BIO372 : Behavior : Sexual Dimorphism : Lesson

I. Human mating preferences

1. Observations
   • 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
   • Survey of 37 groups of individuals across 33 different societies

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

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

2. 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

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

E-mail Professor Gaud at William.Gaud@nau.edu
or call (520) 523-7516