Functions of mammalian penile bones

 For mammals, the penile bone is one of the most mysterious physiological structures. So far, no one really knows what function it has in evolution, or why humans, horses, elephants, and several other species have lost it during evolution.

The penile bones of existing mammals have a variety of shapes. Like the bones of a male honey badger, it looks very much like an ice cream scoop. Others have sharp points, or have barbs, or are like octagonal pyramids in cold weapons...

Although previous studies have shown that the wider the penile bone of male house mice, to some extent, the more offspring, but we still don’t know the underlying reason.

Biologists hypothesized that the bones of the male penis can extend the time of sexual intercourse to some extent and outperform other male competitors in mating competition, but the relevant evidence is very ambiguous.

Now, after 3D analysis of 82 penile bones of different animals, it is found that the most peculiarly shaped penile bone (not the elongated tip) can prolong sexual life, induce female ovulation or scrape from the genitals. Competitor's sperm.

For example, the ice cream spoon of the male honey badger mentioned above may have evolved gradually in competition to ensure the continuation of its offspring.

In this study, the complex shape has nothing to do with the size of the animal’s testicles, but is weakly correlated with longer mating time and ovulation induction, indicating that these bones help reduce sexual competition.

The lead author and biologist Charlotte Brassey of Manchester Metropolitan University said in 2018: "The reason why natural historians are so fascinated by these bones is that they have many unusual features-strange ridges and grooves, inexplicable. Curvature and variety of ends."

Although the size and shape of penile bones vary with species, the latest analysis by Brassey and his team shows that most "complex-shaped" penile bones, such as those with tips, hooks, shovel, and urethral groove, are relatively common among carnivores; The simple and slender forms are mostly found in monogamous species.

People used to think that the simpler and thinner bone shape evolved due to fierce sexual competition.

"However, contrary to our expectations, the complex blunt tip is the result of evolution."

The author explains that social monogamy is not the same as genetic monogamy. For example, African wild dogs are classified as monogamous, but there is evidence that they are not such honest creatures anyway.

Although gregarious carnivores seem to be evolving toward more simplified rod-shaped penile bones, the authors found that socially monogamous species are evolving toward highly complex shapes.

The current research is mainly based on the X-ray scan results of samples from modern museums, which is one of the most rigorous analysis of carnivore bone function. However, because of the lack of modeling of the female animal's reproductive system-a piece missing from research for a long time, we cannot fully determine the actual function of the penile bone during mating.

So we may overemphasize the bones themselves and ignore the complexity of the glands or cartilage at the top.

"As mammals, and more specifically apes, it is unusual for humans to have no penile bones. We hope to further clarify why certain mammals, including humans and hyenas, can successfully reproduce without that."

The paper was published in Proceedings of the Royal Society B.