sexual selection
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SEXUAL SELECTION
Hareesha A S
University of agriculture sciences, Bangalore , GKVK
9739210339
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Sexual Dimorphism
Males and females often are strikingly different
in size and appearance.
Why does sexual dimorphism occur?
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Size
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Fig. 12-06
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Teeth and size
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Horns
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Humans Males
are average
10% taller then
female
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SEXUAL
SELECTION
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Types of sexual selection
• Intrasexual – where males compete for
females.
– males will fight with each other.
– Females mate with the winners
• Intersexual –Where males cannot control access to females –
– the males advertise for mates
– the female chooses
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Intrasexual
• Combat
• Sperm Competition
• Infanticide
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COMBAT
• large body size
• Weaponry
• Armor
Walruspangolin-desert
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Male marine iguanas in combat
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Territories are only a few square meters.
Numbers identify the males which own the territory.
59 and 65 were the most successful males
Prime territories are near the water
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Competition between males has led to
extreme sexual dimorphism when males
can potentially control large harems.
Male and
female
seals
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weaponry
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pangolin-desert
Armor
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Male-Male competition often does
not stop with successful mating.
There is often post-copulatory
competition.
This type of intrasexual competition
is called,
SPERM COMPETITION
DAMSELFLY
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• Occurs with internal fertilization where one
female mates with more than one male within
a short period of time
• Sperm are in a race to the egg
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Other strategies developed due to sperm competition
• prolonged copulation
• copulatory plugs
• application of pheromones to the female to reduce her attractiveness to other males
• Scoop out the sperm left by a previous suitor
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INFANTICIDE • Animals that live in groups.
• Ex: Pride of lions
• Pride composed of a group of closely related females with 2 or 3 adult males related to each other but unrelated to the females
• average time a male holds a pride is a little over 2 years.
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Hanuman Langur (Semnopithecus entellus)
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Intersexual
1.Gift
2.Display
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Gifts:
Male Hanging Flies present their
female partners with insect food
items. The size of the gift is
correlated with the duration of
copulation and the number of
sperm transferred.
HANGING FLIES
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2.Display
Frigate Bird
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Female red-collared widowbird prefer long-tailed males
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What is the motive for female choice?
What benefit is gained?
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Possibility -1
Choosy females may get better
genes for their offspring
• The males that give better displays are genetically superior.
• This was tested in an experiment with the Gray Tree frogs
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Females get better genes
• Collected eggs from gray tree frogs and
fertilized half with sperm from long-calling
and half with sperm from short-calling males.
• Reared half of young on generous diet, others
on restricted diet.
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Females get better genes
• Measured five aspects of offspring performance:
– larval growth
– time to metamorphosis
– mass at metamorphosis
– larval survival
– post-metamorphic growth
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Results: Long calling males have
significantly higher fitness
Table 10.3
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House mice (Mus musculus musculus)
Salmonella enterica
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Results
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Possibility -2
Choosy females may benefit through acquisition of resources
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Hangingflies
• Randy Thornhill studied hangingflies.
• When a male hangingflycatches an insect it releases a pheromone to attract a female.
• Male offers prey to female. If she accepts, they mate while she feeds.
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Hangingflies
• The larger the prey the longer the female eats
and the more sperm the male transfers. After
20 minutes male has transferred the maximal
quantity of sperm.
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Hangingflies
• Male must offer a meal that lasts at least 5 minutes or no sperm are transferred.
• If meal lasts less than 20 minutes female breaks off copulation.
• At 20 minutes male breaks off copulation and seeks another female to offer the remains of the meal to.
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Hangingflies
• Female preference for males providing large
meals benefits her in two ways.
• 1. Provides nutrients and energy that allows
her to lay more eggs.
• 2. Saves her from having to hunt for herself.
Hunting is dangerous. (Males twice as likely
to be caught in spider webs.)
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Possibility -3
• Choosy females may have pre-existing sensory biases
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Choosy females may have preexisting
sensory biases.
• Proctor (1991,1992) work on water mites.
• Tiny freshwater animals that catch copepods.
Very poor vision. Depend on smell and touch.
• Females hunt copepods by standing on four
hind legs with four forelegs spread in net-like
fashion.
• Mating does not involve copulation. Instead
male deposits a spermatophore.
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Water mites
• When male smells a female he approaches
female while lifting and vibrating his front
legs. Frequency of vibrations similar to
copepod frequency.
• Female turns towards male and clutches. Then
male deposit spermatophore.
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