Human evolution theory utilizing concepts of neoteny & female sexual selection
An etiology of neuropsychological disorders such as autism and dyslexia, and the origin of left handedness.

 

 Library of Excerpts

Sexual Organs and Heterochronic Theory


"More convincing vestiges of a sexual selective history in which females mated polyandrously can be found in the human male. Perhaps the clearest such vestige is testis size (Short, 1977). Men's testes are substantially larger, relative to body size, than those of gorillas, a species in which males are polygynous but females mate monogamously so that "sperm competition" within the female reproductive tract is absent. (goes on to talk of chimpanzees) (Wilson, M., Daly, M. (1992) The man who mistook his wife for a chattel: The Adapted Mind; (Barkow, J.H. & Cosmides, L. & Tooby, J. eds.), Oxford Univ. Press, New York. p. 299)

“An excessively large penis produced by epigamic selection would be no less effective in delivering sperm than one of optimal length, but a substantially shorter (than optimal) penis would obviously place its owner’s ejaculates at a disadvantage in competition with those deposited by a longer organ. Therefore it may be possible to test some predictions of the ejaculate delivery/sperm competition hypothesis for human penis length and distinguish this function from one of display.” (Robert L. Smith (1984) Human Sperm Competition. in Sperm Competition and the Evolution of Animal Mating Systems pp. 631)

“The direct utilization of the male genitals and their anatomical modifications as display characters is, as Wickler (1967) shows, a feature of Ceboid rather than Cercopithecoid behavior. Of particular interest is the finding that just as males mimic female genital characteristics in the Cercopithecoidea so in certain Ceboids do the females mimic the male characteristics. The female squirrel monkey (Saimiri) uses the male type genital display as an expression of dominance and shows mimicry of male genitalia through possession of a large clitoris and a pseudoscrotal enlargement dorsal to it.” (Sexual selection in the primates (1972) John H. Crook in Sexual selection and the descent of man 1871-1971 Campbell, Bernard (ed.) pp. 245)

"One characteristic among primates has been clearly targeted for possible selection by Fisherian female choice--male penis size. Primate males living in groups with many females and many males, groups in which promiscuity is the mating rule, have long penes (Dixon 1978). Male chimps, in fact, use their penis for display toward estrous females. Because a longer penis would give a female pleasure (note that the human male has the longest and thickest penis of any primate), female choice might have been a factor driving penis length to extremes among primates." (Small, Meredith F. (1993) Female choices: Sexual behavior of female primates. Cornell Univ. Press, Ithaca, 1993 pp. 109)

"The differences in dizygotic twin frequency, and presumably ovulaton rate, are in the same direction as the differences in testis size. The frequencies of dizygotic twins are even higher (up to 49 per 1,000 births) among African blacks. ... Yoruba women, with the world's highest frequency of dizygotic twins, have higher FSH and LH levels at the time of ovulation than do Japanese women, who have the lowest frequency of dizygotic twins. This variation in female hormone levels may contribute to the distribution of the incidence of breast cancer, which is known to be related to oestrogen levels. Even after all other risk factors for breast cancer have been taken into account, the incidence among Japanese women remains inexplicably low. Perhaps this puzzel, the so-called 'Japanese factor' of (breast cancer, is related to the low double-ovulation frequencies and low hormone levels." Diamond, JM (1986) Variation in human testis size. Nature (London) 320: 488-489)

"In a sense, humans are more like bonobos, with a lack of any real cycle in sexual behavior. The point is, humans may not have any real cycle at all (Blaffer Hrdy 1983). Our lack of a swelling may just be part of a shared ancestral history with all those other primate females who didn't have flamboyant swellings in the first place." (Small, Meredith F. (1993) Female choices: Sexual behavior of female primates. Cornell Univ. Press, Ithaca, 1993 pp. 129)

