Almost immediately, and simultaneously, Mo and Valin correctly deduced that it was a baculum, otherwise known as an os penis or “penis bone.” This rapid response says something encouraging about the readers of this blog, and their familiarity with the anatomy of animal genitals.
For those not in the know: the baculum is a “heterotopic” bone that, like the patella, forms from the ossification of connective tissue in the penis of many mammals. You could sorta think of a baculum as a knee-cap in your penis — except that you, as human, don’t have one, even if you’re a boy. Many other primates do have a baculum as do most bats, rodents, carnivorans etc. There is a wonderful (but sadly, probably apocryphal) reading of the Genesis creation narrative that suggests the “rib” (צלע or tzela in Hebrew) removed from Adam to make Eve in Genesis 2 was actually a baculum – which would explain why humans don’t have one.
Anyway, the critter from whence this baculum came has remained elusive. Kari and Gretchen in turn were able to work out that it came from a carnivoran and more specifically from a mustelid–the family that includes badgers, wolverines and weasels. But my “hint” about its potential lethality proved unhelpful. Various guesses that it came from a marine mammal also deserve partial credit, though it’s worth noting that cetaceans, like their artiodactyl relatives (cows, hippos, muntjacs and the lot), lack penis bones.
So to whom does this impressive piece of equipment belong (or used belong to anyway)?
I’m sure the suspense is just killing you. Below, the big reveal:
Behold: the penis bone of Enhydra lutris, more familiar to most as that dirty, thieving, locust of the sea, the sea otter.
Compared to some of its relatives, Enhydra has a pretty vanilla baculum, eschewing the “french tickler” tips that make the penis bones of some mustelids look more like medieval torture devices An assortment of mustelid penis bones is shown in the picture at left from Baryshnikov et al. 2003, the sea otter’s is second from the bottom on the left.
What it lacks in ornamentation however the sea otter baculum makes up for in size. At around 15 cm in length, it is probably the longest baculum of any living mustelid*. This is not all that surprising, given the fact that Enhydra is among the largest members of its family in terms of overall body size. But even with size taken into account, the sea otter baculum is proportionally huge: longer than the skull and more than 10% of total body length! For comparison, a freaking POLAR BEAR has a baculum around 16 cm long, just a bit bigger than the sea otter’s, despite being well over twice as long and more than 10 times heavier in terms of overall body size.
Interestingly, another group of marine mammals, the seals are also proportionally well-endowed: the world record baculum holder (wait, that doesn’t sound right) among living mammals is the walrus, whose 50 cm baculum is enough to make anyone feel a little self-conscious. The putative ancestral seal Puijila also has a notably respectable baculum which raises the question as to whether there is some aspect of aquatic life that selects for a large baculum among carnivorans that return to the water, as well as whether bacular size or shape can be used to draw inferences about the reproductive biology of extinct mammals.
* A few extinct mustelids including Plesiogulo and Sardolutra were also packing some serious heat.
Believe it or not, being blessed with a frighteningly huge bone in your penis does have some drawbacks however. Not so much for the male otters as for the objects of their affection which turn out occasionally to be unfortunately misguided. One amorous otter has been blamed with the death of a dozen or more harbor seal pups. While the reported MO in that case is drowning, other accounts suggest that trauma inflicted to the internal organs of victims played a role. Apparently some female otters have suffered the same fate (Miller 2008[pdf]).
Which gives you something to think about the next time you are fawning over an adorable little sea otter at the beach or aquarium, though of course trying to apply human moral standards to nature is a loosing game.
Nature: frequently impressive, often disturbing, but always interesting.
Miller, EH 2008 “Baculum” Encyclopedia of Marine Mammals 2nd edition Perrin Wursig and Thewissen eds. Elsevier [pdf]