© Ned Kock
Slide 1
Costly mate choice traits
should be fairly rare and
particularly attractive to
members of the opposite sex
Ned Kock
Texas A&M International University
© Ned Kock
Slide 2
Costly and non-costly traits used in
mate choice
• Evolved traits used in mate choice can be costly or
non-costly.
• Costly traits are often referred to as handicaps, and
non-costly traits as indices.
• Costly traits used in mate choice are defined as
traits that increase mating success while at the same
time decrease survival success.
• Non-costly traits used in mate choice are defined as
traits that increase mating success and that do not
have a negative effect on survival success.
© Ned Kock
Slide 3
Example of costly trait used in mate
choice
• The tail displayed by the male of the peacock
species is an example of costly trait used in mate
choice.
• Having large tails with numerous eye spots
significantly increases the mating success of male
peacocks, while at the same time making them
more vulnerable to predators.
• A related behavioral trait is the propensity to
display the tale to females in leks during the mating
season, without which the tail itself would not be of
much use.
© Ned Kock
Slide 4
Example of non-costly trait used in
mate choice
• The ability and motivation of males of the fruit fly
species Drosophila subobscura to engage in the
species’ rapid courtship dance are examples of noncostly traits used in mate choice.
• Males that show the ability and motivation to dance
quickly and vigorously, in response to lead
movements by females, increase their mating
success.
• Those are traits have no known impact on the
survival success of males.
© Ned Kock
Slide 5
Both costly and non-costly traits are
used in human mate choice
• Human males and females employ costly and non-costly
traits for mate choice.
• A costly mate choice trait displayed by males and used by
females in mate choice is the presence of facial features
indicating high levels of testosterone (e.g., angular facial
features), apparently because testosterone suppresses the
immune system and thus is an indicator that the males in
question have a strong immune system.
• Non-costly mate choice traits seem to be more common than
costly traits in both human males and females; examples are
height and upper-body musculature in males, and hip-towaist ratio and skin condition in females.
© Ned Kock
Slide 6
The fundamental requirement for
evolution of traits through selection
• Price (1970) has shown that for any trait to evolve through
selection in any population of individuals the trait must
satisfy the following equation:
• W is a measure of the fitness of an individual that possesses
the trait (e.g., number of surviving offspring); and Z is a
measure of the manifestation of the trait in the individual
(e.g., Z = 1 if the trait is present, and Z = 0 if it is absent).
• The trait in question can be any morphological,
physiological or behavioral trait; examples could be
opposing thumbs, aggressiveness, or a large tail with many
eyespots.
© Ned Kock
Slide 7
A variation of the fundamental
requirement
• The fundamental requirement can be rewritten as shown below in terms of the
standardized measures of W and Z, referred
to as w and z.
© Ned Kock
Path analysis and the evolution of
costly and non-costly traits
• Path analysis is a statistical analysis method
that has been developed by Sewall Wright,
one of the founders of the field of population
genetics.
• It relies on the development of path models,
which are diagrams that facilitate the
visualization of the relationships between
variables measuring phenotypic traits, any
intermediate variables, and fitness.
Slide 8
© Ned Kock
Slide 9
Path model showing costly (y) and noncostly (x) mate choice traits
Effect of mating
success on fitness
Mating success
measure
Path
coefficient
Attractiveness
of trait x
Non-costly
trait measure
Fitness
measure
Attractiveness
of trait y
Effect of survival
success on fitness
Survival success
measure
Survival cost of
trait y
Costly trait
measure
© Ned Kock
Slide 10
Path model with mate choice traits
showing some hypothetical values
(.24)
(.21)
(.32)
(.09)
(.43)
(-.17)
© Ned Kock
Slide 11
Requirements for the evolution of
costly and non-costly traits
That is, a non-costly mate
choice trait will evolve if its
attractiveness is greater than 0.
The covariance between any two variables in
a path model equals the sum of the products
of the path coefficients in all paths linking the
two variables, therefore ...
A costly mate choice trait will
evolve only if its attractiveness
is above this threshold.
© Ned Kock
Slide 12
The threshold for evolution of costly
traits
This is a measure of
the survival cost of
the trait; it is by
definition negative.
This is a measure of the
effect of survival success
on mating success; it is
largely species-specific,
and always non-negative
because an individual
must be alive to mate.
This term (the ratio of the effects of survival success and mating success on
fitness) is key in defining the threshold for evolution of costly mate choice traits; it
is largely species-specific and increases with:
•The level of offspring dependence on parents for survival to reproductive age.
•The age at which reproductive maturity is reached.
•The probability of death before reaching reproductive maturity.
© Ned Kock
Slide 13
Key conclusions based on the threshold
for evolution of costly traits
There was a threshold for
evolution of costly, but not
for non-costly, traits among
our ancestors. Thus costly
traits should be rarer than
non-costly traits in modern
humans.
The attractiveness of costly traits
must have been above this
threshold for those traits to evolve
among our ancestors; no such
condition existed for the
attractiveness of non-costly traits.
