The Zoologist/4th series, vol 4 (1900)/Issue 711/The Adder-swallowing Theory from an Anatomical Point of View, Leighton
THE ZOOLOGIST
No. 711.—September, 1900.
THE ADDER-SWALLOWING THEORY FROM AN
ANATOMICAL POINT OF VIEW.
By Gerald Leighton, M.B.
The main objections of scientific naturalists to the belief that the Adder-mother (Vipera berus) swallows her young are the following:—First, that no Adder has been dissected by a competent authority with the young found in the gullet. Secondly, that the cubic capacity of the gullet is not sufficient to hold all the young ones in a litter. Thirdly, some (F. G. Aflalo, e.g.) hold that there is no adequate reason for the phenomenon, i.e. that the young ones would have a better chance of escape if they were not thus swallowed. Of these objections, the second is by far the most pressing to my mind, for if it could be shown that the gullet of the Adder is not capacious enough to hold the average number of young, then indeed one could not have much belief in the occurrence. It is this point that I wish to deal with at present. One must settle—
1st. What is the average number of young at a birth?
2nd. Is the anatomical structure of the gullet adapted for the reception of young?
3rd. Is the capacity of the gullet sufficient?
These are practical questions, only capable of decision by dissection of a sufficient number of adult female Adders. Take them seriatim.
1st. The average number of young Adders at a birth.
Various authorities give different figures: thus Dr. Stradling puts the litter at from 15 to 40, M.C. Cooke at 10 to 20. My own experience leads me to doubt very seriously whether Adders ever have forty young at once. I have dissected now a considerable number of full-grown female Adders in young (three this week), one of which I will now describe in detail.
Dissection of Gravid Female.—On Monday, July 23rd, I dissected a large gravid female Adder for the purpose of counting the number of young that had developed. A mesial incision was made from the position of the ovaries to the cloaca, and the skin turned outwards. Both oviducts were full of eggs. The upper and lower ends of the oviducts were tied with string, and the other organs gently separated. Both oviducts were then lifted out of the body cavity. The dissection showed the blood supply very beautifully, and the young were evidently only a day or two from birth. I took a photo of the full oviducts, as one cannot often get an Adder at this stage. The right oviduct contained seven and the left six eggs. (An egg often contains two embryos.) The specimen was such a perfect one that I thought I would preserve it in toto to harden, so as to observe later the precise position of the embryo in the egg. This being so, I could not be quite certain as to their number, but I think it is 19 or 20. The average of this series of dissections works out at thirteen embryos to each female Adder. (The eggs often contain more than one embryo; in this latter case there were thirteen eggs containing the eighteen embryos.) Possibly a larger series of dissections might give slightly different results, but I do not think the difference is likely to be great. To be on the safe side (as far as the question of capacity is concerned), let us consider the average to be fifteen.
2nd. The anatomical fitness of the gullet for their reception.
This is soon disposed of, as the Adder's gullet is, of course, adapted to its food. There is no difficulty whatever for a Mouse, a young Water Vole, or a Blindworm to pass into the gullet, and it is quite easy to press them out again if they are contained therein. (A short time ago I killed an Adder which appeared very full in the throat, and, on gentle pressure, I got out two young Water Voles.) The mechanism of the jaws is indeed peculiarly fitted for the passage of bulky articles of food, a feature common to the Ophidia group.
3rd. The cubic capacity of the œsophagus (gullet).
This is a crucial point. If the Adder does swallow her young it is to be presumed that there must be room in every average-sized Adder's gullet for an average-sized family.
The capacity of the gullet can be estimated by the simple process of dilating it to its full extent with air through a blowpipe. I did this in the presence of a naturalist last week; his comment was, "There is room for twenty young ones, but for forty I think not." The average length of the gullet is 9 in., and the average diameter, when distended, 1 in., the average circumference 3⅓ in. Young Adders can curl themselves up in a remarkable way, and I have no hesitation in saying that the capacity of the œsophagus is sufficient to hold the young. My conclusion is that, theoretically considered, there is no anatomical reason why the Adder-swallowing theory should not be true.
The third objection I alluded to, viz, that there is no need for the process, is a mere matter of opinion. Very curious instances could be given of the expression of maternal instincts in nature.
It only remains, then, for the first objection to be removed, viz. that a competent authority should have the opportunity of dissecting an Adder which has been observed to swallow the young. Until this is done scientific naturalists will continue to regard the question as one capable of proof, if true, but hitherto unproved.
This work is in the public domain in the United States because it was published before January 1, 1929.
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