A fun post I found reposted many times:


xoxoxoxox


Anonymous asked: They theorize that human males have much larger penises than other primates, because whereas, let's say, gorillas have harems and the females are monogamous, human females are more promiscuous and it's much more likely that a human male mating with a human female would've/will have had another male before him. The human penis is designed to optimize chances of one male's sperm winning out against another's.


xoxoxoxox


Theoretically, I mean.

It’s a good suggestion as to why gorillas and chimpanzees have different sized penises. Unfortunately, it fails to answer the question for humans.

Lets compare gorillas and chimpanzees to start with. Female gorillas tend to be in harems and mate with only one male (exceptions exist, but this is generally true). In these types of systems, there is great size dimorphism; the males will be larger to ensure that they can protect their group, and win in any intrasexual competition to keep their position.

Chimpanzees, on the other hand, are highly promiscuous. Rather than competing with each other, it’s their sperm that’s going to need to compete - so bring on the large testes and penises.

So we can now compare the stats between them.

Gorillas have a size dimorphism of 1.8, their testes are 0.02% of their body weight, have 50 million sperm per ejaculate, and their penis is erect penis is 1.25” long.

Chimpanzees have a size dimorphism of 1.3, their testes are 0.3% of their body weight, have 600 million sperm per ejaculate, and their erect penis is 3.15” long.

Bring on humans. Humans are more promiscuous than gorillas, less promiscuous than chimpanzees. This means that we’d expect humans to be somewhere between them. Humans are the most monogamous by far, in that singular pair-bonds will emerge (which may lower size dimorphism). If you want your sperm to win in a mating, you need to let off as many swimmers as possible. If you want to mate sequentially with different females and still be in for a chance, you need larger testes to hold the microscopic tadpoles.

Humans have a size dimorphism of 1.1, their testes are 0.06% of their body weight, have 250 million sperm per ejaculate, and their penis is 5.11” long.

One trait is not like the others…

If it was about sperm competition, the number of sperm released per ejactulate would be higher. To have a penis so much larger than chimpanzees but less than half the amount of sperm per ejaculate (and tiny bollocks) flies in the face of the sperm competition idea. We also lack copulatory plugs, and penile spines which function to or scrape out previous sperm, although the enlarged glans could function to scoop out previous semen.

Now it may still be the case - depth of thrusting has been shown to give your own sperm a better chance and disrupt sperm of previous males. But why the tiny bollocks? If we were adapted to have a great amount of sperm competition, we should have balls like balloons.


There is however another alternative to the sexual-selection hypothesis to consider. Humans are bipedal, meaning that gravity is working against the male. It may aid sperm retention post-copulation if it is deposited high in the vagina.