Males and females often look different Female Male

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

Examples of Sexual Dimorphism

Black Widow bugguide.net/ node/view/7426

Examples of Sexual Dimorphism

Gila Topminnow Desert Pupfish Great-tailed Grackle 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, rivals gets to mate – Examples: creosote bush seal, hummingbird

Intrasexual Selection Strategies

• Scramble Competition – Whoever gets to most mates first gets to mate sexual dimorphism – Examples: cactus bees, 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 female male

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

• Polygamy • Lek

Mating Systems

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