Males and females often look different - PowerPoint PPT Presentation

Male

Female

• 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

Black Widow

bugguide.net/ node/view/7426

Great-tailed Grackle

Gila Topminnow

Desert Pupfish

Pheasant

Elephant Seal

In Elk?

In Great-tailed Grackles?

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.

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.

• Intrasexual selection
• within sex
• usually male-male competition
• Intersexual selection
• between sexes
• usually female choice

• Mate Defense
• Whoever defends mates best against rivals gets to mate
• Examples: deer, elk, beetles

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

• 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

• 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.

• Runaway Selection
• Also called Indirect Fitness Benefits
• Chooser’s offspring gain better sexual attractiveness, often at expense of survival

• 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

www.ownbyphotography.com

The gift

• Prenuptial Gifts
• Whoever offers best “gifts” (e.g., food)
• Example: shrike

The suitor

• 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.

nw-ar.com

• Good Looks
• Whoever looks/smells, sounds, etc. the best
• Example 2: northern mockingbird
• Females select male with largest song repertoire.

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

• Competition: Mate Defense
• Males defend groups of females from other males

• Choice
• Females just choose more brightly colored males (so do predators!)

Monogamy

Polygamy

Polyandry

Polygyny

Promiscuity

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

http://www.paulnoll.com/

• Polygamy: multiple mates per mating season
• Pros: choose best partners
• Cons: single parent care
• Example: most mammals

• Polyandry: one female with multiple male mates (sets up for competition/choice)
• Example: phalarope, seahorse

male

female

• Polygyny: one male with multiple female mates (sets up for competition/choice)
• Example: deer, elk

www.peopleandwildlife.org.uk

www.museum.state.il.us

• Promiscuity: males and females both have multiple mates (sets up for competition/choice)
• Example: snowshoe hare

• 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