Postcopulatory Sexual Selection
Long Miao (lmiao@moon.ibp.ac.cn)
Institute of Biophysics, CAS
April 25, 2011
Male-Male Competition
• Andersson, 1994 “Sexual Selection”
• 5 fundamental areas of male-male competition
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Scrambles
Endurance Rivalry
Contests
Sperm competition
Mate choice (Female choice)
Sperm Competition
• Competition between the sperm of 2 or more
males within the reproductive tract of the female
• Typically Post copulatory sperm competition
– Competition between males after copulation
• With few ova= huge battle
– promiscuity go for broke
• Involves
– Physiological events that occur after insemination
– Behaviors before & after copulation
– Morphology
The Father of Sperm Competition
• Geoff Parker, 1970
• Studied sexual selection in Yellow Dungfly
• Looked at patterns of male-male
competition
Yellow Dungfly
What’s poo got to do with it…
• Female arrives at dung patty ready to mate
• After copulation lays eggs on dung
• Males mount & copulate with female as soon as
she arrives
• Larger males removes male from female &
copulates with her
• Females mate with multiple males throughout
day
Cow Patties
Yellow Dungfly
Parker’s Observations
• Under such extreme male male competition
Parker realized competition between males
continues through action of their sperm
• Copulation is not enough, sperm must make it
to the egg
• Defined sperm competition
– The competition between ejaculates of different
males for the fertilization of a female’s eggs
Parker’s Observations
• Sexual selection occurs in the reproductive tract
– Fertilization not copulation is the driving force
– Males “win” by
• Inseminating more sperm per ejaculate
• Faster swimming sperm
• Inseminate moderate amounts of sperm & remove sperm
from the last guy
• Disabling rival sperm
• Sperm competition results in opposing selection
forces in males
The Trade-off
• Selection favors BOTH
– Males that fertilize females that have been inseminated by other
males
– Males who prevent females they have inseminated from being
fertilized by another male
• Results in counter adaptations among competing male
strategies
– Ex producing larger ejaculates vs better mate guarding
strategies
• What?
– Males want to go around & fertilize as many females as possible,
but this is hard to do while they are mate guarding
Darwin & Sperm Competition
2 General categories of Sperm
Competition
• Adaptations that preventing rival sperm from
copulating with the mate
– Mate guarding, sequestering, frequent copulation,
production of mating plug, displacing rival sperm
• Adaptations that increase the Ability of one
male’s sperm to outcompete the sperm of
another male
– production of abundant sperm, fast sperm
Cuckoldry
• Female infidelity
• Derived from cuckoo
• Male loses some of
his reproductive
fitness to another
male
Sperm Precedence
• The sperm that
fertilizes the egg
effected by the order
of fertilization
Sperm Precedence
• First male precedence
– First male fertilizes the majority of eggs
• Last male precedence
– Last male fertilizes the majority of eggs
• promiscuity
Sperm Competition
• Adaptations that preventing rival males
from copulating with the mate
– Mate Guarding
Mate Guarding
• Male increases the amount of time he spends in close
association with a fertile female
• Male guards female partners from mating with other
males
• Tactic for reducing or preventing sperm competition
• Typically occurs when female is receptive for a short
time period
• Costs of mate guarding
– Interferes with male foraging
– Vulnerable to predators
Sand Martin
Sand Martin
• Male guards female 7-8 days
• Begins few days before she lays first egg
• Male remains next to female during her
fertile period
• Male presence reduces risk other males
copulating with female
Red-winged Blackbird
• David Westneat
• Male guards females
– Male experimentally
removed from territory
– Female copulations with
other males increased
100X
• Males Compromise
guarding for foraging time
– When males given food
paternity went up
Mate guarding strategies
• Mate grasping
• Prolonged
copulations
• Frequent Copulations
• Mate sequestering
• Mating plugs
Yellow Dungfly
Mate Grasping
Male remains on female’s back as she lays eggs
Pre-copulatory Guarding
• Occurs in species where the first male has
the best chance fertilizing the female’s egg
Freshwater Shrimp
• Males use specialized
legs to carry females
around
• Males attach to
female 9 days before
she is receptive
Mate Guarding- Amplexus
• Salamanders & Frogs
Mate Guarding- Prolonged
Copulation
• Male & females remain in copulation
• May last hours to days
• Male prevents other males by staying
attached
Yellow Dungfly
Prolonged copulation…male remains with female 40min
Prolonged Copulation
Frequent Copulations
• Male who increases copulation events with
female prevents mating with other males
• Animals frequently mate more often than
necessary for fertilization
Mate Guarding- Frequent
Copulation
• Giant water bug aka “toe biter”
• Male fertilizes females eggs
• As female lays eggs she sticks them to the
males back
– Lays 1-2 eggs at a time
• Cycle can last up to 36 hours (144 eggs)
• Male fertilizes over 99% of eggs
Giant Water Bug
Days….
