Saturday, 1 April 2017

Deadly Demon Ducks of Doom

Since today is 1st April, although I'm not doing a spoof post, I am taking my annual break from mammals to talk about birds. Specifically demon ducks, although I nearly went with flightless boobies. It's that sort of day.

Australia is the most isolated continent to possess native mammals, so perhaps it shouldn't be surprising that the mammals that live there are particularly unusual, often only distantly related to those elsewhere. Birds, however, have an advantage that most mammals don't, in that they can fly long distances between land masses without dying. (They may not be doing so deliberately, of course, but being blown off course in a storm is the sort of risk they have to put up with). It's notable, in fact, that one of the two groups of native placental mammals in Australia are the bats, which are also also found further out in the Pacific islands.

Anyway, the upshot of this is that the birds of Australia frequently fall into taxonomic groups familiar from elsewhere in the world; Australia has owls, doves, seagulls, and parrots, among many others. At the same time, however, it is isolated enough that it does include some unique kinds of bird that are not found on other continents - less than the number of unique mammal groups, to be sure, but a number nonetheless. For example, lyrebirds are endemic to Australia, and are considered to form the oldest living branch of the songbird family tree.

One of the most distinctive of all Australian birds, however, is one that can't escape by flying - the emu. Given how long the continent has been isolated, and how unusual emus are, by bird standards, it's reasonable to expect that they, and their ancestors, must have had a lengthy history there. In fact, we don't really have many fossil emus, and those we do have are all pretty recent as such things go.

This hasn't stopped people looking. In fact, way back in the 1830s, what appeared to be the fossilised leg bone of a large flightless bird was discovered at caves in New South Wales. The specimen was sent to the great palaeontologist Richard Owen, but he didn't really do anything with it for the next few decades. In 1872, however, a second specimen of the same species was discovered in Queensland, and Owen named it, on the basis of the new sample, as Dromornis australis. That literally translates as the "southern swift-running bird", but it's clearly also a reference to emus, which have the scientific name Dromaius novaehollandiae.

Certainly, describing this animal as a relative of emus made sense. Now that we have reasonably complete skeletons, we know that they were, indeed, flightless, with small wings, and long, powerful legs, and that they lack the keel on their breastbone that is found on all other living birds, save only the emu's own relatives, from ostriches to tinamous. Given that it was Australian, what else could it be but some sort of early emu?

Except it wasn't. That was confirmed in 1998, when a new analysis of the skull bones showed that it couldn't be related to ratites such as emus, but was instead some kind of waterfowl. There are three living families of waterfowl, of which by far and away the largest, in terms of number of species, is the duck family, Anatidae. Dromornis and its relatives - we currently know of at least eight species - were promptly christened the demon ducks (Dromornithidae).

Demon ducks are, of course, not literally "ducks". For one thing, the duck family also includes geese and swans, and, since the demon ducks aren't actually members of that family, but merely close relatives of it, it's at least as accurate to call them giant flightless razor-billed geese. It's also still not absolutely clear where, within the waterfowl in general, the demon ducks truly fit. In fact, the most likely bet is that their closest living relatives are a small group of South American waterfowl called screamers, rather than the ducks themselves. It has also been proposed that they represent a very early branch within the wider group of fowl, making them as equally related to gamefowl (such as chickens and pheasants) as they are to ducks and their kin.

However, that isn't by any means certain, ducks are better known than screamers, and the alternative name of "mihirung" (which comes from an aboriginal word meaning something like "big bird") is nowhere near as fun, so "demon ducks" it is.

Okay, but why "demon" ducks? Simply put, it's because the birds looked decidedly fearsome. The very smallest species of demon duck we know about weighed around 90 kg (200 lbs), which is pretty impressive for a bird. Moreover, they had large, sharp beaks that look ideal for slicing meat up, and, as their scientific name suggests, they could probably run very fast. Whether they really were predators, or even scavengers, is still the matter of some debate, although the general opinion seems to be that they ate a lot more plant matter than one might expect from the beak, and may even have been pure herbivores.

But just how big did they get? It's generally agreed that the largest species was Dromornis stirtoni, a close relative of the first species to be discovered. (And, for what's its worth, also of the slightly smaller species that was actually dubbed "the demon duck of doom" by the press, D. planei). It was undeniably huge, but then so were a number of other extinct flightless birds, such as the moas of New Zealand, the terror birds of South America, and, largest of all, the gigantic elephant-birds of Madagascar. Where, compared with these creatures, do the demon ducks fit?

