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

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

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 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

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