Ecology of Sexual Behavior
From Dr. Tatiana’s Sex Advice to All Creation
Adapted by Dr. JodyLee Estrada Duek
The History of Sexual Assumptions
• A. J. Bateman published a paper in 1948 in the
journal Heredity
• Claimed he had proved
– males evolved for lovemaking
– females evolved for making babies
• Work based on Drosophila melanogaster
• NOTE: there are over 2000 species of Drosophila
Bateman’s Study
• Kept equal numbers male & female in small
bottles for 3-4 days
• Males still keen to mate as often as possible
• Females rejected advances after 1 or 2 events
• RESULT:
– For males – more partners = more offspring
– For females – it made no difference
Bateman’s Explanation #1
• Males produce tiny, cheap sperm
• Females produce a few large, expensive eggs
• Females in many species store sperm for days,
months, even years; 1 mating can last a lifetime
• One male could fertilize eggs of many females
Bateman’s Explanation #2
• Limits to reproduction are:
– For females, how fast can make eggs and/or
rear the young
– For males, how many females they can find
and mate with
THEREFORE…
• Males naturally philanderers
• Females naturally chaste
childraisers, indifferent or
hostile to mating more than
absolutely necessary (the “I
have a headache” theory)
Bateman’s Principle
• Men are cads, women are
saints… or
• Males optimize genetically
with multiple mates
• For females mating more
than absolutely needed is a
waste of time
Since 1948…
• Patriarchal types agreed
• Feminists complained about unfairness
• Scientists expounded, espoused it, added extra
reasons for female chastity
– Fear of venereal disease
– Risk of being caught by a predator
• Humans fought about it
Examples cited
• Species where females mate only once
– Alfalfa leaf-cutter bee (a solitary bee)
• Species where males attempt to mate
constantly when in season
– Frogs and toads
However
…And now for
something
completely
different…
In D. melanogaster…
• Females will multiple mate, but only a few times
each week, and…
• Females who mate once have fewer eggs than
those who mate several times
• So Bateman’s experiment was too short, he quit
too soon…
• In a related species, D. hydei, females mate
several times every morning
Why didn’t anyone speak up?
• Argument sounded reasonable
• Seemed to be supported by
thousands of observations on
hundreds of species
• Some scientists noticed that some
females mated with multiple males
but it was assumed
– Females had “malfunctioned”
– Males had “led them astray”
Enter genetic studies, stage left
• In the 1980s parentage could be established
• Females were hardly ever faithful to one mate
• Rampant promiscuity was everywhere
Female benefits
• Higher rates of conception or more eggs
with multiple partners
– Rabbits, Gunnison’s prairie dogs
– Sand lizards
• More eggs fertilized (in a large clutch) if
spawning promiscuously
– Slippery dick (a coral reef fish)
This female promiscuity leads to…
• A true war between the sexes
• The last male to mate probably has a better chance
of siring offspring
• Males compete to have their sperm be what counts
Males try to
• Deposit more of their sperm
• Remove the sperm of rivals
– Damselfly penis has horns and bristles to scour
inside female before leaving his sperm
• Get female to eject prior sperm
– Moth Olceclostera seraphica rubs penile parts
together on female’s body, vibrations convince
females to eject
– Dunnock: a small bird, gently pecks genitalia
• Block further sperm entry
– Ghost spider crab makes “jelly”, seals off prior sperm
One example
• An African Bird: red-billed buffalo weaver
• Female is extremely promiscuous
• Male has a pseudophallus; stimulates female
with this for about ½ hour
• He then ejaculates from genital opening
• Male who stimulates female “most vigorously”
presumably has his sperm utilized for her eggs
A general rule…
• In species where females practice serial
monogamy, penises generally small compared
to body size, unornamented
– Male gorilla – about 500 pounds, 2 inch penis
• If females are promiscuous, penises adapted
– Argentine lake duck – about 4 pounds, 8 inch penis
with spines – see next slides
Pseudophallus
• Most birds do not have an intromissive organ
• Typically male touches cloaca to female,
passes sperm or sperm packet
• Ducks and geese have a pseudophallus
• Forcible “rape” is only known in ducks and
geese
Argentine lake duck
• 42.5 cm in length is the record
• The male may use the brushlike tip of its penis to scrub the
sperm of previous mates from
the female's oviduct.
Another mechanism
• Blockage
– In honeybees, male genitals break off in female, he
explodes with a loud snap – he dies, she is blocked with
a “chastity belt”
– Dozens to thousands of males chase 1 queen
– He is not likely to mate again, this is his only chance
• However, a Queen bee needs multiple mates
– because of the sex-determining gene
– if a male has her allele she’ll have sterile sons
– she must outcross & be heterozygous
So if you’re the second bee…
• Honeybee penis has a structure on the tip
to remove the genitalia of prior male
• How did this evolve?
–
–
–
–
–
1 queen was promiscuous, was successful
Her genes multiplied in the population
1 male exploded, prevented rivals from mating
His genes multiplied
1 queen could remove plug herself, or 1 male
could remove rival plug…
A genetic race…
• Males evolve mechanisms to control access to the
female
– Rat penis is nearly prehensile, does a “toilet plunger”
maneuver to suction out a plug
• Females evolve mechanisms to evade male control
– Fox squirrel reaches around and removes a male plug and
eats it
So…
• Just seducing all the females
doesn’t work if they don’t use your sperm
• Better off trying to maximize eggs fertilized
– Splendid fairy wren male – 8 billion sperm
• Sperm competition
– Experiments with yellow dung flies shows that if
there is sperm competition average male testes size
increases in 10 generations
– Seahorses (female deposits eggs in his pouch, he
fertilizes, carries) have extremely low sperm count
And…
• Sperm die in huge numbers
as they move through
female’s body
– May be digested, ejected,
rounded up
– Possibly a few saved and
stored, rest discarded
A Human Example
Vagina usually acidic – only 10% of sperm survive
Cervical mucus – perhaps 10% pass
White blood cells attack
In Fallopian tubes have gone from 180 million to a few
hundred sperm
Other ideas
• Sperm size and shape
– D. bifurca produces sperm 20X longer than it
is (3 mm, 60 mm)
– Tandem sperm – opossum, water beetles,
millipedes
– Hooked sperm – koalas, rodents, crickets
– Wheel sperm – crayfish
– Corkscrew sperm – land snails
– 100 tail sperm, crawling sperm
– Sperm packets
For more ideas… read the book