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 – 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 DungflyMate 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 anti-aphrodisiac 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

• 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