Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
You always know who your mother is • in some parts of Italy it is widely believed that a man must shoot a honey buzzard every year if his wife is to remain faithful • to fulfill this ritual many men usually leave their wife behind • some times the wife encourages him to go • for a male to leave his partner behind is risky business • most times we implicitly assume all sperm come from the same man • more frequently than we would like to think, this is probably wrong • sperm from different males compete to fertilize the egg • this idea forms the basis of sperm competition • what are the consequences for the female within whom this competition occurs? • what are the consequences for the male that inseminates the female? • sperm competition is widespread across the animal kingdom • this concept has changed the way biologists view the world Sperm competition: a form of sexual selection 1 Sperm competition in ancient history - the Greeks • before Christ was born, Aristotle and his predecessors knew about sperm competition • they saw that if a bitch copulated with two males during a single heat she could produce pups in the litter fathered by each male • the Greeks understood male anatomy very well because war provided lots of male bodies for dissection, but dissection of women was forbidden • Aristotle correctly illustrated the male uro-genital tract, but his knowledge of female anatomy was abysmal. • Aristotle was confused about menstruation - egg comes from mix of blood and female semen, but 400 years later Galen (130-201 A.D.) proposed that male and female semen coagulated to form an embryo (right!) • Aristotle also saw that if a hen copulated with two cockerels of different types the chicks usually resembled the second male that mated - combining two modern ideas of genetic markers and last male sperm precedence Aristotle was right about things he could see Darwinian origins of sperm competition • little else happened with sperm competition theory for about 2,000 years after Aristotle died • eggs and sperm were discovered as well as fusion to form embryos • sperm competition resurfaces with Charles Darwin • the problem: problem: if natural selection favors those who leave lots of offspring, then how could structures or behaviors that were obviously detrimental to their bearers evolve by natural selection? • the example: example: gaudy plumage or songs of male birds that made them conspicuous to predators • Darwin’ Darwin’s grandfather (Erasmus) wrote about some attributes of males that helped in fighting to obtain females • Charles thought that maybe females were attracted to beauty and preferred attractive males, but wasn’ wasn’t sure exactly why • Darwin identified two forces at work: competition between males and selection by females that he termed sexual selection • although initially neglected, by 1930’ 1930’s this concept was acknowledged on par with natural selection as a great theory Darwin advanced sperm competition with sexual selection 2 Next: sperm competition meets parental investment • Darwin assumed sperm competition only operated until an individual had acquired a mate • in late 1960s Bob Trivers asked what drives sexual selection and poses the idea that the effort put into raising the offspring determines the intensity of sexual selection • Put another way: sexual selection operates most intensely upon the sex that invests the least in rearing offspring • most of the time this is the male - contributes sperm, doesn’ doesn’t carry fetus or care for young after birth The sex that invests the most (females) become limiting for the one investing less (males), therefore increasing sexual selection pressure on the males Example of Trivers idea: the peacock • male provides few million sperm • female does everything else • once inseminated, female is out of game • male can inseminate again in minutes • sexually active males ALWAYS outnumber receptive females • this leads to intense competition and females can afford to be choosy • upshot: all females get inseminates, but reproductive success of males is highly variable The most aggressive or attractive peacock always wins out, hence the peacock’ peacock’s strong sexual dimorphism 3 Trivers idea still works in role reversal: magpies and phalaropes • magpie males have huge investment • feed incubating female and young • parental duties limit his ability to inseminate other females • net result: similar reproductive success among all males and very little sexual dimorphism - female and male magpies look almost identical (to us anyway) • phalarope males incubate and care for young • females compete for males and males choose • female phalaropes are larger, more aggressive, and more colorful The level of investment by each sex determines the intensity of sexual selection Trivers big idea was rooted in a little fruit fly (D. (D. melanogaster) melanogaster) • Angus Bateman used genetic marker of eye color to track reproductive success of male and female flies • he determined