Of course, when we get into detail, there are many exceptions to this. For instance, the default pattern for the paws of mammals is that they all have four digits with three bones each, and one with just two bones (the thumb and big toe in humans). But, obviously, this isn't true of all mammals. For instance, dogs have no big toe on their hind feet, and, while they do have a full set of ankle bones, the metatarsal that would normally connect to the big toe isn't there, either. There are rather more dramatic alterations in, say, horses and two-toed sloths, let alone dolphins.
But, still, when it comes to primates, we'd expect an even greater correlation between their skeletons and ours. And, generally, we'd be right - if a bone is there in humans, it's probably also there in other primates, and vice versa.
Humans do not, however, normally have a bone in their penis.
This sort of bone, however, is reasonably common among mammals, although we humans are by no means unique in lacking one. The bone in question goes by a number of different names, although baculum is probably the most common - others include os penis, or simply "penile bone". The bone has been observed in carnivoran mammals, including cats and dogs, as well as rodents, lagomorphs, and bats, among others.
Possibly the first naturalist to notice that at least some primates had one, too, was Alfred Grandidier, who published this fact in his humungous 40-volume history of Madagascar, completed in 1887. This was based on his dissection of a number of lemurs, including for example, the eastern woolly lemur (Avahi laniger). In his dissection of what was then the newly discovered lemur species Milne-Edwards sifaka (Propithecus edwardsi), he also noticed something else which, while known in scientific circles, has tended to receive less attention: females of that species also have the same bone, known in their case as a baubellum or os clitoris.
This seems to be less common than the baculum, although we do know that at least some bats, rodents, and carnivores do also have one.
The presence of the baculum in so many species raises a number of questions, of which perhaps the most obvious (especially when it comes to females) is "what the heck is it for?" After all, humans, and many other species of mammal besides, manage to do perfectly well without one. And, quite frankly, it's a bit of a mystery.
One thing we can say is that it must be doing something fairly specific, since its shape and size vary considerably between different species, and not in a way that suggests the variation is simply the result of genes for something else being shuffled around and having no more than a knock-on effect. At least in mice, we have even begun to home in on some of the genes responsible for the variation. But it's been difficult to find any real pattern that might help us understand what's going on - probably because the baculum has multiple possible functions, the relative importance of which may vary between species.
The only clue we do have is that it is now well-established that the longer the sex act lasts in a given species, the longer their baculum tends to be. This explains why, for example, the baculum of dogs is proportionately larger than that of cats, and the rule seems to broadly hold in primates, too. There is, however, likely to be more to it than this, because it doesn't fully explain why gorillas and chimps have an (admittedly unusually small) baculum, and their closest relatives - ourselves - have lost it altogether. Conceivably, the ability of a male human to sustain a high blood pressure for prolonged periods is something that gives a hint to their overall genetic fitness... but that would plausibly be true for at least some other species, too.
While this may be the most obvious mystery about this strange bone, it's not necessarily the only one. While the detailed shape and size of penis bones in bats are often used to diagnose different species (quite possibly even by the bats themselves) descriptions of the structure in the literature refer almost entirely the external appearance, once it's removed from the animal. But the internal structure of the bone may also be relevant.
Bacula do tend to be more flexible than other bones, which makes sense when you consider that, being narrow, they might otherwise be at risk of snapping. Which, you know, would probably be bad.
Fortunately, these days, we have such things as CT scans to see what's going on inside a bone or other structure without having to break it open ourselves. Using the bodies of animals that had died of natural causes, a team of scientists recently used a micro-CT scanner to examine the genitals of nineteen different species of primate. The sample included bushbabies and lorises, both of which are related to lemurs, a tarsier, some South American monkeys such as marmosets, a variety of Old World monkeys including baboons and macaques, and two apes - an orangutan and a chimp.
With a limited number of animals to examine, most of those they were able to obtain belonged to females, which limited how much they could learn about the baculum. Even so, they were able to confirm that the bushbabies and lorises had relatively long penis bones, occupying the entire length of the shaft, while the monkeys had much smaller bones, found only in the half of the organ towards the tip. There was also, as previous studies had shown, there was some variation in the shape, some being simple rods, but those of the baboons being hook-shaped, and one of the bushbaby species having a Y-shaped structure.
The internal structure of the bones varied somewhat, with some containing a 'hollow' interior, presumably filled with bone marrow, others lacking a marrow cavity altogether, and some somewhere in between. But what this shows is that, despite not being attached to the rest of the skeleton in any way and presumably forming separately from it, these are true living bones, with all of the usual microscopic cellular structure, blood vessels, and so on that distinguish them from simple foci of calcification.
Just one of the females - a tufted capuchin (Sapajus apella) - seemingly possessed a baubellum, a tiny 3mm structure, almost entirely solid inside but still with at least some spaces for living cells. The purpose of that, if it even has one, is still a mystery.
[Photo by Basile Morin, from Wikimedia Commons.]
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