SECTION: HUMAN MALES COMPARED TO OTHER MALE PRIMATES
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EVOLVED MORPHOLOGICAL CHARACTERISTICS OF HUMAN MALES
HUMAN MALES COMPARED TO OTHER MALE PRIMATES
Compared to males of many other species, human males are relatively drab morphologically. They possess no specialized, intimidating physical structures for fighting, such as antlers or horns. They have not been physically "deformed" by strong epigamic pressures to present females with impressive, but otherwise biologically useless and costly structures, such as the peacock’s large tail or the eye-catching colors of male mallard ducks. Nor is the sexual dimorphism of human males particularly impressive. Human males have been shaped by intra-sexual competition – as reflected by their slightly larger size compared to females – yet they are not three times the size of their female partners, as are male elephant seals. Nevertheless, compared to other primate males, human males stand out morphologically in several respects. Interestingly, of all the primates, human males have the longest erect penis, but not the largest testicles. The following sections explore several theories that attempt to explain why human males have evolved specific morphologies.
MALE SIZE AND UPPER BODY STRENGTH
Bones of australopithecines, Australopithecus afarensis, were unearthed near Hadar, Ethiopia. The males of some of these ancestors were nearly twice as large as the females…
- --Lynn Margulis and Dorian Sagan, Mystery Dance, 1991, p 14.
As we have learned from previous chapters, the female form of the human body is the "generic" human form. The average adult female bodyweight is 80% to 89% of the weight of the male. Ancestral female choice has shaped present male appearance. Females typically mated preferentially with males who exhibited good genes directly through qualities such as visible health and vigor (face symmetry is one cue to this), and indirectly through the ability to achieve high status and amass resources. Larger, stronger males typically won most contests for sexual access to females or resources. These larger, stronger males left larger numbers of offspring who, in turn, inherited the larger and stronger physiques of their fathers and grandfathers.
HUMAN TESTES SIZE COMPARED TO OTHER PRIMATE SPECIES
Male gorillas, who typically weigh about 450 lbs., possess testes that weigh, on average, an ounce and a quarter. Male chimpanzees, weighing in at around 100 lbs. on average, have the largest primate testes – about four ounces. Why does such a relatively small primate, such as chimpanzee, have testicles that outweigh those of the gorilla? The answer has very little to do with the size of the primate and very much to do with the degree of sperm competition within a species.
The polygynous chimpanzee enjoys a relatively very active and promiscuous sex life. A chimpanzee may find opportunities to copulate virtually every day. Compare that to a gorilla’s modest sex life, and the answer to the mystery of the testicle size differential begins to materialize. A gorilla, even one with a large harem of females, has limited mating opportunities – usually only a few a year (Diamond, 1985). This is due to the fact that female gorillas remain sexually unreceptive for about three to four years after giving birth. Even after this lengthy sexual hiatus, she is only receptive for a few days a month, until she is impregnated again.
With relatively minimal sexual activity among gorillas, there is little sperm competition. A copulating male has a significant chance of impregnating the female, because there is little chance of the presence of sperm from another male. Compared to the promiscuous chimpanzee copulating on an almost daily basis, large testicles are not necessary for gorillas. With such a high frequency of copulation, it is in the best interest of the male chimpanzee to evolve large testicles to store and deliver a large amount of sperm in order to outperform other males and increase the chances of winning the competition to fertilize the receptive and promiscuous female.
The testes of human males are intermediate in size between the gorilla and chimpanzee. The average weight of the testes in a man is approximately an ounce and a half (Diamond, 1985). Humans’ medium-size testicles (relative to chimps and gorillas) suffice because the average human male copulates with a frequency less than that of the chimp, but more than that of the gorilla. Humans are less promiscuous than chimpanzees. A receptive human female does not copulate as often as a female chimpanzee, and therefore does not force a significant degree of sperm competition among several different males (Diamond, 1985).
SPERM COMPETITION
DelBarco-Trillo and Ferkin (2004) said, “Sperm competition occurs when a female copulates with two or more males and the sperm of those males compete within the female’s reproductive tract to fertilize her egg” (p. 446). In other words, an increased amount of sperm will increase chances of a male being able to father offspring. In essence the study was trying to determine whether male mammals use chemical signals to assess and respond to sperm competition (Trillo & Ferkin, 2004). Using meadow voles in their first study, the authors employed either a control group with just a male and female in a cage, or the risk of sperm competition (RSC) group in which there was a male and female in a cage that had the odor of another male. They found that males in the RSC group invested significantly more sperm than the control group. There was, however, variation in sperm investment based on the weight of the individual males, so that the larger the male the more sperm he produced. In a subsequent study they tested whether using odors from males of different species of mouse effected the sperm investment. They concluded that males increase sperm investment when they encounter odors of conspecific (same species) males, but not in the presence of males of another species.