"...in the chimpanzee, several males mate frequently with the oestroud females, so that each male has to deposit enough sperm to compete with the presence of sperm form other males. For the chimpanzee, therefore, we hypothesize that selection will favour the male that can deposit the largest number of sperm; thus the volume of spermatogenic tissue and hence the testis size is far greater in the chimpanzee than in the gorilla or orangutan. If this is correct, it implies that primates in which more than one male mates with each oestrous female should have larger testes relative to their body weight than those which single-male breeding systems. We have tested this prediction across a wide range of primates, and the results support the hypothesis. The relative size of testes may, therefore, provide a valuable clue to the breeding system of a primate species." (Harcourt AH, Harvey PH, Larson SG, Short RV (1981) Testis weight body weight and breeding system in primates. Nature 293: pp. 55)

"Although measurements of testis size by orchidometry in living subjects are difficult to standardize, they suggest smaller testes in Japanese and Korean men than in Caucasions. Weighing at autopsy is more accurate and showed that the size was twofold lower in two Chinese samples compared with a Danish sample. Differences in body size make only a slight contribution to these values." (Diamond, JM (1986) Variation in human testis size. Nature (London) 320: 488) [note: larger African testes supports promiscous origins hypothesis, yet if African testosterone levels are higher, it refutes inverse relationship between sperm and T production]

"We do not know why men have conspicuous genitals, but a male chimp solicits a female by opening his legs, displaying an erect penis and flicking his phallus with a finger as he gazes at a potential partner. A prominant, distinctive penis helps broadcast one's individuality and sexual vigor, which may lure female friends. In many species of insects and primates, males have exceptionally elaborate penises, and scientists think these evolved specifically because females chose those males with elaborate, sexally stimulating genitals. So perhaps as Lucy's ancestors became bipedal some four million years ago, males began to parade their genitals in order to make special friends with favored females--selecting for those with large organs." (Fisher, H. (1992) Anatomy of Love: The Mysteries of Mating, Marriage, and Why We Stray. Simon & Schuster, New York, 1992. pp. 177)

"As we have seen, chimpanzees live in groups where several males may share a female, and therefore there is a premium on the ability to ejaculate often and voluminously---he who does so has the best chance of being the father. This conjecture holds up across all the monkeys and across all rodents. The more they can be sure of sexual monopoly, as the gorilla can, the smaller their testes; the more they live in multimale promiscuous groups, the larger their testes. It began to look as if Short had stumbled on an anatomical clue to a species' mating system: Big testicles equals polygamous females. Could it be used to predict the mating system of species a that had not been studied? For example, very little is known about the societies of dolphins and whales, but a good deal is known of their anatomy, thanks to whaling. They all have enormous testicle, even allowing for their size. The testicles of a right whale weigh more than a ton and account for 2 percent of its body weight. So given the monkey pattern, it is reasonable to predict that female whales and dolphins are mostly not monogamous but will mate with several males. As far as is known, this is the case. The mating system of the bottle-nosed dolphin seems to consist of forcible "herding" of fertile females by shifting coalitions of males and sometimes even the simultaneous impregnation of such a female by two males at the same time---a case of sperm competition more severe than anything in the chimpanzee world. Sperm whales, which live in harems like gorillas, have comparatively smaller testicles; one male has a monopoly over his harem and has no sperm competitors. Let us now apply this prediction to man. For an ape, man's testicles are medium-sized---considerably bigger than a gorilla's. Like a chimpanzee's, human testicles are housed in a scrotum that hangs outside the body where it keeps the sperm that have already been produced cool, therefore increasing their shelf life, as it were. This is all evidence of sperm competition in man. But human testicles are not nearly as large as those of chimps, and there is some tentative evidence that they are not operating on full power (that is, they might once have been bigger in our ancestors): Sperm production per gram of tissue is unusually low in man. All in all, it seems fair to conclude that women are not highly promiscuous, which is what we expected to find." (Ridley 1993: 220-21, The Red Queen)

"We have heard, but have been unable to verify, that undescended testes are common in autistic children." (Geschwind & Galaburda 1987: 173, Cerebral Lateralization)


