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Sexual selection & mating systems

Marine Vertebrates: Lecture 16. Sexual selection & mating systems. Hypothesis 1: “Eggs are expensive, sperm is cheap!” More energy invested per egg than per sperm. Females: limited by egg production Males: limited by number of mates. Why do males usually compete, while females choose?.

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Sexual selection & mating systems

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  1. Marine Vertebrates: Lecture 16 Sexual selection & mating systems

  2. Hypothesis 1: “Eggs are expensive, sperm is cheap!” More energy invested per egg than per sperm Females: limited by egg production Males: limited by number of mates Why do males usually compete, while females choose?

  3. Is sperm really cheap? • It often takes a large number of sperm to fertilize a single egg, due to • Hostile environment within female • Sperm competition among males

  4. Why do males usually compete, while females choose? • Hypoth. 2 (R. Trivers): Individual with the most total parental investment chooses. • Often the female (i.e. mammals) • Sometimes the male… Photo: pbs.org

  5. Sexual selection • Characteristics/behaviors are selected for that maximize the chances of acquiring mates. • What are some of these characteristics? • Which gender is likely to have the most obvious sexually-selected characteristics?

  6. Sexual selection and polyandry in pipefishes and sea horses • Why pipefish and seahorses? • Question • Is there a relationship between the type of mating system and degree of sexual selection? • Species • Gulf pipefish: Polyandry (no polygyny) • Dusky pipefish: Polygynandrous • Broad-nosed pipefish: Polygynandrous • W. Australian seahorse: Monogamous • Expected results?

  7. Gulf pipefish ♀ ♂

  8. Broad-nose pipefish ♀ ♂ Photo: Institute of Biology of the Southern Seas, Ukraine

  9. Hippocampus spp. Pair(Same genus as W. Australian seahorse • Note similar size and coloration of pair. www.divegallery.com

  10. Mating systems • Polygyny: One male mates with several females • Why considered the male “default”? • Monogamy: One male mates with one female • Social vs. genetic monogamy • Polyandry: One female mates with several males • Polygynandry: Male mates with >1 female, female mates with >1 male.

  11. Polygyny • Female defense polygyny • Example: Northern elephant seals Photo: www.driftersister.com Photo: www.wetasschronicles.com

  12. Male-male competition and sexual dimorphism (seals)NOTE: Each point represents a species

  13. Polygyny • Lek polygyny • Example: Marine iguanas Photo: Martin Wikelski

  14. Polygyny: Cetacea • Sperm whales • Highly skewed sex ratios on mating grounds • Sexual dimorphism & battle scars • Females: synchronous estrus • Adaptive value?

  15. Polygyny: Cetacea • Humpback whales • Skewed sex ratios • Asynchronous ovulation • Asynchronous arrival on mating grounds • Paternity studies • Many males with no offspring • No male with >3

  16. Polygyny? Orcas • Matrilineal groups • Males associate with mother’s group • Mate outside groups (based on genetic studies using “skin darts” for sampling) www.pacificwhaler.com

  17. Alternative male strategies • Plainfin midshipmen • Dominant male features and behavior • Sneaker male features and behavior Photo: Cornell University

  18. Alternative male strategies • Salmon • Dominant males • Top two in photo • Jacks (smaller, less time at sea) • Bottom photo by Ian Fleming yusukekoseki.michikusa.jp/research.htm

  19. California Sheephead and sex reversal California sheephead female Photo: UC Santa Cruz California sheephead male Photo: V. Sanchez

  20. Other sex reversers: bluehead wrasse Bluehead wrasse, intial phase Bluehead wrasse, terminal phase male Photo: P. Humann

  21. Monogamy: alternate hypotheses • Mate assistance: it takes two parents to raise the offspring • Example: Adelie penguins • Both parents needed for chick survival Photo: Karen Haberman

  22. Monogamy: alternate hypotheses • Danger “theory” • Leaving  increases chance of dying if predation rates are high. • Example: Blue-spotted jawfish www.reefcorner.com

  23. Social vs. genetic monogamy • Video: blue-footed boobie www.amazingjourneys.net

  24. Polyandry/Polygynandry • Potential advantages of multiple matings by females? • Females reproductive success limited by mates rather than gametes (rare) • Example: polyandrous pipefish • Higher rates of conception (i.e. % of eggs fertilized) • Good genes hypothesis: Insures the highest quality mate/offspring • Genetic variability among offspring • Material benefits

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