The Fast Lives of Early Sperm Whales

Livyatan melvillei, a Miocene species
(It was supposed to be 'Leviathan', but the name was
already taken)

Sperm whales are magnificent and highly distinctive animals. While they may not be quite as large as the great toothless whales, they are still pretty huge: a fully grown male is typically about 16 metres (52 feet) in length and weighs over 40 tons. So distinctive is it, in fact, that today it is usually considered to be the only living species in its family, the Physeteridae.

The caveats in that last sentence - "today", "usually", and "living" - are all significant. On the first two points, the sperm whale family used to be considered to contain no less than three living species, and some researchers still define it that way. The other two species are the dwarf and pygmy sperm whales (Kogia spp.), and they're typically (but not always) placed in their own family these days. As their names suggest they are much, much smaller than the "true" sperm whales, being more like the size of a large dolphin.

Still, no matter where how we choose to name the arbitrary dividing line between them, it's obvious that these two groups are closely related, and so we can include them all as being "sperm whales" in a broader, more inclusive sense. And this brings us to the point about "living" species, since, under the wider definition, there are a great many fossil species as well.

In fact, over 40 have been named, mostly dating to the Miocene epoch. Many seem to be close relatives of the living dwarf and pygmy species, with a smaller number close to the "true" sperm whales. A significant number, however, don't appear to be either, and represent early branches in the sperm whale family tree that have long since died out.

Unfortunately, we don't have many decent fossils of most of these species, to the point that it's unlikely that all of the named species are real, and a number have already been sent to taxonomic oblivion as nomina dubia (or "doubtful names"). On the other hand, what we do have are teeth. Lots of teeth.

Obviously, to really learn about how early sperm whales lived, the more of the skeleton we have, the better. And, in some cases, we do have some impressive skeletal remains. But what can we learn from teeth alone? Perhaps more than you might think.

One site from which literally hundreds of fossil sperm whale teeth have been recovered is located at a mine in North Carolina. The rock deposits here appear to date from the Pliocene epoch, not long before the Ice Ages, although it's hard to know their exact age. At the time, the area seems to have been covered in shallow seas, probably around 100 metres (330 feet), and, judging from the range and number of other fossils there, was a rich and fertile environment with plenty for aquatic animals to eat.

From what we know from other fossils, there are at least four different species of sperm whale that these teeth could belong to. Indeed, given that they're not all the same size, it's entirely possible that the teeth belong to more than one of those of those species. To try and minimise this, researchers looked only at the larger teeth from the site, since only one of the four candidate species is known to be particularly big.

That species is Physeterula dubusi, first named in 1877, but with so few remains that we know virtually nothing about it. From the little we do know, it belongs to the "true" sperm whale family, rather than the pygmy/dwarf group, or one of the extinct lineages, but, of course, the teeth might belong to something we haven't otherwise found yet, so that may not help much. Which means that the best we can really say is that, whatever animal left these teeth, it was an "early sperm whale" of some kind.

It's obviously impossible to get an accurate idea of how large these early whales were from the size of their teeth alone, but we can say that, unless they were drastically out of the proportions we would expect, they were big animals, but not enormous. The best guess is that they would have been around half the length of a modern sperm whale, and weighing something like 6 or 7 tons. Which, for comparison, puts them in the approximate size range of a killer whale, and far bigger than the pygmy and dwarf sperm whales.

We're on rather firmer ground when we come to the structure of the teeth themselves. The three species of living sperm whale only have teeth in their lower jaws, but Physeterula, and most other fossil species, still had teeth in the upper jaw as well. This implies that the loss of these upper teeth happened twice independently (once in the 'true' sperm whales, and once in the kogiids), and it's likely linked with a change in their lifestyles from something closer to modern killer whales to the greater reliance on suction-feeding seen in the modern species.

More significantly, the teeth of living sperm whales grow throughout almost the whole of their lives, only stopping in old age as the root of the tooth closes off. All of the fossil teeth had wide open roots, implying the sort of good blood supply that a growing tooth needs, so these animals did the same, and perhaps continued even once they were elderly.

This is important, because it has been known since the 1950s that sperm whales have growth rings in their teeth. Exactly how and why these appear isn't known for certain, although they appear to be annual, with wider, less dense, bands being deposited at times of the year when feeding is good, and narrower, denser, bands, when the environment is less suitable.

These growth rings were apparent in the fossil teeth that the researchers felt brave enough to saw apart and destroy. Assuming that they are, indeed, annual, and that none of the outer ones get worn away as the animal ages, it turns out that these particular whales cannot have lived for more than about 25 years. This is far less than modern sperm whales, or, indeed, killer whales, all of which can happily live into their sixties.

That's a significant difference, especially since 25 is the apparent maximum age, and most of the fossil teeth belonged to individuals that had died around 12 years of age. A modern sperm whale isn't even sexually mature by that point. In fact, it's more the sort of lifespan you would expect for a dolphin or porpoise, and given that these animals seem to have been quite a lot larger than that, it implies a very rapid early growth spurt (by the standards of whales).

So what we have is an animal that was about the size of a killer whale, and hunted in a broadly similar manner, but that reached that size in a much shorter period of time. This sort of growth rate wouldn't be surprising in a medium-sized cetacean like a beluga, but, if it's right, it is a bit odd for something as large as a killer whale.

Belugas, and even more so dolphins and porpoises, have to grow relatively quickly, and then breed rapidly, because otherwise there is too high a risk that they will be killed and eaten before perpetuating their species. It's something we see even more with small, non-cetacean mammals - there's a reason rabbits breed like rabbits. But, if you're the size of a killer whale, or an elephant, there's relatively little to be afraid of, and you can take your time growing to the requisite, suitably impressive size.

We are, perhaps, piling speculation on top of speculation by this point, but is there any reason why an early sperm whale would have any more reason to grow rapidly than a modern, similarly sized, killer whale? There's not really very much that's going to try and eat a killer whale, even if it's young, but were things different in the Pliocene?

In fact, there is just such a creature, and its remains have been found in the very same deposits that the teeth came from. It's even one that we have evidence attacked whales around half the size of the estimated adults that the teeth came from, giving their young a powerful reason to get large quickly.

While early sperm whales were powerful predators in their own right, even they may have had something to fear from megalodon, possibly the largest shark ever to have lived.

[Photo by "Ghedoghedo", from Wikimedia Commons.]