"Females are born with about 2 million egg cells, but fewer than 500 of them will ripen over a lifetime. Human females begin to make viable eggs at about age fifteen and stop at about age fifty....Human males produce about 3,000 tiny gametes a day." (Small, Meredith F. (1993) Female choices: Sexual behavior of female primates. Cornell Univ. Press, Ithaca, 1993 pp. 24)

"Thus, the human penis shows high species-specificity as an elaborated genital structure. Penis size also varies moderately across populations, being largest among African populations, smaller among European populations, and smallest among East Asian populations, but with substantial overlap (Rushton, 1987, 1988b, 1989b). Penises are highly age-specific, attaining their full resting size only after puberty, and are highly courtship-specific, attaining their full erect size only under sexually arousing conditions. Thus, penises show all the hallmarks of a morphological trait elaborated somehow through sexual selection." (Miller, Geoffrey F. (1994) Evolution of the human brain through runaway sexual selection: the mind as a protean courtship device. unpublished thesis. pp. 185)

"Based on this analysis of the adaptive functions of female orgasm, we can start to understand why the female clitoris remained small and relatively hard to stimulate, while the male penis enlarged to unprecedented proportions in our linage. Maintenance of a small, relatively hidden, and relatively hard-to-stimulate clitoris enhanced female discernment and choosiness with respect to assessing sexual stimulation by males. A larger, more protruberant clitoris which could have been effectively stimulated by almost any tactile contact, however inept, would have made its female bearer less able to judge the relative stimulation capabilities of different males. She might have mated with less sexually competent males and produced less sexually competent sons." (Miller, Geoffrey F. (1994) Evolution of the human brain through runaway sexual selection: the mind as a protean courtship device. unpublished thesis. pp. 215)

"The large average size of the human penis (five to six inches, versus three for chimps, and one-half that for gorillas) may have been, evolutionists speculate, to frighten other males. Or to attract females. Or to enhance their pleasure. Perhaps the best hypothesis is that the longer penis delivers sperm more closely to the eggs: today's biologists claim that for females who mated with several males, the male with the longest penis delivered his sperm more safely." (Margulis, L & Sagan, D. (1991) Mystery Dance, On the Evolution of Human Sexuality: Summit Books, New York pp. 23)

"Large testes and big penises are advantages only under conditions of widespread sexual promiscuity. Among our closest relatives the great apes, only male chimpanzees have testicles more prodigious than those of men. And chimps, with their big, heavy testicles, are more sexually promiscuous than humans. that the sperm-producing organs of chimps and humans are relatively big and heavy strongly suggests that some of our not-so-distant hominid ancestors were far more promiscuous than gorillas and orangutans --- or than many people are today. In the evolutionary past, the competition to reach primate eggs was sometimes between sperm from different male donors. If two or more males copulated with the same female within a period of days, an advantage in begetting offspring accrued to the one who ejaculated the greatest quantity of vigorous sperm cells. Like an auto race won by the driver whose sponsoring company can afford to provide him with the most souped-up car, the male with the best-timed copulation, most far-reaching ejaculation, and biggest testicular "engine" able to produce the greatest quantity of sperm tended to win the "game" of impregnation. souped-up genitals with a lot of spermatic firepower, like incredibly expensive streamlined racing cars, are worth it only if there is some sort of race or contest. Otherwise they seem expensive." (Margulis, L & Sagan, D. (1991) Mystery Dance, On the Evolution of Human Sexuality: Summit Books, New York pp. 33)

"Sexually immature, prepubescent boys enjoy a range of sexual response similar to that of adult women. Kinsey and his coworkers reported, "The most remarkable aspect of the preadolescent population is its capacity to achieve repeated orgasm in limited periods of time. This capacity definitely exceeds in the ability of teen-age boys who, in turn, are much more capable than older males." The response of young boys, including mulitiple climax without losing an erection, "is," according to biologist Donald Symons, "perhaps similar to orgasmic women." Neither prepubescent orgasmic boys nor women ejaculate sperm, and thus the "ability of females to experience multiple orgasms may be an incidental effect of their inability to ejaculate."" (Margulis, L & Sagan, D. (1991) Mystery Dance, On the Evolution of Human Sexuality: Summit Books, New York pp. 86)