That's not as easy a question to answer as one might think. While we do have some reasonably complete fossils, a great many more are fragmentary, making it difficult to get a true size range for the birds. Instead, we have to rely on mathematical methods to estimate the size of individual animals from the pieces we do have, especially the legs. In particular, this allows us to get some idea of the weight of the birds, rather than just their height, since a heavy bird will require thicker leg bones to support it.

Calculations published last year, based on a large number of fossils from a 7-million year old rock bed at Alcoota in the Northern Territory, give an average weight of somewhere around half a ton. Let's not beat around the bush: for a bird, that's enormous. For comparison, the largest bird alive today, the ostrich, weighs up to a maximum of 145 kg (320 lbs), which is also around the average weight of the largest terror birds. Even the mighty moas only weighed something like 230 kg (510 lbs). At about double that weight, the only birds that could compare with the largest of the demon ducks would therefore be the Aepyornis elephant birds, which, at around 3 metres (10 feet), were also around the same height.

Indeed, there is enough uncertainty in the figures to make it distinctly possible that D. stirtoni was the largest bird ever to have lived.

But that general figure masks something rather more interesting. The researchers also found that the birds tended to cluster into two distinct forms. One, relatively small, form had an average weight of 451 kg (1,000 lbs), while other other had longer, heavier legs suggesting a greater overall height, and weighed an average of 528 kg (1,150 lbs). One possibility would be that these were two distinct species, previously thought to be just the one. The same deposits in which they were found do, after all, already include two other known species of demon duck, both of which were around ostrich-sized.

But when the two forms are about equally common, as they were, and in all other respects seemed to be much the same, there is another possibility: they're different sexes of the same species.

In birds, as in mammals, where one sex is larger than the other, it is usually the male. This is certainly true of waterfowl, with geese and shelducks being some of the clearest examples. On the other hand, in emus, moas, and their closest relatives, it is the female that is largest. This clearly isn't a universal rule for large, flightless birds, since it's not true for ostriches, but it does perhaps raise some uncertainty as to which of the forms is the male in the case of the demon ducks.

Fortunately, there's a way to tell.

In birds, as well as mammals, there are two main types of bone tissue, distinguished by their detailed appearance under a microscope. Cortical bone typically forms the outside of individual bones, and is a very hard, if somewhat brittle, material that bears much of the body's weight, provides leverage for the muscles, and so on. Inside this, however, is cancellous bone, a more loosely woven, almost spongy, material. The purpose of this type of bone is primarily metabolic, acting as a store of calcium, as well as protecting and surrounding the bone marrow, where new blood cells are made.

In birds, however, there is a third type of bone, medullary, which is similar to, but more extreme than, the typical cancellous bone. This is the primary source of calcium for making eggshells, and, as such, is only found in female birds, and may actually disappear at times of the year when they are not laying. (Indeed, you can make a male bird produce medullary bone by injecting it with oestrogen). In the case of the demon ducks, it proved possible to examine the microscopic structure of the fossilised bones, showing that the smaller individuals were, indeed, female, and the larger ones male.

The reason that males are so often larger than females in mammal and bird species is that they frequently compete for mates, whether through physical combat or visually impressive displays. How important a role this plays, though, can depend on just how big the difference between the sexes is. The size of this ratio can therefore at least give us a clue as to how the birds lived, and raised their young. In emus, for instance, where the females are larger, female emancipation goes sufficiently far that it's only the males who incubate the eggs and raise the resulting young, while their partners bulk themselves up and fight one another for the next chance to mate.

It turns out that the size ratio between male and female demon ducks is very similar to that in most living waterfowl, with Canada geese, black swans, and some kinds of teal being particularly good analogies. This suggests that their mating behaviour may also have been similar to that of waterfowl. The males likely fought one another for access to females, hence the larger size, but once they did, they may, like ducks and geese, have settled down into a life of monogamy. That's because, if they established harems of some kind, the males would likely have been even larger, in comparison with the females - something that is seen in one of the few waterfowl that does behave like this, the musk duck.

The whole point of monogamy, from an evolutionary perspective, is to keep both parents around to look after the young. So, if this is right (and it is, of course, only guesswork) demon ducks shared their parenting duties, both helping to look after the chicks as they grew.

Not quite so demonic, perhaps...

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

2 comments:

  1. Flightless boobies? What species is that? I tried to look it up on my own, but fossil sulids have almost nothing written about them :(

    This demon duck post is awesome, that thing about the medullary bone is really fascinating! Has that been used to distinguish male and female in other fossil birds?

    ReplyDelete
    Replies
    1. I confess to coining the term for effect. They are in fact suliforms, but not actual sulids - I refer, of course, to the plotopterids. As for medullary bone, I believe it's been used to distinguish sex in some dinosaurs, as well as in birds.

      Delete