2Sexual DimorphismMales and females differ in appearance (secondary sexual characteristics)Primary sexual characteristics: reproductive organsSecondary sexual characteristics: other external features of an organism that differ between males and females
3Examples of Sexual Dimorphism Black Widowbugguide.net/ node/view/7426
4Examples of Sexual Dimorphism Great-tailed GrackleGila TopminnowDesert PupfishPheasantElephant Seal
5Why do males and females often look different from each other? In Elk?In Great-tailed Grackles?
6Background: Natural Selection has 2 components Ecological SelectionSelection for attributes (physical or behavioral) that contribute to survival.Sexual SelectionSelection for attributes (physical or behavioral) that contribute to reproductive advantage.
7How does sexual dimorphism arise? 1. Give ecological selection hypotheses2. Give sexual selection hypotheses3. Give deductions for each4. Devise tests that will allow you to observe whether a prediction for the ecological selection, sexual selection, or both hypotheses occurs.
9Intrasexual Selection Strategies Mate DefenseWhoever defends mates best against rivals gets to mateExamples: deer, elk, beetles
10Intrasexual Selection Strategies Resource DefenseWhoever defends best resources (e.g., food, shelter, etc.) against rivals gets to mateExamples: creosote bush grasshopper, elephant seal, hummingbird
11Intrasexual Selection Strategies Scramble CompetitionWhoever gets to most mates first gets to mateMay or may not lead to sexual dimorphismExamples: cactus bees, ground squirrels, horseshoe crabs
12Intersexual Selection: Choice Good Gene SelectionAlso called Direct Fitness BenefitsChooser gains better nest site, territory, provider (of food/protection), parental care, lack of contagious disease or parasites, etc.
13Intersexual Selection: Choice Runaway SelectionAlso called Indirect Fitness BenefitsChooser’s offspring gain better sexual attractiveness, often at expense of survival
14Intersexual Selection: Choice LekFemale 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
15Intersexual Selection: Choice Prenuptial GiftsWhoever offers best “gifts” (e.g., food)Example: shrikeThe suitorThe gift
16Intersexual Selection Strategies Good LooksWhoever looks/smells, sounds, etc. the bestExample 1: widowbirdFemale selects male with longest tailIs 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.
17Intersexual Selection Strategies Good LooksWhoever looks/smells, sounds, etc. the bestExample 2: northern mockingbirdFemales select male with largest song repertoire.nw-ar.comIs this good gene or runaway selection?Her male offspring will have large song repertoire and be selected first but… also may indicate male longevity
18So, why do male elk and deer have antlers, but not females? Competition: Mate DefenseMales defend groups of females from other males
19So why are male birds often more brightly colored than females? ChoiceFemales just choose more brightly colored males (so do predators!)
20The amount of competition/choice occurring in a species depends on the mating system MonogamyPolygamyPolyandryPolygynyPromiscuity
21Mating: 2 Main Strategies Monogamy: single mate per mating seasonPros: two-parent careCons: out-reproducedExample: ~90% of birds; rare in mammalsCaveat: extra-pair copulations
22Mating: 2 Main Strategies Polygamy: multiple mates per mating seasonPros: choose best partnersCons: single parent careExample: most mammals
23Polygamous Mating Systems Polyandry: one female with multiple male mates (sets up for competition/choice)Example: phalarope, seahorsemalefemale
24Polygamous Mating Systems Polygyny: one male with multiple female mates (sets up for competition/choice)Example: deer, elk
25Polygamous Mating Systems Promiscuity: males and females both have multiple mates (sets up for competition/choice)Example: snowshoe hare
26Mating Systems Monogamy: mate-guarding, mate-assistance Polygamy Polygyny: one male, multiple femalesMate defenseResource defenseScramble competitionLekPolyandry: one female, multiple malesMore material benefits/parental careBetter sperm/fertility insurancePromiscuous: males and females both have multiple mates