Chapter XII: Rodents (2)

It is truly an astonishing fact that animals should engage in such vast architectural labours with what appears to be the deliberate purpose of securing, by such very artificial means, the special benefits that arise from their high engineering skill. So astonishing, indeed, does this fact appear, that as sober-minded interpreters of fact we would fain look for some explanation which would not necessitate the inference that these actions are due to any intelligent appreciation, either of the benefits that arise from the labour, or of the hydrostatic principles to which this labour so clearly refers. Yet the more closely we look into the subject, the more impossible do we find it to account for the facts by any such easy method. Thus it seems perfectly certain that the beavers, properly and strictly speaking, understand the use of their dams in maintaining a certain level of water. For it is unquestionable that in the solid-bank dams, as already observed, a regular opening or trough is cut at one part of its crest to provide for the overflow; and now it has to be added that this opening is purposely widened or narrowed with reference to the amount of water in the stream at different times, so as to ensure the maintenance of a constant level in the pond. Similarly, though by different means, the same end is secured in the case of the stick dams. For 'in most of these dams the rapidity or slowness with which the surplus water is discharged is undoubtedly regulated by the beavers; otherwise the level of the pond would continually vary. There must be a constant tendency to enlarge the orifices through which the water passes,' when the stream is small, and _vice versâ_; otherwise the lodges would be either inundated or have their sub-aquatic entrances exposed.[223] Moreover, a very little consideration is enough to show that in stick dams the tendency to increased leakage from the effects of percolation, and to a settling down of the dam as its materials decay from underneath, must demand unceasing vigilance and care to avert the consequences. And accordingly it is found that 'in the fall of the year a new supply of materials is placed upon the lower face of these dams to compensate this waste from decay.'

Now, it is obvious that we have here presented a continual variation of conditions, imposed by continual variations in the amount of water coming down; and it is a matter of observation that these variations are met by the beavers in the only way that they can be met--namely, by regulating the amount of flow taking place through the dams. It will therefore be seen that we have here to consider a totally different case from that of the operation of pure instinct, however wonderful such operation may be. For the adaptations of pure instinct only have reference to conditions that are unchanging; so that if in this case we suppose pure instinct to account for all the facts, we must greatly modify our ideas of what pure instinct is taken to mean. Thus we must suppose that when the beavers find the level of their ponds rising or falling, the discomfort which they experience acts as a stimulus to cause them, without intelligent purpose, either to widen or to narrow the orifices in their dams as the case may be. And not only so, but the conditions of stimulation and response must be so nicely balanced that the animals widen or narrow these orifices with a more or less precise _quantitative_ reference to the degree of discomfort, actual or prospective, which they experience. Now it seems to me that even thus far it is an extremely difficult thing to believe that the mechanism of pure or wholly unintelligent instinct could admit of sufficient refinement to meet so complex a case of compensating adaptation; and, as we shall immediately see, this difficulty increases still more as we contemplate additional facts relating to these structures.

Thus it sometimes happens that in large dams the pressure of the water which they keep back is so considerable that their stability is endangered. In such cases it has been observed by Mr. Morgan that, at a short distance beneath the main dam, another and lower dam is thrown across the stream, with the result of forming a shallow pond between the two. This pond is--

Of no apparent use for beaver occupation, but yet
subserving the important purpose of setting back water
to the depth of twelve or fifteen inches; . . . and the
small dam, by maintaining the water a foot deep below
the great dam, diminishes to this extent the
difference in level above and below, and neutralises
to the same extent the pressure of the water in the
pond above against the main structure.

'Whether,' adds Mr. Morgan, with commendable caution, 'the lower dam was constructed with this motive and for this object, or is explainable on some other hypothesis, I shall not venture an opinion.' But as, he further adds, 'I have also found the same precise work repeated below other large dams,' we are led to conclude that their correlation cannot at least be accidental; and as it is of so definite a character, there really seems no 'other hypothesis' open to us than that of its having reference to the stability of the main dam. Yet, if this is the case, it becomes in my opinion simply impossible to attribute the fact to the operation of pure instinct.