In a study by Snook (2005), the author discusses the other male and female factors that are contributors to sperm competition success. It was found that sperm length plays a role in chances of fertilization. Thus, there may be a relation between sperm competition and sperm length. Sperm longevity and mobility increases fertilization as well. Females were able to control the process whereby sperm is transported from the place in which it is deposited to where it fertilizes an egg. They could reduce sperm competition by not allowing sperm from one male and instead allowing the sperm from another male to reach the place of fertilization These findings all support the claim that it is not only the number of sperm that leads to increased likelihood of fertilization but many other factors as well. This simply takes the game of sperm competition to a whole new level.
Parker and Ball’s (2005) study, “suggests that testis size (relative to body mass) increases with measures of sperm competition level across species, because of a positive relation between sperm competition and mating rate” (p. 235). In this study they found that sperm allocation (mating rate and testes size) may be unaffected by sperm limitations unless there were biological reasons, such as females mating with more than one male. There are two models that show the range of sperm competition. One involves a female who only mates with one or at most two males during a reproductive cycle and one in which a female mates with many males and there is a great deal of sperm competition. Ejaculate expenditure and testis size increase linearly with sperm competition risk (Parker & Ball, 2005). The average sperm count was found to increase across species with sperm competition risk yet they decreased with sperm competition intensity (Parker & Ball, 2005). Therefore sperm distribution does not necessarily increase every time there is sperm competition. Distribution simply depends on the level of the competition that is present.
One final study by Simmons et al. (2004) focused on human sperm competition in relation to testis size, sperm production, and rates of extrapair copulations (EPC), in which there was more than one sexual partner. In this study they examined patterns of testis size variation, sperm production, and EPCs which were used to assess the general risk of sperm competition. They used 194 women and 222 men in their study in which participants answered a questionnaire about their sexual behavior. The questions asked about such things as whether or not they had ever had sex with someone who was not their partner while in a relationship as well as how many times this occurred. Questions related to permissive attitudes were also asked, such as rating sex without love on a scale of 1 (agree) to 9 (disagree). Males were asked to measure their testes, and 50 of the men were asked to give semen samples. They found no significant difference in testis size between men who had EPCs versus those who did not. The testis size was found not to be related to permissive attitude. The data collected showed that, in relation to nonhuman primates, human sperm competition is relatively low, even with rates of EPCs around 5-27% in people under the age of 30. The “conclusions are consistent with the position of humans in comparative analyses of testis size across primates” (Simmons et al, 2004). Finally there was no evidence to suggest that human males adopt alternative reproductive strategies of sperm competition when engaging in EPCs (Simmons et al, 2004).
Unconcious Competition: Sperm Competition and other Adaptations
It is no surprise that men compete for women. This competition is seen all the time in the form of expensive cars, big muscles, and fancy clothes. Evolution wise, women hold the key to what men want. Men want their genes to continue on and on through the timeline of humanity. This fierce competition for mating possibilities has resulted in the evolution of some very interesting mechanisms in males. Males compete not only visually and physically, but even with their sperm. In the ancestral environment, females might mate with several males as we see in some animal species today. Perhaps while the female’s mate leaves for a long hunt, she attempts to obtain more beneficial genes from another mate. As only one sperm can inseminate the egg and the males have already given as much physical effort as possible, it is up to their sperm to continue the battle.
Some researchers believe this possibility is the motivating factor for a contemporary man wanting to copulate with his partner as soon as possible after being away for an extended period of time. They believe a man who is highly motivated to inseminate his partner after having been away is more likely to pass on his genes than a man who took no action. Goetz et al. (2007) found that the more time a man spent away from his partner since their last intercourse, the greater his desire to copulate with her as soon as possible upon his return. These same men also express a greater amount of worry and pressure on their mate for intercourse if their initial attempt is denied (Goetz). Men who on average copulated more frequently with their partner were less likely to pressure their mate upon denial. This is likely because they know their opportunity will come soon enough. Interestingly, the man’s satisfaction with the relationship had no bearing on his desire to copulate with his mate, or the amount of pressuring he would use. The study supports the evolutionary theory that male’s main concern is to have certainty that their genes are going to be passed on to the next generation. This also ties in to the male adaptation of somehow placing more sperm in his ejaculate the longer he has been away from his mate. The mix of physical and psychological adaptations help the male’s chances that he will be the father of his mate’s children, and not become a cuckold. Not surprisingly, Goetz & Shackelford (2007) also found that a copulatroy ejaculate contains more semen than one from self-pleasuring.
