BIO372 : Behavior : Sexual Dimorphism : Lesson

I. Sexual dimorphism

1. Observations
   • Several animal examples
   • Gender dimorphism occurs in approximately 10% of flowering plant species: example
   • Reverse sexual dimorphism in hawks
   • Skeletal differences between human males and females
   • Alcohol dehydrogenase (see below)
2. Hypothesis
   • Males and females faced different evolutionary challenges during their lives
   • Males and females adapted differently to the different evolutionary challenges, and individuals possessing these adaptations left more offspring than individuals without them
   • The different adaptations were inherited (have a genetic basis) and some of them are exhibited as differences in morphology, physiology, and/or behavior between males and females that we see today

II. Human mating preferences: evidence of sexual dimorphism in behavior?

1. Observations
   • Parental investment in offspring (reproductive effort) differs between human males and females
   • Males and females are attracted to different attributes in their mates
2. Hypotheses
   • Humans pair up on the basis of preferences unique to each individual
   • Any observed preferences result from cultural conditioning
   • Preferences reflect a mating strategy, at least partially genetically based, that has evolved to maximize fitness; males and females have different preferences for attributes of the opposite sex that are related to the different amount of effort required by each sex to produce successful offspring
3. Evidence
   • Surveys of male and female college students in the United States
   • Survey of men and women from 37 countries over 6 years.

II. Sexual selection: Redwing blackbirds

Female	Male

1. Theory
   • Sexual dimorphism results from sexual selection
   • Sexual dimorphism increases fitness
2. Observations of redwing blackbirds
   • Males and female differ in appearance
   • Males and female differ in behavior
   • Other examples: primates, ungulates, pinnipeds, birds, humans
3. Hypotheses
   • Observed differences result from factors unrelated to reproductive success
   • Females choose males which will produce more young
   • Males which are more successful in competing for territories are also more reproductively successful
   • Males also choose females which will produce more young
   • Females which are more successful in competing for nest sites are also more reproductively successful
   • Males and females eat different foods and their morphological and behavioral differences are associated with feeding
   • Males and females employ different modes of locomotion and behavioral differences are associated with movement
4. Evidence
   • Female choice (cf article, "How females choose their mates")
     • Quality of territority: predation and food
     • Parental quality of males
     • Genetic quality of males
   • Male competition
     • Body size
     • Aggressiveness
     • Song
     • Plumage
   • Male choice
   • Female competition

III. Dispersal: ground squirrels Observations Natal dispersal is sexually dimorphic: males move away but females stay home Males mate at locations 10 times farther from home than females do
Hypotheses and evidence
Hypothesis	Name	Evidence
Physiological	Neuronal Hormonal	Blood samples Gonadectomy Hormone suplements
Trigger	Social aggression Avoidance Internal factors:   ontogenetic switch	Observations of agonistic behavior Correlation with litter size Correlation with weight
Fitness	Food shortages Nest burrows Ectoparasites Mate competition Avoid incest	Diets Burrow density Flea and tick infestations Male fighting Kin matings avoided
Evolutionary	Natal dispersal is a direct   result of natural selection	Circumstantial evidence

IV. Alcohol

1. Observations
   • Alcohol has a greater inebriating effect on women than on men
2. Hypotheses
   • Anatomy: Size and fat content
   • Physiological: Enzyme in stomach - alcohol dehydrogenase
3. Evidence
   • Size has an effect
   • Stomach enzymes
   • Alcoholics' enzymes

When you have completed this lesson, go on to Review Questions(in BIO 372 in Vista)