Males and females often look different
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Transcript of Males and females often look different
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
Examples of Sexual Dimorphism
Gila Topminnow Desert Pupfish
PheasantElephant Seal
Great-tailed Grackle
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.
4. Devise tests that will allow you to observe whether a prediction for the ecological selection, sexual selection, or both hypotheses occurs.
How does sexual
dimorphism arise?
3. Give deductions for each2. Give sexual selection hypotheses 1. Give ecological selection hypotheses
Sexual Selection: 2 components
• Intrasexual selection– within sex– usually male-male
competition• Intersexual selection
– between sexes– usually female choice
Intrasexual Selection Strategies
• Mate Defense– Whoever defends
mates best against rivals gets to mate
– Examples: deer, elk, beetles
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
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
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.
Intersexual Selection: Choice
• Runaway Selection – Also called Indirect Fitness Benefits– Chooser’s offspring gain better sexual
attractiveness, often at expense of survival
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
www.ownbyphotography.com
Intersexual Selection: Choice• Prenuptial Gifts
– Whoever offers best “gifts” (e.g., food)– Example: shrike
The giftThe suitor
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.
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
So, why do male elk and deer have antlers, but not
females?
• Competition: Mate Defense– Males defend groups of
females from other males
So why are male birds often more brightly
colored than females?
• Choice– Females just choose more brightly
colored males (so do predators!)
The amount of competition/choice occurring in a species depends on the mating
systemMonogamy
Polygamy
Polyandry
Polygyny
Promiscuity
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
http://www.paulnoll.com/
Mating: 2 Main Strategies
• Polygamy: multiple mates per mating season– Pros: choose best partners– Cons: single parent care– Example: most mammals
Polygamous Mating Systems
• Polyandry: one female with multiple male mates (sets up for competition/choice)– Example: phalarope, seahorse
malefemale
Polygamous Mating Systems
• Polygyny: one male with multiple female mates (sets up for competition/choice)– Example: deer, elk
www.peopleandwildlife.org.uk
Polygamous Mating Systems
• Promiscuity: males and females both have multiple mates (sets up for competition/choice)– Example: snowshoe hare
www.museum.state.il.us
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