Life Science 15 11/13/08 Hormones (& Chromosomes) and Sex Determination Fetal Sexual Development Undifferentiated gonads testes determining factor: if present, become testes. If not, become ovaries SRY/TDF: genes of the Y-chromosomes that cause male-ness Gonads make steroid hormones End of chromosomes determining sex, and now hormones do the rest If ovaries, start producing estrogen If testes, start producing testosterone Testosterone is also produced in adrenal glands Estrogen is also produced in fat cells Undifferentiated ducts connecting gonads & outside of body If testosterone present, become male organs If no testosterone present, become female Undifferentiated external genitals DHT: dihydrotestosterone, a chemical which is important in the development of male organs If testosterone and converted to DHT, become penis/testicles If no DHT present, becomes vagina Embryos develop female internal/external organs unless a Y-chromosome stimulates testes development leading to testosterone production which stimulates male sex organ development. Fetuses ?by default? are female, and become male when stimulated by a Y-chromosome What happens if an XY individual has no androgen receptors? androgen insensitivity syndrome Males could be described as humans who have y-chromosomes Individuals who do not have androgen receptors, however, cannot ?turn on? his genes to respond to androgens, therefore externally looks like a female Develops completely like a girl, but then never has period, and has testes in the place where ovaries should be ?super females?: almost as if they have no testosterone at all, since their receptors can?t respond to them What happens if an XY individual can?t convert testosterone to DHT? 5-?-reductase deficiency Have testes, produce testosterone, develop internal male organs, but NO PENIS Has secondary male characteristics (lower voice, facial hair, growth of genitals) at puberty What happens if a male has an extra T-chromosome? XYY Males: ?super males? Taller than average (~6? taller) Lower IQ than average 20x as likely to be in jail We?re Built Differently Casual sex study ?I have been noticing you around campus. I find you very attractive. Would you: Males Females ?go out with me tonight?? 50% 50% ?come over to my apartment tonight?? 69% 6% ?have sex with me tonight?? 75% 0% Early Nurturing is Necessarily Female Female reproductive investment is initially greater: lactation size of gamete: female gamete is less motile gestation early nurturing of the child/embryo is necessarily female dominated (especially in mammals) in the wild you will more likely see mothers taking care of their offspring Males Have Greater Reproduction Capacity, but Paternity Uncertainty What are the max limits on human reproduction? Females: 69, pregnant 27 times and having twins or triplets Males: 888 After the first mating, additional matings are valuable to males but not very valuable to females Females must wait until after pregnancy to really mate again Is the average number of heterosexual sex partners over a person?s life larger for males or for females? In the US, typically 10 for males and 3 for females. This data, however, has to be the same, because every time a male is having sex with a new partner, the female is having sex with a new partner Is we learn anything from this study, it?s that people tend to lie on surveys related to sexual behavior In which sex is variance in reproductive success greater? For males and for females there is a mean and standard deviation of reproductive success The mean is the same The standard deviation is not Case studies ( variance in male reproductive success Elephant seals December: Males fight to control beach Go without food until march Establish dominance hierarchy January: Females arrive, ready to mate High variance in male reproductive success: Top 4% get 85% of the copulations may have 50-200 babies in one year Most males never copulate Economics 101: those who have valuable resources don?t give them away Sex behavior predictions Sex that invests more will be more discriminating Sex that invests less will compete amongst themselves for access to the high-investing sex (and will seek greater paternity certainty) Physical differences in reproductive investment (constraints) lead to Physical and behavioral differences In females: choosiness In males: competition 11/18/08 Males and Females Are Vulnerable in Different Ways How can female mammals maximize their reproductive success? Invest heavily in offspring? YES Control access to lots of resources? YES Mate a lot? NO How can male mammals maximize their RS? Invest heavily in offspring? NO Control access to lots of resources? YES Mate a lot? YES Sex behavior predictions? in more detail More investment ( more discriminating Demand honest signals of health Honest signs meaning something that is very hard to fake Eg. Peacocks: the healthier and higher RS, the more eye spots; bird: can?t have big red belly unless very healthy; moose: can?t have giant antlers without being very healthy; a person?s height or weight; a person?s age Value fighting ability (so he can control resources) This way he can control more resources Eg. Dung beetles: males will fight until one wins, female will only mate with winner; horses, boars, and elephant seals do the same. Elicit a commitment to invest Courtship dance: goose has to follow female?s every movement for two weeks, shows that he does not spend time with any other female Nuptial gifts: bird builds a little house of straw and feathers, if females like the house then they mate. If not, the male smashes it down and starts over Bush cricket ejaculates 1/3 of his body weight, so very selective Because female reproductive investment is initially higher, they are vulnerable (they can be taken advantage of) early on. We expect the evolution of choosiness and demands for investment Lower reproductive investment More competition Need for paternity certainty Competition for access Mate guarding/extended mating E.g. frogs after mating the male hangs on to the female?s back for months; fruit flies stay together for about an hour; black widow spider breaks off his organ so it stays in her body, then eats him Sperm competition/anti-sperm Sperm Wars Observation: lots of cuckoldry (when males believe they are the offspring of someone else but they are not) Evolutionary response: produce more sperm Physical manifestation: Gorillas? (all females choose to mate with 1 male) Chimps? (everyone has sex with dozens of partners) Humans? (not necessarily 1 male, but not much more) Additional sperm wars Anti-sperm: sperm that links together that keeps the sperm of other males out (>90% in humans) Toxins: sperm kills off other sperm Copulatory plugs/cement glands: sperm clogs up female?s reproductive organs Shovel organs: males scoop out sperm in females and replaces it with their own Seawater injections: kills sperm in females and then replaces it with their own Why do males exhibit more mate guarding than females? To ensure that his sperm is the only sperm that is successful in a female Initial low reproductive investment and paternity uncertainty lead to male-male competition for access to females, indiscriminate male mating, and sperm wars. Females are really choosy and males have to compete against one another Can sexual selection help us understand why so few women run for political office? All presidents of US are male, and mast majority of senators are male, even though there are more women. This could possibly be because females do not compete ? regardless of whether you are the highest status or lowest status female on earth, you have the same amount of reproductive productivity What Are Mating Systems? Sexual dimorphism is considered a very good predictor of mating systems. Why? Sexual dimorphism: peacocks with brightly colored feathers indicate that there is a lot of sexual competition going on So in general, the more sexual dimorphism, the greater the difference between males and females, the more competitive the males are, and the more likely it is that males mate with many females Pidgins, on the other hand, all look the same, because there is an equal number of males and females Most birds are monogamous and there is no real competition Why would we see more monogamy in birds than mammals? Birds lay eggs, and these eggs need to be taken care of more (sitting on eggs). Males can take care of the offspring before they hatch. This also guarantees reproductively. Birds do not lactate either, so males can also help feed offspring. Males can take care of offspring AND feed offspring, so they do not have time to mate with other females
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