the mothers and fathers of 9,500 flies • for a male, the more he copulated, the better his reproductive success • for a female, one copulation was enough, so additional copulation did nothing to increase her reproductive success • therefore males like to copulate as often as possible, but from a reproductive standpoint females have nothing to gain • forms the basis for big ideas about how sexual selection works 4 Trivers big idea lays the foundation for behavioral ecology • Trivers championed Bateman’ Bateman’s studies and opened a new field • also introduced the concept of ‘mixed reproductive strategy’ strategy’: males of socially monogamous species (like birds) form a pair bond with one female but never pass on the opportunity to copulate with another female in order to further enhance reproductive success • males reproductive success not limited by ability to produce sperm • sperm are small and cheap to produce compared to eggs • males seem to have an almost limitless supply • therefore, sexual selection operates more intensely on males, right? • problem: problem: if numbers of males and females are about equal, then mow can most males copulate with many females and each female copulate only once? • presumed answer (wrong): (wrong): females simply acquiesced and put up with repeated matings from other males (but their heart really wasn’ wasn’t in it) In fact, females of many species are pretty interested in copulating repeatedly in search of the perfect mate The Father of Sperm Competition and the yellow dungfly • Geoff Parker spent his graduate career looking at cow pies in England • he noticed intense competition for females by the male flies • as soon as a female lands she is grabbed by a male who copulates, who is then ripped off her by a larger male, who mates again with her until another larger male throws him off • so females regularly copulate w/ several males • maybe the sperm of the two males competes after copulation in the female • similar mating occurs in guppies Sperm of different ejaculates compete between males for fertilization of female eggs 5 Understanding the different meanings of sperm competition • Darwin had thought about acquisition of partners • Parker had thought about acquisition of fertilization by sperm • Parker pointed out that acquiring a partner does no good if your sperm lose out to someone else • So sexual selection can continue right up to the point of fertilization • if females copulate with multiple males, then sexual selection favors the male who can fertilize the most eggs • males could win by having faster swimming sperm or maybe sperm that disabled competing sperm from another male Studying how sperm win this contest for fertilization is the basis for the study of sperm competition Sexual Conflict: the true battle of the sexes • Parker’ Parker’s idea was that selection picks out individuals rather than the population or species as a whole • same concept popularized by The Selfish Gene (Dawkins) • far from being cooperative between the sexes, reproduction is a selfishly motivated exercise • each male and female out to maximize benefits and minimize costs • sometimes interests coincide, but usually it’ it’s everyone out for themselves • this conflict has been the driving force for many behavioral, physiological, and anatomical aspects of reproduction Males and females are out to “screw” each other (literally) to get the best genetic deal 6 Intersexual and Intrasexual Conflict: male versus male versus female • Parker was initially less concerned about the battle between the sexes relative to the battle within one sex: males • sperm competition generates opposing selection pressure on males: • males who successfully fertilize females inseminated by other males win • males who prevent females they have inseminated from being fertilized by sperm from another male win • fertilizing more females is great, but costs of being cuckolded are also great • reproductive success of cuckolded male is depressed, and reduced even further if the male spends time rearing offspring that are not his own Pre and post-copulatory sperm competion have shaped the evolution of all species including our own Sperm competition: the evolutionary arms race • selection pressure of sperm competition drives the evolution of traits that promote male reproductive success • a war of attack and counter-measures to nullify the advantage • as soon as males evolve a characteristic, other males evolve to compensate • example: a larger ejaculate by some males prompts counter-selection on males to better protect their paternity (i.e. decrease female infidelity) • next example: better paternity protection by some males prompts counter selection on males to evolve ways to outsmart paternalistic males and fertilize their mates • Sound familiar? “Running as fast as you can to stay in the same place” 7 Love on the rocks: the complicated sex life of barnacles • Darwin dissected barnacles for eight tedious years • found not all barnacles were hemaphrodites, hemaphrodites, some had separate