1. Males and females often look different

2. Sexual Dimorphism
Males and females differ in appearance (secondary sexual characteristics)
Primary sexual characteristics: reproductive organs
Secondary sexual characteristics: other external features of an organism that differ between males and females

3. Examples of Sexual Dimorphism
Black Widow
bugguide.net/ node/view/7426

4. Examples of Sexual Dimorphism
Great-tailed Grackle
Gila Topminnow
Desert Pupfish
Pheasant
Elephant Seal

5. Why do males and females often look different from each other?
In Elk?
In Great-tailed Grackles?

6. Background: Natural Selection has 2 components
Ecological Selection
Selection for attributes (physical or behavioral) that contribute to survival.
Sexual Selection
Selection for attributes (physical or behavioral) that contribute to reproductive advantage.

7. How does sexual dimorphism arise?
1. Give ecological selection hypotheses
2. Give sexual selection hypotheses
3. Give deductions for each
4. Devise tests that will allow you to observe whether a prediction for the ecological selection, sexual selection, or both hypotheses occurs.

8. Sexual Selection: 2 components
Intrasexual selection
within sex
usually male-male competition
Intersexual selection
between sexes
usually female choice

9. Intrasexual Selection Strategies
Mate Defense
Whoever defends mates best against rivals gets to mate
Examples: deer, elk, beetles

10. Intrasexual Selection Strategies
Resource Defense
Whoever defends best resources (e.g., food, shelter, etc.) against rivals gets to mate
Examples: creosote bush grasshopper, elephant seal, hummingbird

11. Intrasexual Selection Strategies
Scramble Competition
Whoever gets to most mates first gets to mate
May or may not lead to sexual dimorphism
Examples: cactus bees, ground squirrels, horseshoe crabs

12. Intersexual Selection: Choice
Good Gene Selection
Also called Direct Fitness Benefits
Chooser gains better nest site, territory, provider (of food/protection), parental care, lack of contagious disease or parasites, etc.

13. Intersexual Selection: Choice
Runaway Selection
Also called Indirect Fitness Benefits
Chooser’s offspring gain better sexual attractiveness, often at expense of survival

14. Intersexual Selection: Choice
Lek
Female chooses male holding best spot at ritual gathering place (note: males often have to compete for best spot). The spot has no resources.
Example: sage grouse, tarantula hawk

15. Intersexual Selection: Choice
Prenuptial Gifts
Whoever offers best “gifts” (e.g., food)
Example: shrike
The suitor
The gift

16. Intersexual Selection Strategies
Good Looks
Whoever looks/smells, sounds, etc. the best
Example 1: widowbird
Female selects male with longest tail
Is this good gene or runaway selection?
Her male offspring will have long tail and be selected first, both by females and predators, but… Also may indicate the male is free of parasites or disease.

17. Intersexual Selection Strategies
Good Looks
Whoever looks/smells, sounds, etc. the best
Example 2: northern mockingbird
Females select male with largest song repertoire.
nw-ar.com
Is this good gene or runaway selection?
Her male offspring will have large song repertoire and be selected first but… also may indicate male longevity

18. So, why do male elk and deer have antlers, but not females?
Competition: Mate Defense
Males defend groups of females from other males

19. So why are male birds often more brightly colored than females?
Choice
Females just choose more brightly colored males (so do predators!)

20. The amount of competition/choice occurring in a species depends on the mating system
Monogamy
Polygamy
Polyandry
Polygyny
Promiscuity

21. Mating: 2 Main Strategies
Monogamy: single mate per mating season
Pros: two-parent care
Cons: out-reproduced
Example: ~90% of birds; rare in mammals
Caveat: extra-pair copulations

22. Mating: 2 Main Strategies
Polygamy: multiple mates per mating season
Pros: choose best partners
Cons: single parent care
Example: most mammals

23. Polygamous Mating Systems
Polyandry: one female with multiple male mates (sets up for competition/choice)
Example: phalarope, seahorse
male
female

24. Polygamous Mating Systems
Polygyny: one male with multiple female mates (sets up for competition/choice)
Example: deer, elk

25. Polygamous Mating Systems
Promiscuity: males and females both have multiple mates (sets up for competition/choice)
Example: snowshoe hare

26. Mating Systems Monogamy: mate-guarding, mate-assistance Polygamy
Polygyny: one male, multiple females
Mate defense
Resource defense
Scramble competition
Lek
Polyandry: one female, multiple males
More material benefits/parental care
Better sperm/fertility insurance
Promiscuous: males and females both have multiple mates