Penis Evolution: Intersexual and Intrasexual Selection

One of the biggest questions in the scientific world is why the human penis is so different compared with its closest relatives, the great apes: chimpanzees, gorillas and orangutans. The human penis is unusual in many different respects. It is abnormally large, compared to other primate species. The average length of a human penis is five to seven inches(1). The closest genetic relatives to humans are chimpanzees. The average chimpanzee penis is about half of the size of human penises, or about three inches(1). The gorilla's penis is the smallest, at only an inch long. Along with size, the human penis is also unique because of its structure. The human penis lacks an erectile bone, and has a distinctive coronal ridge. Also, the human penis is alone in having a foreskin.

Penises evolved mainly as an intromission device. It has since been modified by many organisms using intrasexual selection. Some have hooks at the end to scoop out sperm while others have bottlebrush features to them. Although human penises have been modified, they still are not the oddest penises in the world. There have been suggestions that all unusual penises are due to female selection. However, it has been shown that there are other evolutional reasons for unusual penis shapes. There is some evolutionary evidence that the human penis evolved as a mechanism to displace sperm(1). By using the coronal ridge to create a suction mechanism, the penis is able to remove sperm already deposited in the female's reproductive tract. This is significant because it would imply that females are not monogamous, otherwise sperm displacement would be useless, because only one male's sperm would be present. Humans are culturally socialized to believe that women do not seek out sex. If the penis did evolve as an instrument to displace sperm, which there is much evidence to suggest, Western culture may have to reassess the way they categorize women sexually.

The most obvious assumption of the evolution of the male penis is female preference. That is males with longer, thicker penises are preferred to males with shorter or thinner penises by receptive females. If this were the case, then there would be evidence that females do indeed prefer big penises to small penises. According to G. Arnqvist, the penis only evolves as a result of sexual selection if the female of the species mates polyandrously(2). This would indicate that since there is evidence that the human penis evolved through sexual selection(1), that human females are polyandrous in their mating choices. Since human females are polyandrous, this leads to the assumption that there is not only intersexual selection, but also intrasexual selection. Intrasexual competition is realm where sperm competition occurs. Sperm competition explains why human testicles are so large. Chimpanzees, whose females are also polyandrous, also engage in sperm competition. However, their penises are small, when compared to humans, while their testicles are larger than humans. So what is the mechanism behind the actual size of the human penis?

There is evidence to suggest that the human penis is capable of displacing seminal fluid from the reproductive tract of human females. Length of the penis may have evolved as a way to place semen deeper in the reproductive tract of the female, making it less vulnerable to displacement(1), which would make the sperm of a male with a longer penis more likely to fertilize the female, which would make the male with the longer penis more fit. This explanation overlooks whether the female prefers a male with a longer penis. It has been suggested that the actual size of the penis has evolved through female selection, and that females have chosen larger penises over smaller penises. Evidence for this hypothesis is unavailable at this point in time, but it bears mentioning that that the coronal ridge of the penis is shaped so that at its most pronounced point, it will come into contact with the g-spot of the female. It has also been suggested that a female who reaches orgasm during intercourse is more likely to conceive. So it may be that a penis long enough to reach the g-spot will be more likely to produce offspring, and that females may select penises that may give them orgasms.

The fact that human females are the primary caregivers of their young leads them the ability and necessity to be choosier when finding a suitable mate. For ancestral males, there was not necessarily a large amount of paternal care given to offspring. This leads males to be less choosy, going for quantity instead of quality. This does not mean that females are less sexually active than males, just that they can be more directed in their mate choices. Since males have to "persuade" females to mate with them, it stands to reason that males are going to compete against each other for the females' attention. This relates to penis size as in an ancestral environment, males may have displayed their penises to entice females. This may be a biological reason behind male "flashing" or exhibitionism. However, research in pornography points out that visual cues are mostly sought after by men, as the heavily male-centered porn industry denotes. This would lead one to believe that ancestral females did not use visual penile cues to choose a mate. This may cause one to infer that the female g-spot may have evolved after the length of the penis, as if the females did not use visual cues to choose a mate, it can be assumed that the main priority of a female choosing a mate is not to achieve orgasm, but to attempt to obtain the best genes possible for her offspring, which may or may not include penis size.

Since humans are bipedal the penis is a highly noticeable feature. It has been suggested that in ancestral environments, males may have selected for bigger penises by using them as a status symbol. This would mean using penises to decide social standing. The male with the largest penis would be the one with the most power, therefore the male with the most access to females. There is not much evidence for this. In a small population of Papua New Guinea, males wear penis sheaths, with the male with the largest penis sheath being in charge. However, the penis sheath does not correlate with the actual size of the penis, and if the lead male falls out of power, his sheath is shortened. This would seem to indicate that it is not the actual length of the penis which is the determinate of power, and conflicts with the hypothesis proposed formerly in this paragraph.

In all, human penises were most likely selected for by the process of sperm displacement, as that is the hypothesis with the most evolutionary proficient explanation. However, other hypotheses may be conjectured in the future which may provide a stronger case for the morphology of the penis.