"Roger Short...had predicted that in {primate} species where females mate with more than one male during a reproductive cycle, the males would have larger testes for their body size than in species whose females had only a single mate per cycle. When the females were promiscous, the sperm of each male would have to compete with those of other males, and the male producing the most sperm was most likely to generate offspring. [quote from] (Harvey and Clutton-Brock 1983, p. 315" (Cronin, Helena (1992) The Ant and the Peacock: Cambridge Univ. Press, Cambridge p. 97)

"The ethnographic record {e.g., A French Army Surgeon (1898/1972), a 30-year specialist in genitourinary diseases} makes reference to numerous anatomical distinctions which show a similar pattern of whites being between blacks and Orientals. These include the placement of female genitals (Orientals front and high; blacks back and low); angle and texture of erection (Orientals parallel to body and stiff, blacks at right angles to body and flexible); salient buttocks, breasts, and muscularity (Orientals least, blacks most); and size of genitalia (Orientals smallest, blacks largest). We averaged the ethnographic data on erect penis and found the means to approximate: Orientals, 4 to 5.5 in. in length and 1.25 in. in diameter; Caucasions, 5.5 to 6 in. in length and 1.5 in. in diameter; blacks, 6.25 to 8 in. in length and 2 in. in diameter. Women were proportionate to men, with Orientals having smaller vaginas and blacks larger ones, relative to Caucasians. Clitoral size differed in length: in European women, 1.2 in.; in African women, 2 in. variations were noted; in French West Indies, the size of the penis and vagina covaried with amount of black admixture; Arab men, who were often mixed with black, had larger penises than Europeans. Recent data show similar patterns. Measurements taken from living subjects as well as those at autopsy, show the size of testes is twofold lower in Asian men than Europeans (9 g vs 21 g), a difference too large to be accounted for entirely in terms of body size (Diamond, 1986; Short, 1984). Concomitantly, as mentioned, Asian women have lower ovulation rates than Caucasian women, as indexed by dizotic twin frequency, with the frequency per 1000 across several Asian populations being < 4, while for Caucasians it is 8, and for blacks 16 per 1000 (Bulmer, 1970; Diamond, 1986). Contrary to the general trend, Freeman (1934) observed that, at autopsy, American blacks had less heavy testes than American whites (13g vs 15g). Freeman (1934), however, did find that black women had heavier ovaries than white women. Subsequently Daniel, Fienstein, Howard-Peebles, and Baxley (1982) found no black-white difference in testicular volume among American adolescents, while Ajmani, Jain, and Saxena (1985) found larger scrotal circumference in Nigerians than Europeans (212.6 mm vs 195.1 mm or 8.37 in. vs 7.68 in.). A French Army Surgeon (1988/1972) also provided early observations that, in speed of sexual maturation, Orientals < whites< blacks. Several subsequent studies are confirmatory. In the United States, blacks are more precoscious than whites as indexed by age at menarche, first sexual experience, and first pregnancy (Malina, 1979). A national probability sample of American youth found that by age 12, 19% of black girls had reached the highest stages of breast and pubic hair development, compared to 5% of white girls (Harlan, Harlan, & Grillo, 1980), although the same survey found white and black boys to be similar (Harlan, Grillo, Coroni-Huntley, & Leaverton, 1979). Subsequently, Westney, Jenkins, Butts, and Williams (1984) found that 60% of 11-year-old black boys had reached the stage of acelerated penis growth in contrast to the white norm of 50% of 12 1/2-year-olds. This genital stage significantly predicted onset of sexual interest, with 2.2% of the black boys experiencing intercourse by age 11. While some surveys found that Oriental girls enter puberty as early as whites (Eveleth & Tanner, 1976), others suggest that in both physical development and onset of interest in sex, the Japanese, on the average, lag 1.5 to 2 years behind white Americans (Asayama, 1975). (Rushton, J.P. & Bogaert, A.F. (1987) Race differences in sexual behavior: Testing an evolutionary hypothesis. Journal Research in Personality 21(4): pp. 536-7)


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