Spermatozoons, floating to ovule - 3d render

Evolutionary Psychology, I think, is an excellent theoretical approach in understanding WHY certain behavioral, structural, physiological, and psychological mechanisms evolved in humans and many other species. This includes offering evolutionary insights into human penis shape and testes sizes, compared to those of our relatives, great apes. See the table below for comparative testes size, where sperms, the basic ingredient in baby-making, are produced.

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The key idea of the table above is that the knowing the testes sizes of the species could predict the mating system (Harcourt et al. 1981).

First, let’s look at the definition of the mating system in the table first (Harcourt et al. 1981). Monogamy in a sense means one partner at a time. Though monogamous mating system is not a 100% practice across human cultures, monogamous relationships are universally practiced by the majority. Humans also have multiple monogamous relationships. For example, we have breakups between boyfriends and girlfriends, and divorces and remarriages even after marriage. The mating system is also coined as “serial monogamy.” Multi-male-and-females system involves having sexual relationships with multiple partners during the intercourse. Uni-male mating systems mean there would usually be alpha males who monopolize/ are being chosen by a group of females as the mate.

Now, consider “sperm competition.” The sizes of testes vary and are dependent on the mating system (Harcourt et al. 1981). The higher the multiple partners during the intercourse, the more intense the competition is to be a father and to pass on the genes. When chimpanzees mate, males would be lining up for the sexual access to the female chimpanzee in heat. But there would only be “one” successful father, so to speak. To give an analogy, there would be only one winner for the lottery, but there are so many potential winners. To win or to increase the chances of winning, one has to buy more lottery tickets. The chimpanzees’ testes can be understood in a way that they have more “tickets” to be chosen by females’ egg as a lucky “father.” Sperm competition is also the reason why gorilla and orangutan testes’ sizes are smaller  than humans and chimpanzees in general. Their mating system usually involves alpha-males, and there would be very few opposite mates who would have sexual access to females. The competitions among these species are usually in a physical, intense and aggressive competition, rather than internal sperm competition in mating females.

However, human males have the longest penis, compared to any other ape species (Short, 1979). Some evolutionary theories were proposed in explaining why it is shaped this way. One proposal is that the increased length sends the ejaculated sperms closer to the cervix, deep inside the vagina so that the chance of fertilizing with the egg would be increased (Baker & Bellis, 1995; Smith, 1984). Smith (1984) also proposed that becoming bipedalism (walking on two feet in humans) also results higher “sperm flowback” out of the female vagina, say when the female gets up and walk after the intercourse. In order to counter that, human has compensated the longer length for the sperms to reach further distance to the egg.

The shape of the human penis head may also be analogous to some insect penises, which remove sperms of other insects (Waage, 1979) and may there being selected as a device that replaces the semen(Baker & Bellis, 1995). To further explore how it works read the cool study in suggested reading II.

Those of you who are interested, here are some suggested readings: 

I. On sperm-competition http://www.toddkshackelford.com/downloads/Shackelford-Pound-Goetz-RGP-2005.pdf 

II. On human penis as a semen displacement device (pretty cool: experiment!)


III. On sperm war in humans! 


IV. On why is the penis shaped that way 




Most of what I wrote in the article has been learned in evolutionary psychology class and readings on evolutionary psychology and biology. I do not take credits for the all the ideas presented here. I am simply presenting them to my readers. Some of the examples I have used came from the classes, but do not know where it was originally mentioned. I apologized for that. I hope you enjoy my article and find it interesting. (For last 2 books, you have to buy or rent)


Baker, R. R., & Bellis, M. A. (1995). Human sperm competition. London: Chapman & Hall.

Harcourt, A. H. Harvey, P. H., Larson, S. G. & Short, R. V. (1981). Testis weight, bodyweight and breeding system in primates. Nature, 293: 55-7.

Short, R. V. (1979). Sexual selection and its component parts, somatic and genital selection as illustrated by man and the great apes. Advances in the Study of Behavior, 9, 131–158.

Smith, R. L. (1984). Human sperm competition. In R. L. Smith (Ed.), Sperm competition and the evolution of animal mating systems (pp. 601– 660). New York: Academic Press.

Waage, J. K. (1979). Dual function of the damselfly penis: Sperm removal and transfer. Science, 203, 916–918.


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