Animal Intelligence / The International Scientific Series, Vol. XLIV.

by George John Romanes

The only other measurement is that of length, and this, of course, varies with the width of water to be spanned. Where this width is considerable the length of a dam may be prodigious, as the following quotation will show:—

Some of the dams in this region are not less remarkable for their prodigious length, a statement of which, in fact, would scarcely be credited unless verified by actual measurement. The largest one yet mentioned measures 260 feet, but there are dams 400 and even 500 feet long.

There is a dam in two sections, situated upon a tributary of the main branch of the Esconauba River, about a mile and a half north-west of the Washington Main. One section measures 110 and the other 400 feet, with an interval of natural bank, worked here and there, of 1,000 feet. A solid-bank dam, 20 feet in length, was first constructed across the channel of the stream, from bank to bank, with the usual opening for the surplus water, five feet wide. As the water rose and overflowed the bank on the left side, the dam was extended for 90 feet, until it reached ground high enough to confine the pond. This natural bank extended up the stream, and nearly parallel with it, for 1,000 feet, where the ground again subsided, and allowed the water in the upper part of the pond to flow out and around into the channel of the stream below the dam. To meet this emergency a second dam, 420 feet long, was constructed. For the greater part of its length it is low, but in some places it is two and a half and three feet high, and constructed of stick-work on the land, and with an earth embankment on its outer face. In effect, therefore, it is one structure 1,530 feet in length, of which 530 feet in two sections is artificial, and the remainder natural bank, but worked here and there where depressions in the ground required raising by artificial means.

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:—