Again, Mr. Morgan observed one case in which, higher up stream than the main dam, there was constructed another dam, ninety-three feet long, and two and a half feet high at the centre:--

A dam at this point is apparently of no conceivable
use to improve the lake for beaver occupation. It has
one feature, also, in which it differs from other dams
except those upon lake outlets, and that consists in
its elevation, at all points, of about two feet above
the level of the lake at ordinary stages of the water.
In all other dams, except those upon lake outlets, and
in most of the latter, the water stands quite near
their crests, while in the one under consideration it
stood about two feet below it. This fact suggests at
least the inference, although it may have but little
of probability to sustain it, that it was constructed
with special reference to sudden rises of the lake in
times of freshet, and that it was designed to hold
this surplus water until it could be gradually
discharged through the dam into the great space below.
It would at least subserve this purpose very
efficiently, and thus protect the dam below it from
the effects of freshets. To ascribe the origin of this
dam to such motives of intelligence is to invest this
animal with a higher degree of sagacity than we have
probable reason to concede to him, and yet it is
proper to mention the relation in which these dams
stand to each other--whether that relation is regarded
as accidental or intentional.

As before, we have here to commend the caution displayed by the closing sentence; but, as useless dams are not found in other places, the inference clearly is that the dam in question, both as regards its exceptional position and exceptional height, can only be explained by supposing the structure to have been designed for the use which it unquestionably served. That is to say, if we do not entertain this explanation, there is no other to be suggested; and although in any ordinary or occasional instance of the display of animal intelligence in such a degree as this I should not hesitate to attribute the facts to accident, in the case of the beaver there are such a multitude of constantly recurring facts, all and only referable to a practical though not less extraordinary appreciation of hydrostatic principles, that the hypothesis of accident must here, I think, be laid aside. To substantiate this statement I shall detail the facts concerning the beaver-canals.

As Mr. Morgan, who first discovered and described these astonishing structures, observes,--

Remarkable as the dam may still be considered, from
its structure and objects, it scarcely surpasses, if
it may be said to equal, these water-ways, here called
canals, which are excavated through the low lands
bordering their ponds for the purpose of reaching the
hard wood, and for affording a channel for its
transportation to their lodges. To conceive and
execute such a design presupposes a more complicated
and extended process of reasoning than that required
for the construction of a dam, and, although a much
simpler work to perform when the thought was fully
developed, it was far less to have been expected from
a mute animal.

These canals are developed in this way. One of the principal objects served by a dam thrown across a small stream, is that of flooding the low ground so as to obtain water connection with the first high ground upon which hard wood is to be found, such connection being convenient, or even necessary, for the purposes of transport.

Where the pond fails to accomplish this fully, and
also where the banks are defined and mark the limits
of the pond, the deficiency is supplied by the canals
in question. On descending surfaces, as has elsewhere
been stated, beavers roll and drag their short
cuttings down into the ponds. But where the ground is
low it is generally so uneven and rough as to render
it extremely difficult, if not impossible, for the
beavers to move them for any considerable distance by
physical force. Hence the canal for floating them
across the intervening level ground to the pond. The
necessity for it is so apparent as to diminish our
astonishment at its construction; and yet that the
beaver should devise a canal to surmount this
difficulty is not the less remarkable.

The canals, which are made by excavation, are usually from three to five feet wide, three feet deep, and perhaps hundreds of feet long--the length of course depending on the distance between the lodge and the wood supply. They are cut in the form of trenches, having perpendicular sides and abrupt ends. All roots of trees, under-brush, &c., are cleared away in their course, so as to afford an unobstructed passage. These canals are of such frequent occurrence that it is impossible to attribute them to accident; they are evidently made, at the cost of much labour, with the deliberate purpose of putting them to the use for which they are designed. In executing this purpose there is sometimes displayed a depth of engineering forethought over details of structure required by the circumstances of special localities, which is even more astonishing than the execution of the general idea. Thus it not unfrequently happens that when a canal has been run for a certain distance, a rise in the level of the ground renders it impossible to continue the structure further from the water supply or lodge-pond, without either incurring a great amount of labour in digging the canal with progressively deepening sides, or leaving the trench empty of water, and so useless. In such cases the beavers resort to various expedients, according to the nature of the ground.

