previously discussed how this works in some detail, but, in general, there are two approaches. In "fore-gut fermenters", the stomach is divided into multiple chambers, allowing the animal to ferment its food, then bring the solid parts back up to the mouth to re-chew ("chewing the cud"), before sending it back to finish the job. This is the system found in cows, deer, camels, and many other animals, collectively known as "ruminants".
The second approach is to place the fermentation chamber behind the stomach, down in the colon. This is less efficient, but quicker, and is the arrangement found in horses, rhinos, and a number of other animals. Rabbits are also "hind-gut fermenters" of this sort, but take the additional step of re-eating the fermented food once it passes out of their back end for the first time.
I mentioned in passing in my previous post that kangaroos are fore-gut fermenters, but that they are not ruminants, and have a slightly different system. Today, I'm going to explain what it is that they do do. (Much of what follows applies equally to wallabies, and, to a lesser extent, rat-kangaroos).
Kangaroos and wallabies are the large grazing animals of the Australian continent, the native equivalent of the cows, antelope, and sheep found in Africa, Eurasia, and North America. Their teeth, when you get a good look at them, have a resemblance to those of rabbits, and they are used in much the same way, for clipping and chewing vegetation - mainly grass, in the case of grey kangaroos, although the details vary among the other species. Koalas, wombats, and most herbivorous possums (i.e. not the American sort, which are a different thing entirely) are hind-gut fermenters, but kangaroos and their relatives are not, instead fermenting food in their stomachs.
But these are not the multi-chambered stomachs of ruminants. In fact, they look really rather odd. They take the shape, not of a simple sac, but of a wide, somewhat elongated, tube. The oesophagus enters part way along the tube, with the result that one end of if forms a blind-ending sac, while the rest typically curves around through the upper abdomen before emptying into the small bowel at the other end. Muscles are arranged in three strips, technically called "taeniae", along the length of the stomach and are shaped in such a way that the stomach has a progressive series of short bulges along its length.
This, it should be noted, would also be a remarkably accurate description of what the human colon looks like, and, indeed, that of most other mammals. Minus the appendix, admittedly, but that blind-ending bit does resemble a caecum, which is the part of the colon that the appendix would be attached to. The exact degree of resemblance to the colon does vary quite a bit between species, but, in the eastern grey kangaroo (Macropus giganteus), where the stomach curves up in an inverted 'U', then has an almost S-shaped bit at the end, it's particularly striking.
The point of this seems to be to allow the food to pass along the stomach in a conveyor-belt manner, rather than being mixed up together, as it would be in most other animals with a single-chambered stomach. The majority of the stomach is used for fermentation, just as the first three chambers of a cow's stomach are, or, for that matter, the actual caecum in horses and their ilk. The blind-ending bit seems to function mainly to absorb soluble carbohydrates, while the bulk of the tube deals with the roughage. Only once the food reaches the last section of the stomach is it actually exposed to gastric juices, since it's only that last bit (the S-bend in the grey kangaroo) that has a normal stomach lining. So that part is acting like a cow's fourth stomach, without the need to divide the whole thing into multiple chambers.
It has to be said that this arrangement is less efficient than the full set-up found in, say, sheep, or even the three-chambered stomach of camels. It does allow food to pass through the gut more quickly, but, by it's very nature, that gives the animal less time to extract nutrition from it. There are other differences, too, in that kangaroos produce far less methane than regular ruminants, and pretty much all of that which they do produce comes out of the back end, rather than being belched out as ruminants do. This may be, at least in part, because, while the kangaroo stomach does contain fermenting bacteria and the like, just as those of ruminants do, the specific kinds of bacteria found down there are quite different, and likely digest the food in a different way.
But one of the features of true ruminants is chewing the cud, regurgitating the more solid parts of the food at the animal's leisure to give it a second go. As fore-gut fermenters, if not true ruminants, do kangaroos chew the cud?
Certainly, it used to be thought so. As far back as the 1830s, and as recently as the 1950s, at least some scientists regarded kangaroos as cud-chewing animals, on the not unreasonable grounds that they had watched them doing it. But, as people looked into it more, it became clear that what the kangaroos were doing was rather different than what true ruminants do. For one thing, they don't do it as often, and it doesn't seem to be an essential part of their digestive process, just something that they can do if they have to.
So, in the 1960s, a new term was coined for this behaviour: merycism. It wasn't actually a new word, because it has a medical meaning that dates back to the 19th century, in which it refers to the repetitive regurgitation of undigested food, commonly in infants. Indeed, the word itself is derived from a Latin term that literally means "rumination", so it could have been used legitimately as an alternative term for regular cud-chewing. Now, however, it has been taken to mean regurgitation and re-chewing of food by non-ruminant animals, and has since been observed, non-pathologically, in some other herbivores, such as leaf-eating monkeys.
So how does merycism differ from true rumination? Systematic observations of the process in western grey (Macropus fuliginosus) and red (M. rufus) kangaroos reveal some key distinctions. During feeding of these captive animals at a research station near Fowlers Gap, New South Wales, they engaged in merycism only about a third of the time, which shows that it clearly isn't essential, as it is in ruminants.
When they did, the kangaroos typically backed off from the feeding trough, and almost immediately underwent abdominal spasms, threw back their heads and regurgitated the freshly swallowed food straight back into their mouths. This is very different from the usual calm and contented chewing of cattle, who appear completely relaxed through the entire process. Evidently, it's a much more vigorous process in kangaroos - some even went so far as to pump their arms up and down while doing it, as if the whole thing took quite a bit of physical effort. It's also notable that it's something they do while actually feeding, not later on when they have some free time.
Once the mostly liquid material was back in their mouths, the kangaroos did their best to slurp it back and then chew the relatively solid parts before swallowing again. A great many times, the slurping didn't really work, resulting in quite a mess as the liquid spilled out of the animal's mouth.
One is tempted to ask, given all the effort involved, whether this is a normal part of kangaroo behaviour, or something to do with them being in captivity or the nature of the food they're being given. For example, it might be that, as with two dogs being regularly fed in the same room, they're bolting the food down as fast as possible in case it's stolen, and doing so so much that they're practically making themselves sick. Or that the prepared food pellets aren't as good for their digestion as we might suppose.
We can't rule this out, at least as an exacerbating factor, but we do know that kangaroos do this in the wild, so it's unlikely to be the whole reason; this does seem to be normal behaviour, at least some of the time. On the other hand, the fact that they don't do it all the time suggests that it might only be needed when they've eaten something particularly tough that needs re-chewing. Indeed, by all accounts, it almost looks as if the animals are taken by surprise, as a human might be by a sudden belch. Perhaps, for some reason, the food is getting clogged in the first part of that tubular stomach, and has to be forced back up the throat to relieve the pressure before they can swallow it again.
So, to the extent that grass-eating kangaroos chew the cud, it's a very different - and to human eyes, rather more uncomfortable - thing than what sheep go through. Perhaps koalas, with their regular hind-gut fermentation, have it rather easier.
[Photo by enjamint444, from Wikimedia Commons.]