sexes • in many species the males were minute with several living parasitically inside the female • Darwin found 14 in one female! • Darwin described them as mere bags of spermatozoa • it seemed just one male could easily fertilize all the female’ female’s eggs Intense sperm competition occurring inside the female barnacle Why didn’ didn’t Darwin pursue the idea of sperm competition? • maybe he never made the connection - unlikely • illegitimate fertilization in plants was pretty clear • his poems on plant reproduction were considered thinly veiled erotica • Victorian prudery was well establish - taboo subjects and sexual repression • He wrote enthusiastically about the penis of one species being “wonderfully developed” developed”, lying “coiled up, like a great worm” worm” and “when fully extended, it must equal between eight to nine times the length of the animal.” animal.” • his wife Emma and daughter Henrietta acted as censors. Henrietta was described as both closeted Victorian matriarch and fussy moralist • Darwin’ Darwin’s grandfather was very enthusiastic about sex and prescribed it to his patients as a cure for hypochondria. Since Henrietta was also a hypochondriac, maybe she should have taken her grandfather’ grandfather’s suggested treatment Bad timing for sperm competition in the Victorian era 8 Changing the androcentric view of reproduction • battle between the sexes had been seen as very asymmetric • active males versus passive females • females were thought to only chose from different partners • until recently, a mixture of unconscious sexism and biological ignorance perpetuated the idea that sexual selection was male-dominated • a male who copulates with multiple females increases offspring • a female is unlikely to increase number, but maybe increase quality • if evolution is a game of numbers, then quantity is more important than quality • therefore, the belief persists that sexual selection operates more on males • the argument for multiple female mating was acquiescence, maybe because initial sperm competition studies were on insects (hard to know their motivation) • research in 1980s on birds showed that far from passive, females often actively sought multiple male partners • sperm choice by the female was a new idea of post-copulatory selection Females mate often and exercise mechanisms of sperm choice in reproduction Changing the androcentric view of reproduction (part 2) • if females could choose between different sperm, there is sexual conflict • when one sex evolves a method to constrain the reproductive success of the other, selection immediately favors counter adaptation to negate the advantage • example: males evolve a longer penis, females evolve longer uterus • escalating race of adaptation and counter-adaptation • sperm competition was viewed with anger by the feminists in 1980s “A woman without a man is like a fish without a bicycle” bicycle” Gloria Steinem 9 Turning the focus upon humans: sperm competition • Robin Baker and Mark Bellis combined infidelity and evolutionary theory • research on extra-marital sex, used undergraduate students to gather data • reported that their nationwide survey revealed high levels of polyandry • also suggested the idea of “killer sperm” sperm” battling in female reproductive tract • also suggested females use orgasm as a selection method to control sperm uptake and paternity of offspring • claimed a correlation between testis size and success in sperm competition • measured testes of 14 colleagues • took photos and asked women to rate the men on the likelihood they would engage in extra-pair copulation if given the chance • Baker reported his prediction was absolutely correct - significant correlation • sloppy science with poor experimental design: relationship of height, race and other could easily account for the positive correlation • then the ideas of “killer” killer” and “kamikaze” kamikaze” sperm, with blockers, egg getters, and all the rest began to unravel Almost impossible to perform controlled studies on humans So what can we learn about and from sperm competition? 1. 2. Why should an individual copulate with more than one partner? If a female copulates with more than one male, then how’ how’s the father chosen? What determines which male fertilizes her eggs? 3. How do sexual conflicts arising from sperm choice and sperm competition get resolved (antagonistic co-evolution)? • Why questions: Why do males inseminate with a particular number of sperm? Why do some males remain particularly close to females before and after copulating? Why do females of some species initiate copulations with so many males? Why do females seek extra-pair copulations? • How questions: How do physiological processes including sperm choice transform copulations into offspring? How do the sperm of different males interact in the reproductive tract? How important are sperm choice and sperm competition in determing which male fertilizes the egg? Sexual conflict challenges longcherished assumptions about male and female roles in sex 10