Sexual Conflict and Sperm
Competition
Sperm Competition
• Sperm compete to fertilize an egg
• Occurs in animals with external as
well as internal fertilization
• Probably the first type of sexual
competition to evolve due to
anisogamy
• Can be present in relatively
simple and sessile animals
Sperm of territorial and
sneaker males differs in quality
in male bluegill sunfish
(Lepomis macrochirus). Sneaker
male sperm is higher quality
and fertilizes more eggs
Sperm Competition
• With internal fertilization, sexual selection can
continue after copulation if females mate
multiply
• Sperm from rival males compete for
fertilizations…
• … OR cryptic female choice
Sperm Competition
• Males clearly benefit from
extra copulations
• Females can get all the sperm
needed to fertilize their eggs
from a single copulation
• So, why do females copulate
with multiple males?
Number of partners
Female Multiple Mating Hypotheses
1. Costs of resistance exceed the costs of
acquiescence
– No reproductive benefit, but may be harassed or
harmed if she resists
Female Multiple Mating Hypotheses
2. Direct (material) benefits
– Ensure all eggs are fertilized
– Increase amount of parental care
(males may co-operate in care if
they have a chance of paternity)
– Reduce male harassment of
herself and her young by giving all
males a chance of paternity (e.g.
primates)
Concealed ovulation
coupled with promiscuous
mating may help to prevent
infanticide in primates
Female Multiple Mating Hypotheses
3. Indirect (genetic benefits)
– Increase the genetic quality of her offspring
– E.g. Socially monogamous birds frequently engage
in extra-pair matings
• Colourful plumage and displays are viability indicators
that function to attract other males’ females for extrapair matings
• Females paired to attractive males are more faithful
– Different sires are better for sons versus daughters
Sexual Conflict
• Occurs when the optimal outcome is different
for males and females
• Male and female adaptations and counteradaptations
Enforced copulation in
Cimex lectularius
Waterfowl
• Length of male phallus is correlated with frequency
of forced extra-pair copulations
– Length varies from 1.5 to 40 cm
• In species where phallus is long and elaborate,
females have more elaborate vaginal morphology
– Reduce chances of male intromission without co-operation
Vagina
Mallard duck
(Anas platyrhynchos)
2 cm
Phallus
In species where males have a longer phallus, females have a more elaborate vagina
Forced Copulations – Water Striders
• Male pounces on female and
tries to secure a mating by
grasping her
• Across species there is
correlated evolution of male
morphology to increase
grasping (elongation of
grasping genitalia) and female
morphology to resist
(elongation of abdominal
spines to thwart the male)
MALE
Gerris incognitus
FEMALE
MALE
Gerris thoracicus
FEMALE
Male grasping genital segment and female abdominal spine correlate across species
Hemipteran beetle – Xylocoris
maculipennis
• Male simply pierces the body wall of the
female to inject sperm
– Swim around until they meet an egg
• Does this to other males too
– Sperm make their way to victim’s testes where
they can be passed on to females
Mate Guarding by Males
• Ensure paternity by guarding mates or by
copulating frequently to swamp the sperm of
rivals
• E.g. Socially monogamous birds
– Males follow females closely during the time she
is fertile
– Reduce efforts in feeding the chicks if female is
absent during this time
Bottlenose Dolphins
• 2 – 3 males form alliances to herd females and
defend them from other males
• Males sometimes use force to ensure that
females stay with them
Arthropods
• Prolonged copulation in arthropods is a form
of mate guarding
Male Adaptations
• Post-copulatory adaptations of males include:
– Sperm removal
• Remove sperm of
rival males before
depositing own
– Sperm displacement
• Insemination flushes
out previous
inseminations
Uninflated penis
Inflated penis
Flagellum caries
barbs to remove
sperm
Sperm removal in Odonata: Penis of Crocethemis
erythraea (top) and Orthetrum cancellatum (bottom)
Male Adaptations
• Post-copulatory adaptations of males include:
– Copulatory plugs
• Male “cements” up female’s genital opening to prevent
other males from mating with her
• E.g. Moniliformis dubius (parasitic acanthocephalan
worm in rats) seals up both females and other males
– Anti-aphrodisiacs
• Males deposit a scent after mating that discourages
other males
Male Adaptations
• Post-copulatory adaptations of males include:
– Sterile sperm
• Males produce two types of sperm – eusperm (fertile)
and parasperm (sterile)
• Parasperm is a “cheap filler” of female sperm storage
structures and also helps protect eusperm from
spermicide
Male Adaptations
• Post-copulatory adaptations of males include:
– Accessory gland proteins (Acps)
• Ejaculate contains a cocktail of proteins that influence
female behaviour and physiology
• Functions include incapacitating rival sperm, protecting
sperm, increasing egg laying rate in female, and
decreasing female’s propensity to re-mate
• Reduce female longevity
Acps decrease longevity in female Drosophila
melanogaster (dark blue line) relative to controls
Male Adaptations
• Post-copulatory adaptations of males include:
– Strategic allocation of sperm
• Male fowl increase their sperm investment when more
male competitors are around, and when females have
larger combs (produce larger eggs)
• Males may use less sperm or reject
females with whom they have
already mated
Gallus gallus
Female Adaptations
• Extra-pair copulations (EPC)
– Females of many seemingly monogamous or
polygynous species engage in infidelity
– Common in birds
Frequency (+SE) of extrapair young in relation to
the social mating system (M = monogamy, P =
polygyny) of 40 species of passerine birds
Female Adaptations
• Cryptic female choice
– Females may have some control over which male’s
sperm fertilizes her eggs
• E.g. Feral fowl (Gallus gallus):
– Subordinate males may force copulations
– Females contract cloaca to eject sperm
Field Crickets (Gryllus bimaculatus)
• Mated females with two males:
– Both siblings
– One sibling and one non-sibling
– Both non-siblings
• Hatching success decreased only if
both males were siblings
• DNA markers showed that females
preferentially stored sperm of
unrelated males
Sexual Cannibalism
• Females eat the males they copulate with
• Common in mantids and spiders
• Males may or may not deliberately sacrifice
themselves
Redback Spiders
• Males offer themselves as a meal after copulation
• Males that get eaten fertilise more eggs than
uneaten males, and thus benefit from sexual suicide
• Cost to male is low
– Little chance of mating again