Ecology of Sexual Behavior

1. Ecology of Sexual Behavior From Dr. Tatiana’s Sex Advice to All Creation Adapted by Dr. JodyLee Estrada Duek

2. 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

3. 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

4. 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

5. 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

6. THEREFORE… • Males naturally philanderers • Females naturally chaste childraisers, indifferent or hostile to mating more than absolutely necessary (the “I have a headache” theory)

7. 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

8. 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

9. 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

10. However …And now for something completely different…

11. 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

12. 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”

13. Enter genetic studies, stage left • In the 1980s parentage could be established • Females were hardly ever faithful to one mate • Rampant promiscuity was everywhere

14. 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)

15. 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

16. 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

17. 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

18. 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

19. 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

20. Argentine lake duck • 42.5 cm in length is the record • The male may use the brush-like tip of its penis to scrub the sperm of previous mates from the female's oviduct.

21. 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

22. 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…

23. 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

24. 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

25. 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

26. 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

27. For more ideas… read the book