The phenomenon of reingestion, as it is perhaps best termed, is one that appears to have been "discovered" on several (if not many) occasions throughout the centuries. The reference in Leviticus, xi,6 to the hare's chewing the cud may relate to reingestion. In his New Naturalist volume, on British Mammals, /p. 27/ Dr. L. Harrison Matthews quotes Gesner as having written in 1602 "Cuniculus aliquando vorat urinum et stercus suum," and it is difficult to believe that reingestion has not in fact been observed many times. Capt. Cartwright in his Journal on the Coast of Labrador, published in 1792, stated that English rabbits eat their own dung. It was not until 1882, however, that Morot published in a French veterinary journal the results of experiments that provided a satisfactory explanation of the origin and probable nature of the mucus-coated pellets often found in rabbits' stomachs. These pellets had hitherto been regarded as foodballs formed in the mouth. By feeding different coloured foods to rabbits Morot showed that this was not the case. He was able to observe that a blinded rabbit took pellets direct from its anus; and when he confined rabbits in a form of strait-jacket that prevented their gaining access to their anus he found that they no longer had pellets in their stomachs.
In the cages of the strait-jacketed animals Morot found two types of droppings. The first was the usual rounded hard type that is familiar to all who keep rabbits or who walk over land where rabbits run. The second were smaller, softer, mucus-coated pellets, identical with those found in the stomach. Morot therefore concluded that since the stomach pellets were so frequently present, they represented part of the normal digestive process of the species.
It is usually stated that Morot's work was completely overlooked for over half a century; although in a German text-book of physiology, published in 1911, we have found a reference to the reingestion process in the rabbit and a suggestion that a similar, if less regular, rhythm occurs in the guinea-pig also. Moreover, the naturalist Drane had, in 1895, provided a detailed description of the process of reingestion in the hare, and this in turn had been repeated in detail in the third volume of Millais's work on The Mammals of Great Britain and Ireland, published in 1906. For many years Drane kept hares in captivity, and his observations showed that there was an "undoubted second passage of food through the hare's digestive system." He realised that, although the method was different, the results of this process /p. 28/ might be similar to those of chewing the cud as practised by ruminants, and therefore anticipated the term "pseudo-rumination" applied nearly 50 years later.
It is a remarkable fact that naturalists, fanciers, and research workers were on the whole quite unaware of the phenomenon of reingestion until 1939, when Madsen of Copenhagen repeated and confirmed Morot's work. Madsen employed a large wooden collar to prevent reingestion of the soft pellets, and found that these were normally passed by night in contrast to the hard droppings which were produced by day. It might have been supposed that the hard pellets were formed solely of material that had been "twice round," but Madsen showed that the hard pellets were still being passed by animals that had been prevented, by collaring, from eating the soft pellets for over a month.
One of us happened at that time to be working in the Veterinary Laboratory at Weybridge, Surrey, which was asked to check up on Madsen's findings; and can remember the considerable scepticism with which they were greeted by some workers. But they provided an explanation of the somewhat baffling results obtained by Eden, who had found that it took over 5 weeks for single doses of copper to pass through rabbits; and this worker and also Taylor were able to confirm the existence of reingestion, at least in domestic rabbits. They (and later observers) have shown that the soft pellets, after being swallowed whole, tend to accumulate at the cardiac end of the stomach. Sometimes---especially at the start of the reingestion period---the rabbit swallows a semi-liquid material in which the individual pellets are almost or entirely unformed. Watson, in New Zealand, has stated that soft pellets may be found in the stomach up to 9 hours after they have been eaten by the rabbit. Usually at the beginning of the reingestion period the soft pellets are produced in small batches, alternating with hard pellets; but sometimes the change-over from the hard to the soft type is an abrupt one.
Such was the scepticism aroused by the publication of Eden's and Taylor's confirmatory work, that reingestion was stated to be an abnormal form of behaviour even in domestic rabbits. /p. 29/ Dr. Tom Hare, a pathologist with both medical and veterinary qualifications, provided a reasonable argument for the case that reingestion represented a depraved appetite or pica, and was indicative of disease, and later in the Veterinary Record of 25 May 1940 (52: 406) he playfully wrote of Dr. E. L. Taylor's work as follows, the reference to the Civil Service being due to the fact that the laboratories at Weybridge were those of the Ministry of Agriculture and Fisheries:---
Two years later, however, Southern of Oxford observed wild rabbits in the act of reingestion; and the subsequent studies of our colleague Marie Stephens in Wales and of Watson in New Zealand established beyond reasonable doubt that the process is a normal one practised daily by all wild rabbits. Indeed, the habit is formed before the animals are fully weaned. Marie Stephens examined 49 suckling rabbits and found that 22 of them had well-formed pellets in their stomachs. These pellets were firm and unmistakable, and consisted of a white curd-like substance. In a few of them a little greenstuff was present also, giving a speckled appearance. Watson also found pellets in animals weighing as little as 170 gm. (about 6 ounces) which must have been under 3 weeks of age. In these rabbits, however, the stomach contained chiefly milk with some vegetable matter, and a number of small dark pellets similar to those seen in the rectum.
Those who have seen or photographed a rabbit in the process of reingestion describe how the animal will sit and bend its head down between its legs (or sometimes round its flank) to procure the pellets.
Though the times at which reingestion occurs in wild rabbits appear to vary slightly, there seems to be a definite rhythm, the /p. 30/ peak period of reingestion being immediately after the main feeding period. Sometimes there is evidence of a further period of reingestion during the evening. The data collected by Marie Stephens are summarized in Table XII, Appendix B. Her animals did not include any caught at night; or they might well have shown that there is a second if less marked peak of reingestion during the evening. Southern's data, based on 192 rabbits, taken mainly during March from a woodland area of Gloucestershire, are shown in Table XIII, Appendix B. They indicate that, at this time of the year, reingestion must certainly occur mainly during the day when the animals are in their burrows. In domestic rabbits, which are without enemies and are fed by day, the rhythm is reversed and reingestion occurs at night---hence the designation "night faeces" applied to them a few years ago.
It seems fairly obvious that reingestion ensures the better use of food, by allowing micro-organisms in the lower parts of the alimentary tract to act twice upon it. In cattle, sheep and other ruminants, the coarse food and roughage of the natural diet are attacked in the complicated rumen before the "true stomach" is reached; and only after being regurgitated into the mouth from the rumen does the non-comminuted food pass to the "true stomach." During its stay in the rumen the cellulose of the plant tissues is broken down and many other changes take place, owing to microbial activity, including the synthesis of various vitamins. To a degree reingestion replaces rumination; and certainly the "night faeces" are a rich source of certain B vitamins---as was ascertained soon after the phenomenon was rediscovered, and as American workers have now established in some detail. Reingestion is likely, however, to be considerably less efficient than rumination, partly because, in the ruminant, important materials (including volatile fatty acids) formed from the breakdown of plant materials in the rumen are absorbed through the walls of that organ and do not have to find their way into the "true stomach" and on into the intestine before being utilised. Recent American studies indicate that an actual mucous secretion from the caecum contributes substantially to the makeup of the "night faeces." /p. 31/
Reingestion may have a physical as well as a chemical function. Blount believes that it provides bulk for the stomach muscles to act upon, permitting compression of the remainder of the stomach contents so that more will pass into the small intestines. He considers that reingestion developed as a compensatory habit, to help tide over the time spent in the burrows. Southern also considers that reingestion may be a factor during "lying-up" in the burrow (which may last for at least a week, and which we have once recorded as lasting almost 11 days), and suggests that the rabbit may be able to increase the degree of reingestion when it is scared into "lying-up" for long periods.
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