Males and females often look different

Male

Female

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

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Black Widow

Examples of Sexual Dimorphism

Gila Topminnow Desert Pupfish

PheasantElephant Seal

Great-tailed Grackle

In Elk?

Why do males and females often look different from each other?

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.

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