Transcript Polygyny

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


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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
• Plainfin
midshipmen
 Dominant male
features and
behavior
 Sneaker male
features and
behavior
Photo: Cornell University
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,
intial phase
Bluehead wrasse,
terminal phase male
Photo: P. Humann
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: Bluespotted 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