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Why do males usually compete, while females choose? 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
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Marine Vertebrates: Lecture 16
Sexual selection & mating systems
Why do males usually compete, while females
choose?• 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
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
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
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?
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?
Gulf pipefish
♀
♂
Broad-nose pipefish
Photo: Institute of Biology of the Southern Seas, Ukraine
♀
♂
Hippocampus spp. Pair(Same genus as W. Australian
seahorse•Note similar size
and coloration of pair.
www.divegallery.com
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.
Polygyny•Female defense polygyny
Example: Northern elephant seals
Photo: www.driftersister.com Photo: www.wetasschronicles.com
Male-male competition and sexual dimorphism (seals)
NOTE: Each point represents a species
Polygyny•Lek polygyny
Example: Marine iguanas
Photo: Martin Wikelski
Polygyny: Cetacea
•Sperm whales Highly skewed sex
ratios on mating grounds
Sexual dimorphism & battle scars
Females: synchronous estrus•Adaptive value?
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
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
Alternative male strategies
Photo: Cornell University
•Plainfin midshipmen Dominant male
features and behavior
Sneaker male features and behavior
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
California Sheephead and sex reversal
California sheephead female Photo: UC Santa Cruz
California sheephead male Photo: V. Sanchez
Other sex reversers: bluehead wrasse
Bluehead wrasse, terminal phase male
Photo: P. Humann
Bluehead wrasse, intial phase
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
Monogamy: alternate hypotheses
• Danger “theory”
Leaving increases chance of dying if predation rates are high.
Example: Blue-spotted jawfish www.reefcorner.com
Social vs. genetic monogamy
•Video: blue-footed boobie
www.amazingjourneys.net
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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