The baculum is a poorly researched and diverse bone in the penis of many animals / What is it good for? /.
Charlotte Brassey smiles into the webcam, exchanges a brief greeting and immediately disappears from the scene again. A rattle and dig can be heard, as well as the voice of the British zoologist at Manchester Metropolitan University: She has some penis bones here that she can show. A moment later she is back on the screen, waving something in her hand that looks like … – well, how? Like a misshapen tonewood? Like a giant fossilized nudibranch? “It’s a seal’s penis bone,” says Brassey.
Immediately afterwards she dives again and continues to rummage in her box. “It’s pretty big, isn’t it?” She asks after she’s back in front of the screen with a long stick in her hand. It’s the penis bone of an elephant seal. “Unfortunately, I don’t have a copy of the walrus at the moment. But it wouldn’t even fit into the picture here. It’s so big that it is often used as a walking stick.”
Instead, many more Baculum specimens follow: some are tiny as a grain of rice, others have so many edges, bumps and dents that it looks like someone has tried pouring lead. “Oh yes, and here we have a weasel,” says Brassey just before her head pushes back into the camera. In her hand she is holding a small thing with a barbed end. “The marten family has very strange bacula,” says Brassey – and one can only agree with her.
What, one wonders now, do weasels need bony barbs in their penis for? “There are different hypotheses about what these bones are for,” says Brassey. To test these hypotheses, she collected the penile bones of more than 80 different predators, copied them as 3D printouts, and compared them in a comprehensive study published in the Proceedings of the Royal Society.
In doing so, she soon discovered that, in addition to size, it is almost always the tip of the bone that differs from family to family. Brassey suspects that these differences are related to the social and mating behavior of the predators.
“There are predators in which the females automatically ovulate, like us humans,” explains Brassey. “But other female predators have to be stimulated to ovulate through the sexual act.” Like the weasel. In fact, in her analysis, Brassey found that these species tended to have the more “complex” penile bones. It is therefore likely that the shape of the baculum – and with it the apparently pointless dents, bumps or hooks – also determine the reproductive success of the male.
But that is far from the end of Brassey. She is holding a canine family bone at the camera. Rather plain and boring from above, this baculum has a small groove on the underside in which the urethra usually runs. This type of bone is mainly found in animals that mate for a long time and change their positions in the process. Ferrets and ermines, for example, can copulate continuously for up to two hours.
A very special mating behavior can also be observed in domestic dogs. “It often looks like they’re tied together,” explains Brassey. “And sometimes the males turn from the back of the bitch by 180 degrees in the opposite direction.” It could well be that these sex practices are a burden on the genitals of the animals. The bone could have a stabilizing effect and the groove could protect the urethra, so the hypothesis.
That the baculum can both protect and stimulate are well-known assumptions that Brassey has now been able to support with her comprehensive analysis. When comparing the many shapes, however, she also noticed another variant for which she assumes a different function: “The honey badger is the classic here. The tip of the bone looks like an ice cream scoop.” According to Brassey, this bone could serve to clear the sperm of the predecessor and rival out of the way. Because she noticed that this form is mainly found in those animals in which the female mates with several males. So far, however, this is nothing more than a hypothesis.
However, this strategy would not be completely unusual. They are already known from the animal kingdom: there are birds that peck at the rear of the female until the female excretes the parcel of its predecessor. And there are also fish that use their tail fin to wave their rival’s sperm away. And then there is even a study that claims that the shape of the – boneless – human penis is also designed to eliminate other sperm. The shape of the acorn wreath makes it look like a suction piston that conveys other men’s sperm out again, wrote a team led by the American researcher Gordon Gallup in 2004. In this respect, it would not be surprising if the honey badger’s ice cream scoop is also made for serving the soup of the Spoon out the previous one and then bring your own juice to its destination.
“When we think of male competition we usually think of antlers cracking,” summarizes Brassey. “But when females mate with several males, there is an extra level of this competition. And that often takes place in the reproductive tract of the females.” Whether the penis bone can also be a weapon in this sperm competition has yet to be clarified.
The best way to do this, says Brassey, is to look at the bones in action. This is exactly what the British zoologist is doing now: She keeps a few ferrets, which she takes with an X-ray film during the mating season. “Conveniently, these animals don’t let themselves be disturbed by the rather sterile ambience while filming. They immediately attack each other,” says Brassey. She is not allowed to tell much more about it yet, because the research results have not yet been published. “All I can say is that the bone is very much in motion,” says Brassey. And you can see that it is not easy for her to remain silent about everything else.
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Why do people not have a penis bone?
Almost all predators, many rodents, most primates and bats have it: the baculum, in English “penis bones”. This bone, a few millimeters to over half a meter long, can extend from the shaft of the penis to the glans or even beyond. It is not connected to other bones, but mostly to many muscle cords. The baculum arose from an ossification of the penile erectile tissue – presumably independently of one another at different times in different families or species.
It is not entirely clear why humans, like elephants, whales or ungulates, have to get along without bones in the penis. There is the hypothesis that the bone regressed when Homo sapiens became monogamous and the act of mating could therefore be shorter without decreasing the likelihood of fatherhood. But there are also scientists who suspect that women could read the fitness of their partner better from a swelling penis and that this was therefore preferred in evolution.
The female counterpart to the penis bone – the clitoris bone – is also not found in humans, but in some mammals. However, its forms and functions have so far hardly been researched.
DESIRED PENIS BONES
The difficult search for the good piece
Finding all the penis bones of the various predators was more difficult than expected for Brassey. Because they are missing in many museums and natural history collections. Sometimes, Brassey suspects, the penis bones, some of which are very small, would probably simply be overlooked and disposed of with the tissue when the skeletons were prepared. In cats, for example, the baculum is often not much larger than a grain of rice. “So we mostly had to get carcasses from cats, tigers and lions and get the baculum out ourselves,” explains the zoologist.
But even if the skeleton taxidermist has done a great job, it may well be that the baculum is still missing in the end. Because with their peculiar shapes and their aura, the penis bones are coveted objects. “I think there must be museum curators with closets full of them at home,” says Brassey. It was said to have been in collections in which 20 male whale skeletons were kept. “And you go in and see, yes, there are 20 skeletons, with every single one of their bones – only the baculum is missing,” says Brassey. “I think some of them are thrown away because we overlook them, some of them are collected privately, especially in America. People wear them as jewelry because they are so bizarre.”
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