ContestsMate Choice

- 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

http://www.youtube.com/watch?v=DU4xW79ASsg

http://www.youtube.com/watch?v=VDhNutbXpFE

“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?

Mean number of nests per male

2

1

Before

after Anderson (1982)

Mean number of nests per male

2

1

shortened I II elongated

Tail treatment

After

controls

Control I – unmanipulatedControl II – cut and glued back on

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 AsymmetryHandicap PrincipleParasite Load (Hamilton-Zuk Hypothesis)

Direct Selection – Females are attracted to mates for reasons having to due with their own welfare or reproductive success.

Laughing Gull – courtship feeding

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

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 ownnourishment via a spermatophore, whichprovides nutrition to the female

This clearly benefits the female

The spermatophore may be so valuable that sex role reversal occurs

Var

ianc

e in

rep

rodu

ctiv

e su

cces

s

Environmental quality

Males

Females

When food is common:spermatophores are easily produced

and many males are available

females are choosy

When food is scare:spermatophores limit female reproduction

and females should compete for males

males 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 moreextravagant the more attractive

2 theories: Runaway SelectionSensory Bias

Link between an exaggerated male trait and the preference for it – i.e., such amating produces males with the trait and daughters with the preference for it

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 thefact that they lack an evolutionary history

Platyfish diverge from swordtailsprior 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 theyelicit 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

Males

Females

50 males mate with 25 random females10 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

Stalk-eyed flies - CyrodiopsisEye span

Preference

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., increasedresistance) 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 signaltheir contribution to prospective mates

Predicts: Ornaments/displays viewed as visible indicators of the underlyinggenetic quality of males

3 concepts: Handicap PrincipleHamilton-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 signaltheir contribution to prospective mates

Signal intensity

Fitn

ess

cost

or

bene

fit

Cost – low qualityCost – high quality

Benefit

EQ low

EQhigh

Handicap Principle – The size and conspicuousness of the males’s handicapis 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.

Honest y arises through Costly signaling

Long-tailed widowbird

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/jacksons-widowbird/

euplectes-jacksoni/video-00.html

Honest Signal?? – Testosterone influences the development of malesecondary characters, e.g.,

song ratecomb – jungle fowltail length – barn swallowintensity of plumage, bare skin color

Testosterone has a negative effect on the immune system – maximal elaboration of tes-driven traits simultaneous increases the risk ofinfection by disease/parasites

It has to be COSTLY!!

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 resourceholding power, its forging aptitude, etc…

e.g., brightness of red pigmentation ofmale House Finches used by femalesin mate choice, redder males are better parents, survive better, and thatplumage color is related to dietaryintake of cartenoids.

- Signal nutritional status/health - May function in immune responses

Mates of sunscreened male produced fewer male offspring, supporting the notion that UV reflectance plays a role in male attractiveness

Visible light

UV light

Control Sunscreen

Male Blue Tits have a UV-reflecting crown patch

In Carolina wrens (Bewick’s wren is

pictured) the BMR increases ~2-fold.

Sage grouse displays: 2-4 fold increase in BMR

Costly Signal Part IV. – Songs and displays are costly in terms of increasedmetabolic rate and missed opportunities – time allocated to singing is “lost”

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 charactersindicates 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 betweenornamental 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 less-parasitized (or non-parasitized) possess and display more highly developed ornamental traits. Females should prefer to mate with malesthat produce the most ornamental traits while simultaneously coping withand resisting the deleterious effects of parasites

1 2 3 4 5 6 7

Breeding plumage score

32.4% cestodesinfection

12.2% Plumage variation

in the male bar-tailed godwit

400

200

0

# malescaptured

60

30

0

# malesseen in

later years

true parents foster parents

mites on offspring raised by foster parents

(2) parasite load is heritable

(3) parasite load is correlated w/tail length

10050

10

0

90 110 130

mites on offspring raised by foster parents

Tail length

10

20

0tail treatment

days betweenarrival to pairing

(1) Female choice is based on male tail length

S

E

after Møller (1990)

# on parent

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.

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

• 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