Male. Female. Males and females often look different. Sexual Dimorphism. Males and females differ in appearance (secondary sexual characteristics) Primary sexual characteristics : reproductive organs
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Male Female 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 Black Widow bugguide.net/ node/view/7426
Examples of Sexual Dimorphism Great-tailed Grackle Gila Topminnow Desert Pupfish Pheasant Elephant Seal
Why do males and females often look different from each other? 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.
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.
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
The gift Intersexual Selection: Choice • Prenuptial Gifts • Whoever offers best “gifts” (e.g., food) • Example: shrike The 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.
nw-ar.com Intersexual Selection Strategies • 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
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 system Monogamy 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 male female
Polygamous Mating Systems • Polygyny: one male with multiple female mates (sets up for competition/choice) • Example: deer, elk www.peopleandwildlife.org.uk
www.museum.state.il.us Polygamous Mating Systems • Promiscuity: males and females both have multiple mates (sets up for competition/choice) • Example: snowshoe hare
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