Mr. Morgan gives an interesting sketch of one such case, where the canal is excavated through low ground for a distance of 450 feet, when it reaches the first rise of ground, and throughout this distance, being level with the pond, it is supplied with water from this source. Where the rise begins a dam is made, and the canal is then continued for 25 feet at a level of one foot higher than before. This higher level reach is supplied with water collected from still higher levels by another dam, extending for 75 feet upon one side of the canal and 25 feet on the other, in the form of a crescent with its concavity directed towards the highlands, so as to collect all the drainage water, and concentrate it into the second reach of the canal. Beyond this larger dam there is another abrupt rise of a foot, and the canal is there continued for 47 feet more, where a third dam is built resembling the second in construction, only having a still wider span on either side of the canal (142 feet), so as to catch a still larger quantity of drainage water to supply the third or uppermost reach of the canal. We have, therefore, here presented, not only a perfect application of the principle of 'locks,' which are used in canals of human construction, but also the principle of collecting water to supply the reaches situated on the slope by means of elaborately constructed dams of wide extent, and of the best form for the purpose. There is thus shown much too great a concurrence of engineering principles to the attainment of one object to admit of our attributing the facts to accident. On this structure Mr. Morgan observes:--

The crests of these dams where they cross the canals
are depressed, or worn down, in the centre, by the
constant passage of beavers over them while going to
and fro and dragging their cuttings. This canal with
its adjuncts of dams and its manifest objects is a
remarkable work, transcending very much the ordinary
estimates of the intelligence of the beaver. It served
to bring the occupants of the pond into easy
connection by water with the trees that supplied them
with food, as well as to relieve them from the tedious
and perhaps impossible task of transporting their
cuttings 500 feet over uneven ground unassisted by any
descent.

Again, in another case, also sketched by Mr. Morgan, another device is resorted to, and one which, having reference to the particular circumstances of the case, is the best that could have been adopted. Here the canal, proceeding from the pond to the woodland 150 feet distant, encounters at the woodland a rising slope covered with hard wood. Thereupon the canal bifurcates, and the two diverging branches or prongs are carried in opposite directions along the base of the woodland rise, one for a distance of 100 and the other for 115 feet. The level being throughout the same, the water from the pond supplies the two branch-canals as well as the trunk. Both branches end with abrupt vertical faces. Now the object of these branches is sufficiently apparent:--

After the rising ground, and with it the hard wood
trees, were reached at the point where it branches,
there was no very urgent necessity for the branches.
But their construction along the base of the high
ground gave them a frontage upon the canal of 215 feet
of hard-wood lands, thus affording to them, along this
extended line, the great advantages of water
transportation for their cuttings.

One more proof of engineering purpose in the construction of canals will be sufficient to place beyond all question the fact that beavers form these canals, as they form their dams, with a far-seeing perception of the suitability of highly artificial means to the attainment of particular ends, under a variety of special circumstances. Mr. Morgan observed one or two instances where the land included in a wind or loop of a river was cut through by a beaver canal across the narrowest part, 'apparently to shorten the distance in going up and down by water.' Judging from the figures which he gives, drawn to measurement, there can be no question that such was the object; and as these structures may be one or two hundred feet in length, and represent the laborious excavation of some 1,500 cubic feet of soil, the animals must be actuated by the most vivid conception of the subsequent saving in labour that is to be effected by making an artificial communication across the chord of an arc, instead of always going round the natural curve of a stream.

Regarding now together all these facts relating to the psychology of the beaver, it must be confessed, as I said at the outset, that we have presented to us a problem perhaps the most difficult of any that we have to encounter in the whole range of animal intelligence. On the one hand, it seems incredible that the beaver should attain to such a level of abstract thought as would be implied by his forming his various structures with the calculated purpose of achieving the ends which they undoubtedly subserve. On the other hand, as we have seen, it seems little less than impossible that the formation of these structures can be due to instinct. Yet one or other hypothesis, either singly or in combination, must be resorted to. The case, it will be observed, thus differs from that of the more wonderful performances of instinct elsewhere, such as that of ants and bees, inasmuch as the performances here are so complex and varied, as well as having reference to physical principles of a much more recondite or less observable nature. The case from its theoretical side being thus one of much difficulty, I think it will be better to postpone its discussion till in 'Mental Evolution' I come to treat of the whole subject of instinct in relation to intelligence.