Thus costly traits should be
generally more attractive than
non-costly traits in modern
humans.
This term was likely much higher
for ancestral human females than
males. Thus costly mate choice
traits should be rare (or
nonexistent) in modern women.
© Ned Kock
Slide 14
Key examples of these conclusions in
modern humans
• Rarity of costly mate choice traits in humans:
– The literature suggests the existence of significantly fewer costly than
non-costly traits used in mate choice by humans.
• Attractiveness of costly mate choice traits:
– Creative intelligence in men (as displayed by successful artists) is a
highly attractive trait.
– Creative intelligence is positively correlated with schizophrenia, which is
likely to have impaired survival among our ancestors.
• Rarity of costly mate choice traits among women:
– Apparently there are no costly traits among women that are used for mate
choice by men.
– Among men there a few costly traits, such as facial testosterone markers
and creative intelligence.
© Ned Kock
Slide 15
What about the peacock species?
Costly trait rare
Costly trait attractive
Costly trait only in males
Tail: Costly (and by far the most attractive
for females; Petrie, Halliday & Sanders,
1991 ).
Body symmetry: Non-costly.
Head crest: Non-costly.
Dark eye ornamentation:
Non-costly.
Brightly colored head feathers:
Non-costly.
Brightly colored chest feathers:
Non-costly.
© Ned Kock
Limitations
Slide 16
• The discussion about the ratio pws / pwm is based on life history
theory; actual values for the ratio can only be obtained empirically.
• This discussion assumes that there is no bias in preferences in
favor of costly or non-costly traits.
– For any costly or non-costly mate choice trait to evolve, a preference for the
trait must have evolved first.
• Mate choice traits, unlike traits selected by a stable external
environment, maintain much of their variance over time; therefore:
– They crowd the variance space that can be used up by new traits (especially
regarding the endogenous variable m), making costly traits even more
unlikely to evolve.
– They can become correlated with one other, which requires much more
complex models and algebraic operations for their proper analysis.
• The mathematical formulations do not explicitly incorporate the
full complexity of mate choice patterns in humans – e.g.,
differences in short- and long-term mate choices made by women.
© Ned Kock
Slide 17
Key references
Final slide
• Maynard Smith, J. (1998). Evolutionary genetics. New York, NY:
Oxford University Press.
• Maynard Smith, J., & Harper, D. (2003). Animal signals. New York,
NY: Oxford University Press.
• Petrie, M., Halliday, T., & Sanders, C. (1991). Peahens prefer
peacocks with elaborate trains. Animal Behaviour, 41(2), 323-331.
• Price, G.R. (1970). Selection and covariance. Nature, 227(1), 520521.
• Wright, S. (1934). The method of path coefficients. The Annals of
Mathematical Statistics, 5(3), 161-215.
• Zahavi, A. (1975). Mate selection—A selection for a handicap.
Journal of Theoretical Biology, 53(1), 205-214.
• Zahavi, A. & Zahavi, A. (1997). The Handicap Principle: A missing
piece of Darwin’s puzzle. Oxford, England: Oxford University Press.
© Ned Kock
Slide 18
Conditional probabilities of evolution
of costly and non-costly traits
Notes:
•Traits are assumed to be independent.
•Preference for the traits are assumed to have evolved in the sex using the trait for
mate choice.
© Ned Kock
Slide 19
Conditional probabilities that costly
trait will be more attractive
Notes:
•Horizontal axis: threshold for evolution of costly trait.
•Traits are assumed to be independent.
•Bar height: probability that costly trait will be more attractive than non-costly
trait.
© Ned Kock
Slide 20
Ratios of mean attractiveness of costly
and non-costly traits
Notes:
•Horizontal axis: threshold for evolution of costly trait.
•Traits are assumed to be independent.
•Bar height: measure of how much more attractive a costly trait should be in
comparison with a non-costly trait; e.g., 1.5 means 50% more attractive.
© Ned Kock
Slide 21
Visualizing how a medium threshold
affects the evolution of traits
This representation is only
schematic and somewhat
misleading, because
threshold values will vary for
each new costly trait.
Magnitude of the effect of the
trait (x or y) on mating
success; or the attractiveness
of the trait.
Threshold value.
x5
y5
x3
x2
x1
y2
x4
y3
y1
y4
Outcomes:
•Non-costly traits that will evolve: x1, x2, x3, x4, and x5.
•Costly trait that will evolve: y5 only.
•The costly trait will be one of the most attractive.
© Ned Kock
Slide 22
Visualizing how a high threshold
affects the evolution of traits
This representation is only
schematic and somewhat
misleading, because
threshold values will vary for
each new costly trait.
Magnitude of the effect of the
trait (x or y) on mating
success; or the attractiveness
of the trait.
Threshold value.
x5
y5
x3
x2
x1
y2
x4
y3
y1
y4
Outcomes:
•Non-costly traits that will evolve: x1, x2, x3, x4 and x5.
•No costly trait will evolve.