There are adaptations involved even in the act of intercourse once the man returns to his mate. Sperm from another mate in the vagina is displaced by the frenulum of the coronal ridge when a man thrusts deeply and quickly. A study by Crews et. al. (2007) showed men thrust significantly faster, and deeper when having intercourse for the first time after being away. They have also found men copulate longer when they have been away. Although they may be acting subconsciously, all these factors help to insure the man that his mate’s children will be his.
There are even more subtle psychological adaptations which have been found as well. A man who has been away from his mate will find his mate more attractive than before, and this will likely increase the motivation for immediate copulation. However, other men also view a female whose mate has been away as more attractive. They likely see this as a good opportunity to pass on their genes without the responsibility of raising a child. Also men found a woman who was not married, and not engaging in casual sex as the most desirable short-term sex partner. These women are the least likely to have been inseminated by other men (Goetz). Most of this seems to be common sense, but it certainly seems possible, and even likely, that these natural reactions are in fact due to our evolution over thousands of year
References
Crews, D., Domjan, M., Matthews, N. R., & Ramsey, M. (2007) Learning Effects On Sperm Competition And Reproductive Fitness. Psychological Science, 18, 758-762
Goetz, A. T., McKibbin, W. F., Shackelford, T. K., & Starratt, V. G. (2007) Absence Makes The Adaptations Grow Fonder: Proportion Of Time Apart From Partner, Male Sexual Psychology And Sperm Competition In Humans (Homo Sapiens). Journal of Comparative Psychology, 121, 214-220.
Goetz, A. T., & Shackelford, T. K. (2007). Adaptation to Sperm Competition in Humans. Current Directions in Psychological Science, 16, 47-50.
Websites
Science Daily: Primate Sperm Competition: http://www.sciencedaily.com/releases/2007/09/070925090250.htm
Semen Displacement as a Sperm Competition Strategy in Humans: www.epjournal.net/filestore/ep021223.pd
HUMAN PENIS SIZE COMPARED TO OTHER PRIMATE SPECIES
The average length of the erect human penis is about six inches, longer than that of any other primate. The erect penis of a gorilla measures about one and a quarter inches, an orangutan an inch and a half, and a chimp three inches (Diamond, 1985). Why is the human penis so long? Various theories attempt to explain penis length, from ornamentation to attract females, to an aggressive display to challenge other males, to enhanced pleasure for females, to enhanced reproductive capability.
One possibility is that the human penis evolved as a display, much like the peacock’s colorful tail (Diamond, 1985). However, the peacock’s tail evolved because peahens found it attractive. In order for the human penis to have evolved for purposes of display, human females would have to find it attractive, and research indicates that the penis is not a significant source of attraction for females. According to a survey conducted by Village Voice, only 2% of women indicated that they admired the penis. Women found other male anatomical features much more attractive, such as the buttocks (39%). So, the "attraction/display" theory proves unsatisfactory.
Another theory suggests that human penis size evolved as an aggressive display towards other men, suggesting that males with larger penises would intimidate males with smaller penises (Parker, 1985). This theory was quickly deflated. According to this "scare" theory, a large and muscular man with a less than average penis length would be dominated by a small and weak man with a large penis. This theory proves unlikely.
Enhanced pleasure for females has also been proposed as an explanation of penis length (Parker, 1985). But this "pleasure" theory also has critical weaknesses. Female enjoyment and orgasm rate has little to do with penis size, according to Masters and Johnson (1966).
Sperm competition is another possible explanation for penis length. A penis that is long enough to deliver sperm as close as possible to the cervix could aid in fertilization. The vagina is a hostile environment for sperm; many sperm do not survive vaginal acidity. A male possessing a penis that is just long enough to reach the rear of the vagina is more likely to have the sperm in his ejaculate survive to reach and fertilize the ova. This theory seems to have merit, but it faces at least one important counterexample. The theory would predict that chimpanzees, a species with greater sperm competition, would have longer penises than humans.
There is no widely accepted theory of primate penis length – it remains a mystery.
GRAPHIC: Insert Morphological differences between man and primate representation here.