Frequent Copulations
• Mountain Lion
• Ave 9 copulations per
hour
Mate Sequestering
• Male limits female contact with other
males by isolating her
– Physical isolation
– Chemical isolation
Mate Sequestering Physical
Isolation
Mate Sequestering Physical
Isolation
• Douglas Fir Bark Beetle
• Male sequesters female in gallery
Mate Sequestering Chemical
isolation
• Reducing Attractiveness of mates
• “Anti-aphrodisiac”
• Substances that reduce the females
attractiveness
• Ex. Make her smell like a male
• Reduce her pheromonal output
Chemical Mate Guarding
• Green veined white butterfly
• Males inject methyl salicylate along with
sperm
– Chemical compound derived from host plant
• Smell repels other males
Green veined white butterfly
Spider- Linyphia litigiosa
• Male disrupts female’s
chemical signals
• Male that finds virgin
female will reduce
evaporation of attractive
pheromones from her
web by packing it into a
tight mass
Central American Bee
• Mandibular gland
rubs secretions onto
female makes her
smell like a male
• Conceals female
Darkling Beetle
• Male rubs antiaphrodisiac
pheromone with
genitals
• Makes female less
attractive to other
males
Mating Plugs
• Aka copulatory plugs
• Male produced substances or structures
which are transferred to the female after
copulation
• Seal genital opening
• Ex guinea pig, chimpanzee, lemur,
masked palm civet,
Red-sided Garter Snake
Red-sided Garter Snake
• Intense male competition
• Male produces gelatinous secretion in
ejaculate
• Secretion hardens in the female oviduct
Ground Squirrels
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Intense competition for females
Male emerges from hibernation first
Testes grow
Females emerge & within 4 days are
receptive
– 1-2 hours
Mating Plugs
Mating Plugs
Orb Spider Argiope bruennichi
Orb Spider Argiope bruennichi
• Male transfers sperm packet to female
reproductive tract using pedipalp
• Pedipalp breaks off in female & created
mating plug
Orb Spider Argiope bruennichi
Honey Bee
• Uses own body as a
copulatory plug
• Male dies after
reproduction
• Harvester ant
Fly with no common name
• Ceratopogonid
Johannseniella nitida
• Tiny male crawls into
female genital
opening to create
mating plug
Ring-tailed Lemur
Other Functions of Mating Plugs…
• Sperm storage reservoir
• Reduce leakage
Displace Rival Sperm
• displace a previous male’s sperm
• Mechanism for last male precedence
– Last male fertilizes the majority of the
offspring by removing previos males’ sperm
• Adaptations
– Physical
– Chemical
Black-winged Damselfly
• Females mate with multiple males within a few
hours
• Sperm storage in spermotheca
• Male penis shaped like scrub brush
• During copulation male abdomen pumps up &
down “scrubbing” out the sperm of other males
– 90-100% of sperm removed
What’s a Spermatheca?
Black-winged Damselfly
Fertilization success depends on:
1. Physical traits of sperm:
numbers of sperm inseminated, quality,
size, morphology, speed, cooperation of
close-related
2. Seminal fluid and its components produced
by several accessory glands in male body
3. Cryptic female choice