I must not, however, conclude this epitome of the facts without alluding to the only other publication on the habits of the beaver which is of distinctly scientific value. This is a short but interesting paper by Prof. Alexander Agassiz.[224] He says that the largest dam he has himself seen measured 650 feet in length, and 3-1/2 feet in height, with a small number of lodges in the vicinity of the pond. The number of lodges is always thus very small in proportion to the size of the dam, the greatest number of lodges that he has observed upon one pond being five. It is evident from this that beavers are not really gregarious in their habits, and that their dams and canals 'are the work of a comparatively small number of animals; but to make up for the numbers the work of succeeding inhabitants of any one pond must have been carried on for centuries to accomplish the gigantic results we find in some localities.'

In one case Prof. Agassiz obtained what may be termed geological evidence of the truth of an opinion advanced by Mr. Morgan, that beaver-works may be hundreds if not thousands of years in course of continuous formation. For the purpose of obtaining a secure foundation for a mill dam erected above a beaver dam, it was necessary to clear away the soil from the bottom of the beaver pond. This soil was found to be a peat bog. A trench was dug into the peat 12 feet wide by 1,200 feet long, and 9 feet deep; all the way along this trench old stumps of trees were found at various depths, some still bearing marks of having been gnawed by beavers' teeth. Agassiz calculated the growth of the bog as about a foot per century, so that here we have tolerably accurate evidence of an existing beaver dam being somewhere about a thousand years old.

The gradual growth of these enormous dams has the effect of greatly altering the configuration of the country where they occur. By taking levels from dams towards the sources of streams on which they occur, Agassiz was able ideally to reconstruct the original landscape before the growth of the dams, and he found that, 'from the nature of the surrounding country, the open spaces now joining the beaver ponds--the beaver meadows where the trees are scanty or small--must at one time have been all covered with forests.' At first the beavers 'began to clear the forest just in the immediate vicinity of the dams, extending in every direction, first up the stream as far as the nature of the creek would allow, and then laterally by means of their canals, as far as the level of the ground would allow, thus little by little clearing a larger area according to the time they have occupied any particular place,' In this way beavers may change the whole aspect of large tracts of country, covering with water a great extent of ground which was once thickly wooded.

FOOTNOTES:

[209] It is particularly remarkable that if under these circumstances a rabbit bolts and, seeing the sportsman, doubles back into its burrow, being then certain that the sportsman is waiting, it will usually allow itself to be slowly and painfully killed by the ferret rather than bolt a second time. This is remarkable because it proves the strength of an abiding image or idea in the mind of the animal.

[210] See Watson's _Reasoning Power in Animals_, and _Quarterly Review_, c. i., p. 135.

[211] See especially Jesse, _Gleanings_, &c., iii., p. 206; and _Quarterly Review_, c. i., p. 135.

[212] Thompson, _Passions of Animals_, p. 368.

[213] _The Rat, its Natural History_, p. 102.

[214] Mrs. Lee, _Anecdotes of Animals_, p. 264.

[215] Jesse, _Gleanings_, &c., ii., p. 281.

[216] _Reasoning Power in Animals_, p. 293.

[217] _Loc. cit._

[218] Jesse, _Gleanings_, iii., p. 176.

[219] _Introduction to Arctic Zoology_, p 70.

[220] Dr. Henderson, _Journal of a Residence in Iceland in 1814 and 1815_, vol. ii., p. 187.

[221] _The American Beaver and his Works_ (Lippincott & Co. 1868).

[222] To obviate this possibility, they often select as their site a place where a spring happens to rise in the bottom of the lake or pond.

[223] In times of considerable 'freshet' the former case sometimes occurs; the beavers not being able to provide for a very considerable overflow through their dams, the latter become then wholly submerged. When again exposed, the animals take great pains in repairing the injuries sustained.

[224] Note on Beaver Dams (_Proc. Boston Soc. Nat. Hist._, 1869, p. 101, _et seq._).