Slide 1© Ned Kock

Costly mate choice traits should be fairly rare and particularly attractive to

members of the opposite sex

Ned Kock

Texas A&M International University

Slide 2© Ned Kock

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.

Slide 3© Ned Kock

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.

Slide 4© Ned Kock

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 non-costly 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.

Slide 5© Ned Kock

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-to-waist ratio and skin condition in females.

Slide 6© Ned Kock

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.

Slide 7© Ned Kock

A variation of the fundamental requirement

• The fundamental requirement can be re-written as shown below in terms of the standardized measures of W and Z, referred to as w and z.

Slide 8© 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 9© Ned Kock

Path model showing costly (y) and non-costly (x) mate choice traits

Mating success measure

Survival success measure

Path coefficient

Attractiveness of trait x

Attractiveness of trait y

Survival cost of trait y

Fitness measure

Effect of mating success on fitness

Effect of survival success on fitness

Non-costly trait measure

Costly trait measure

Slide 10© Ned Kock

Path model with mate choice traits showing some hypothetical values

(.24)

(.43)

(.09)

(.21)

(-.17)

(.32)

Slide 11© Ned Kock

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.

A costly mate choice trait will evolve only if its attractiveness is above this threshold.

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 ...

Slide 12© Ned Kock

The threshold for evolution of costly traits

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.

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.

Slide 13© Ned Kock

Key conclusions based on the threshold for evolution of costly traits

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.

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.

Slide 14© Ned Kock

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.

Slide 15© Ned Kock

What about the peacock species?

Tail: Costly (and by far the most attractive for females; Petrie, Halliday & Sanders, 1991 ).

Head crest: Non-costly.

Brightly colored chest feathers: Non-costly.

Brightly colored head feathers: Non-costly.

Dark eye ornamentation: Non-costly.

Body symmetry: Non-costly.

Costly trait rare Costly trait attractive Costly trait only in males

Slide 16© Ned Kock

Limitations• 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.

Slide 17© Ned Kock

Key references• 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), 520-

521. • 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.

Final slide

Slide 18© Ned Kock

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.

Slide 19© Ned Kock

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.

Slide 20© Ned Kock

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.

Slide 21© Ned Kock

Visualizing how a medium threshold affects the evolution of traits

x5

x1

x2

x3

x4y1

y2

y3

y4

y5

Magnitude of the effect of the trait (x or y) on mating success; or the attractiveness of the trait.

Threshold value.

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.

This representation is only schematic and somewhat misleading, because threshold values will vary for each new costly trait.

Slide 22© Ned Kock

Visualizing how a high threshold affects the evolution of traits

x5

x1

x2

x3

x4y1

y2

y3

y4

y5

Magnitude of the effect of the trait (x or y) on mating success; or the attractiveness of the trait.

Threshold value.

Outcomes:•Non-costly traits that will evolve: x1, x2, x3, x4 and x5.•No costly trait will evolve.

This representation is only schematic and somewhat misleading, because threshold values will vary for each new costly trait.