR
Honest Signals Part I – Fluctuating Asymmetry (NOT COST BASED):
When normally bilaterally symmetric traits that are under the control of a single gene
do not undergo identical development on both sides of the body
Considerable evidence exists that a large degree of FA in morphological characters
indicates an individual has been unable to cope with stress during development of the
trait
Sexually-selected traits are often displayed
so as to “show-off” their degree of
(a)symmetry.
If low-quality males attempted to grow
extravagant traits (e.g., long tails), they
would possess a conspicuous degree of
asymmetry
A role for asymmetry in humans?
Hamilton-Zuk Hypothesis (Parasite load hypothesis) – Relationship between
ornamental traits and parasite load.
2 Components/predictions:
(1) Interspecific (between spp) – in more heavily parasitized spp, females
should discriminate more among males, sexual selection is more intense,
and males become more ornamental
Evidence is equivocal
(2) Intraspecific (within spp) – Among males of a given spp, the lessparasitized (or non-parasitized) possess and display more highly
developed ornamental traits. Females should prefer to mate with males
that produce the most ornamental traits while simultaneously coping with
and resisting the deleterious effects of parasites
400
# males
captured
200
0
32.4% cestodes
infection
# males
seen in
later years
12.2%
60
30
0
1
2
3 4
5
6
7
Breeding plumage score
Plumage variation
in the male
bar-tailed godwit
true parents
after Møller (1990)
foster parents
mites on
offspring raised
by foster parents
(2) parasite load
is heritable
# on parent
(1) Female choice is based
on male tail length
mites on
offspring raised
by foster parents
S
20
days between
arrival to pairing 10
0
E
(3) parasite load is
correlated w/tail length
100
50
10
0
90
tail treatment
110
130
Tail length
In conclusion:
(1) Direct Selection - is most appropriate in monogamous relationships; male
quality per se is not of interest, but rather his contribution or territory
(2) In Arbitrary Mate Choice ornate traits do not signal male quality, rather they
arise and become exaggerated through sensory bias/runaway selection
(3) In contrast, the Good Genes Hypothesis females use male ornaments to
honestly signal the genetic quality of males.
• Hamilton-Zuk Hypothesis
Ornamentation reveals Parasite Load and hence genetic quality
• Handicap Principle
Ornamentation reveals male vigor (survivorship or energetic state)
• Fluctuating Assymetry
Ornamentation reveals developmental stress
Honest signals may arise thru
•
•
•
•
•
testosterone-driven ornamentation and immune system suppression
Carotenoid-based pigmentation
Physical handicaps and predation risk
Energetically-costly singing and displaying
Expression of Fluctuating Asymmetry