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Males and females often look different Male Female.

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Presentation on theme: "Males and females often look different Male Female."— Presentation transcript:

1 Males and females often look different Male Female

2 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

3 Examples of Sexual Dimorphism bugguide.net/ node/view/7426 Black Widow

4 Examples of Sexual Dimorphism Gila TopminnowDesert Pupfish Pheasant Elephant Seal Great-tailed Grackle

5 In Elk? Why do males and females often look different from each other? In Great-tailed Grackles?

6 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.

7 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 each 2. Give sexual selection hypotheses 1. Give ecological selection hypotheses

8 Sexual Selection: 2 components Intrasexual selection –within sex –usually male-male competition Intersexual selection –between sexes –usually female choice

9 Intrasexual Selection Strategies Mate Defense –Whoever defends mates best against rivals gets to mate –Examples: deer, elk, beetles

10 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

11 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

12 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.

13 Intersexual Selection: Choice Runaway Selection –Also called Indirect Fitness Benefits –Chooser’s offspring gain better sexual attractiveness, often at expense of survival

14 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

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

16 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.

17 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

18 So, why do male elk and deer have antlers, but not females? Competition: Mate Defense –Males defend groups of females from other males

19 So why are male birds often more brightly colored than females? Choice –Females just choose more brightly colored males (so do predators!)

20 The amount of competition/choice occurring in a species depends on the mating system Monogamy Polygamy Polyandry Polygyny Promiscuity

21 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

22 Mating: 2 Main Strategies Polygamy: multiple mates per mating season –Pros: choose best partners –Cons: single parent care –Example: most mammals

23 Polygamous Mating Systems Polyandry: one female with multiple male mates (sets up for competition/choice) –Example: phalarope, seahorse male female

24 Polygamous Mating Systems Polygyny: one male with multiple female mates (sets up for competition/choice) –Example: deer, elk

25 Polygamous Mating Systems Promiscuity: males and females both have multiple mates (sets up for competition/choice) –Example: snowshoe hare

26 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


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