Mating Systems Adapted from Dixon, A., Dixon, B., Anderson, M., 2005, p. 3 Presented by: Megan Dorrian Stefanie Galich Sean Ivester

Sexual Selection and the Evolution of Visually Conspicuous Sexually Dimorphic Traits in Male Monkeys, Apes, and Human Beings  Quantitative approach  Measured sexually dimorphic characters  Compared development in monogamous, polygynous, and mulitmale-mulitfemale mating systems

Darwin’s Sexual Selection  Traits that improve reproductive success have been selected during the course of evolution, especially among males, primarily via intrasexual selection  Examples: sexual attractive adornments, antler, horns, enlarged canine teeth and other masculine weaponry  Such features also provide info to females regarding masculine reproductive condition

The Different Mating Systems  Monogamy: one male and one female form a durable reproductive partnership, tend to have little sexual dimorphism  Polygyny: some males mate with more than one female  Polyandry: some females mate with more than one male  Multimale-Multifemale: males and females mate with members of the opposite sex with no long-term bonds  Order of visual trait development among systems: Polygyny > Monogamy > Multimale-multifemale Polygyny > Monogamy > Multimale-multifemale

Non-Human Primates  Exist in polygynous and multimale- multifemale mating systems  Primates have pronounced sex differences in canine size and body size Largest physical sex differences among males Largest physical sex differences among males Males of polygynous primate species tend to be much larger than females Males of polygynous primate species tend to be much larger than females

Apes  Mandrill: Polygynous or multimale-multifemale mating system Polygynous or multimale-multifemale mating system Mate guarding of females Mate guarding of females High sexual dimorphism illustrating extreme selection pressure to signal rank to other males and their attractiveness to females High sexual dimorphism illustrating extreme selection pressure to signal rank to other males and their attractiveness to females Alpha males suppresses secondary sexual development in subordinate males Alpha males suppresses secondary sexual development in subordinate males  Orang-Utan: Do not form groups Do not form groups Extreme sexual dimorphism and adornments of males as a result of competition for females Extreme sexual dimorphism and adornments of males as a result of competition for females Alpha male suppresses secondary sexual development in subordinate males Alpha male suppresses secondary sexual development in subordinate males

Humans  Primarily monogamous Low sexual dimorphism and body weight. Low sexual dimorphism and body weight. Relatively high score for masculine visual traits Relatively high score for masculine visual traits Canine teeth of all human beings are small, unlike the large canines of polygynous monkeys and apes Canine teeth of all human beings are small, unlike the large canines of polygynous monkeys and apes Males have relatively small testes in relation to body weight Males have relatively small testes in relation to body weight Two sexes are of similar size: Two sexes are of similar size: Men are somewhat larger, stronger and more muscular than womenMen are somewhat larger, stronger and more muscular than women Women have more body fatWomen have more body fat  Some polygynous, usually among men of higher social status Have higher ratings for masculine visual traits Have higher ratings for masculine visual traits Sexually dimorphic visual traits positively correlated with the degree of sexual dimorphism in body weight mainly in polygynous mating systems Sexually dimorphic visual traits positively correlated with the degree of sexual dimorphism in body weight mainly in polygynous mating systems

Humans (continued…)  Men: Taller men have greater reproductive success Taller men have greater reproductive success Deep masculine tone voice, more muscular torso with broad shoulders and narrow waist is more attractive by some women Deep masculine tone voice, more muscular torso with broad shoulders and narrow waist is more attractive by some women Mesomorphic (muscular) males are rated most attractive by womenMesomorphic (muscular) males are rated most attractive by women Consistent with the theory that sexual selection has an effect on visual signals advertising masculine strength, physical health, age and underlying endocrine condition in the human male Consistent with the theory that sexual selection has an effect on visual signals advertising masculine strength, physical health, age and underlying endocrine condition in the human male Mesomorphy may be an indicator of superior cardiac function and metabolic health Mesomorphy may be an indicator of superior cardiac function and metabolic health May be favored because they offered females greater protection and had physical advantages in huntingMay be favored because they offered females greater protection and had physical advantages in hunting Endomorphy (heavier built) has been correlated with greater risk of heart disease and poor general health Endomorphy (heavier built) has been correlated with greater risk of heart disease and poor general health Hair on the trunk and face is a sexual dimorphic character Hair on the trunk and face is a sexual dimorphic character Do not know why have evolvedDo not know why have evolved

Women’s mean ratings for sexual attractiveness based on somatotypes Adapted from Dixon, A., Dixon, B., Anderson, M., 2005, p. 13

Peacocks  Females are directing evolution through choice of mates  Male competition  Sexual selection theory Healthy, fit, strong peacocks have larger tails Healthy, fit, strong peacocks have larger tails Evolution operating on sexual selection Evolution operating on sexual selection Natural Selection Tale of the Peacock

MAIN POINTS  Overall complexity of masculine secondary sexual traits are greatest in polygynous mating systems  Male human secondary sexual traits are associated with monogamous mating systems  Among anthropoids studied males had the highest visibility of sexaully dimoprhic traits compared to females

Critique  Greatly pointed out sexual dimorphism among primates, monkeys and apes, but needs more cross cultural studies to determine the function of human dimorphic traits  There needs to be a strategy to quantitatively measure the sexual dimorphic visual traits. (i.e. hair, red skin)