HUMAN PENIS AND SPERM COMPETITION
Why is the tip of the human penis shaped differently than other male primates? The human male penis has evolved to displace the semen of other male competitors. It is one of the defense mechanisms to remove previous sperm from the cervixcal area and to ensure their own Reproductive success. Birkhead (2000) noted the role of the penis as a tool for internal fertilization, and emphasized the differences in morphology between species.
One theory explains these physical differences between species by suggesting that sperm competition played an important role in shaping the human penis. Gallup and Burch (2003) examined some of the possibilities for why the human penis is shaped differently than that of many other primates, such as the shape of the cervical end, human males having a larger glans, and the shape of the coronal ridge. Drawing on work by Baker and Bellis (1995), which stated the action of repeated thrusting would draw some of the semen away from the cervix, Gallup and Burch did a simulation on the effects of the human penis shape using different artificial models of the vagina and different sizes and shapes of the penis. Semen was placed in the vagina and they measured how much sperm would be removed with the insertion of differently shaped penises. Results indicated that the shape of the coronal ridge and frenulum significantly reduced the amount of semen present in the models.
Goetz et al, (2004) conducted a study on the behaviors that occur as a result of the presence of sperm competition. Their study used 305 males from universities. An independent group of males rated the physical attractiveness and personality characteristics of partners of male participants to boost jealousy. Participants were also asked to report mate retention tactics they used to prevent female infidelity. Results indicated the recurrent risk of sperm competition and mate retention tactics were positively correlated with one another. In other words, men who were at a higher recurrent risk of sperm competition reported more frequent use of mate retention tactics (e.g., vigilance, concealment of mate, monopolization of mate, jealousy inducement, punishment of threat to infidelity, emotional manipulation). The study also introduced the idea that higher recurrent risk of sperm competition positively correlated with sperm displacement (numbers of thrusts, deepest thrust, depth of thrusts, and duration of sexual intercourse). Hence, the higher the risk of sperm competition, the more males tended to thrust stronger and deeper. Furthermore, these authors explored whether the risk of sperm competition correlated with sperm displacement behaviors, and again found a positive correlation.
What happens when the effects of circumcision come into play? Circumcision is often (but not always) accomplished for religious reasons. On the other hand, Ohara (1999) notes that circumcision has an important effect on semen displacement. This study involved 139 women who had experienced intercourse with a number of circumcised or non-circumcised men. Results indicated that 73% of the women experienced a harder and deeper thrust by the uncircumcised men. Among the women who enjoyed non-circumcised men, the most common benefit noted by the women was “prolonged intercourse,” but the complaints included loss of vaginal secretion. Uncircumcised men thrust deeper, thereby displacing more vaginal fluids. Ohara concluded the reason why “loss of vaginal lubrication and discomfort” may occur as a result is because of tight penile skin around the coronal ridge of the glans.
Taking a different approach, Gallup Jr., Burch, and Mitchell (2006) conducted a study on the displacement of one's own semen. Heterosexual undergraduates responded to a survey that included, yes or no questions (“have you ever experienced sexual intercourse?”), open ended questions, Likert scales, and frequency scales. Their hypothesis asked whether, during the post ejaculation period, there are some factors that minimize self-semen displacement. Their findings indicated that a great majority of participants experience thrusting changes after ejaculation. They also found the mean refractory period (the time to regain the ability to copulate) was 59 minutes, with a wide range of 5 to 480 minutes. Females also noted they know when their partners ejaculate because the change of thrusting speed and depth falls drastically, changes in facial expression occur, and the partner states that he has ejaculated. The number one response was women know by the changes in their partner's facial expression after ejaculation.
From these findings, we can conclude that the male penis is shaped such that it scoops out any sperm that may be inside the female vaginal tract. Furthermore, although the male penis has evolved this shape as a device to remove sperm, males also have the ability to decrease semen displacement after ejaculation through behavioral changes.
WHY DID HUMAN MALES LOSE THE PENIS BONE?
Unlike other primates, human males do not possess a penis bone. Possibly, as Dawkins (1989) suggested, the absence of the penis bone is a result of female preference. Dawkins suggested that ancestral females may have preferred males without an os penis, or penis bone, because the ability of a male to achieve an erection is an unfakable and valuable indicator of health, perhaps of confidence, and perhaps also of social status. The penis of a male with an os penis offered, versus the penis alone, offers no such indication. Apparently this is a manifestation of "handicap theory." In essence, a boneless penis indicates to a female: "Copulate with me – I am so healthy, confident, and of such high social status that I can maintain a firm erection without a bone." Hypothetically, this is another example of the morphological effects of epigamic selection, with female choice literally modifying the male body.
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