What Happened To Our Penis Bone?

Technically speaking, a human penis can’t actually be ‘broken’. That’s because it’s made up not of bone, but of soft, spongy tissue. But what about animals? Do our close and not-so-close relatives possess boney penises? And if so, what is the function of a penis bone? The answer isn’t as simple as you might think.

Having taken a somewhat technical approach to posts recently, I’ve decided to take a step in another direction, and for this post I stumbled upon this interesting animal fact and went with it. 

It turns out that almost every mammal (except humans, obviously) possess a penis bone. WHAT? Yes it’s true, and it is known as a baculum – pretty wacky right? But exploring how the taxonomy and potential behaviour of our ancestors led to us evolving without a baculum is even more interesting.


Great apes, despite their size, have extremely tiny baculum. Ha ha.

Firstly, what is the role of a penis bone, which we’ve done reasonably well without? The problem with research in this area is that the baculum is so diverse in terms of length between even closely related species, that it’s difficult to pinpoint its exact function. Let’s explore the several hypotheses that have been researched:

  1. The extra rigidity that a baculum provides ease of intromission (sex), which would have an even greater benefit in sexually dimorphic species due to the large difference in organ sizes (Vaginal Friction Hypothesis).
  2. Assisting sperm transport – especially in species that take their time during intromission, it would prevent blockages in the ducts (Prolonged Intromission Hypothesis).
  3. Stimulating the reproductive tract of the female to induce ovulation – this is actually a thing in some species! (Induced Ovulation Hypothesis).

Fun fact: Blue whales have the largest penises in the animal kingdom. Here’s one I found earlier. (Source: Richard Gould)

Now let’s take a look and discuss how the lack of the need for these features could have led to the redundancy of a baculum in humans:

Vaginal Friction Hypothesis

The credibility of this hypothesis is that humans are sexually monomorphic and hence do not find it difficult to mate. However statistical analysis of 44 species have shown that there is little correlation between baculum length and ‘degree of dimorphism’.

VF hypothesis

Graph showing baculum length vs degree of sexual dimorphism in 44 North American mammals.

Prolonged Intromission Hypothesis

Could this be the reason why humans have lost our baculum? Many articles and papers have stated that due to humans not having a prolonged mating period (contrary to what individuals might think), evolution had regarded it as unnecessary. This is due in part to a study performed in 1995 which claimed that correlation existed between baculum length and duration of copulation, but failed to use comparative methods and statistical analysis1.

This theory is refuted by a later study which showed little correlation existed (r2 = 0.19).

PI hypothesis

Relationship between baculum length and duration of copulation for 18 North American mammals.

Induced Ovulation Hypothesis

Since human females do not undergo induced ovulation, this theory could hold water. However a study comparing induced ovulators and non-induced ovulators using analysis of covariance (ANCOVA) could not show a significant difference between the proportion of mass:baculum length in the two groups.

IO hypothesis

Baculum length vs male mass in 13 induced ovulators and 22 non-induced ovulators.

tl;dr Nobody really knows what the baculum’s purpose is and hence nobody has a clear idea why humans have lost our baculum somewhere in our evolution – bummer. At least you learnt a little bit about good graph design?


  1. Berrigan, D., Charnov, E. L., Purvis, A., & Harvey, P. H. (1993). Phylogenetic contrasts and the evolution of mammalian life histories. Evolutionary Ecology, 7(3), 270-278.
  2. Lariviere, S., & Ferguson, S. H. (2002). On the evolution of the mammalian baculum: vaginal friction, prolonged intromission or induced ovulation?. Mammal Review, 32(4), 283-294.

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