Читать книгу Natural History of Cottonmouth Moccasin, Agkistrodon piscovorus (Reptilia) - Ray D. Burkett - Страница 8

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The scutellation of the cottonmouth closely resembles that of the other species of Agkistrodon. For example, the nine cephalic shields are characteristic of most species of Agkistrodon, as well as most other primitive crotalids and viperids, and most colubrids. Most individuals have an additional pair of large scales behind the parietals.

The numbers of postoculars, supralabials, and infralabials are variable. On either side the postoculars (three in most specimens) are reduced to two in some specimens. The supralabials (eight in most specimens) frequently vary (usually on one side only) from seven to nine. The number of infralabials is somewhat more variable than the number of supralabials, the usual number being 11, but 10 is also common; 8, 9, and 12 are more rare (Table 1). In 102 snakes in which these characters were examined, four different combinations of supralabials and seven combinations of infralabials were found. Both characters together yielded 16 combinations, considering only the actual number of scales and not taking into account the side of the head on which they occurred (Table 2). The combinations found in a brood of seven young from Houston, Texas, are shown in Table 3 to illustrate the variability of this character. Gloyd and Conant (1943:168) found a variation of 6 to 11 (8) and 7 to 9 (8) supralabials and 8 to 13 (11) and 8 to 12 (10.4) infralabials in samples of 301 leucostoma and 119 piscivorus, respectively (numbers in parentheses represent average). Also of interest is the variability of the scales themselves. In one instance a scale was found that had not completely divided. In another specimen the last supralabial and last infralabial were one scale that completely lined the angle of the jaw. Instances of one scale almost crowding out another were common. In still other instances one or two supralabials were divided horizontally into two scales. Individual variation rather than geographical variation occurs in these characters.

TABLE 1.—Frequency of Occurrence of Various Numbers of Supralabial and

Infralabial Scales in 102 Cottonmouths.

Number of scales	Specimens having number on both sides	Specimens having number on one side	Total	Percentage
Supralabials
7	11	24	35	25.2
8	64	27	91	65.5
9	0	3	3	2.2
Infralabials
8	0	2	2	1.5
9	3	10	13	9.6
10	12	32	44	32.4
11	53	22	75	55.1
12	0	2	2	1.5

TABLE 2.—Numbers of Supralabials and Infralabials of 102 Cottonmouths.

Number of individuals	Number of supralabials	Number of infralabials
37	8	11
15	8	10-11
12	7-8	11
6	7-8	10-11
5	8	10
5	8	9-10
4	7	11
3	7	9-10
3	7-8	10
2	7	9
2	7	10
2	8	10-12
2	8-9	10
2	7-8	8-9
1	7-8	9
1	8-9	10-11

The dorsal scales of cottonmouths are strongly keeled except that those of the two lower scale-rows on each side are weakly keeled. Also they are slightly larger than the others. Two apical pits are present on each dorsal scale. The shape of the scales and number of scale rows vary depending upon the position on the body. Scales on the neck are considerably smaller than those elsewhere on the body and are arranged in two or three more rows than those at mid-body. The skin in the region of the throat, neck, and fore-body is especially elastic and allows the swallowing of large prey. Posteriorly from the mid-body the scales decrease in size and become more angular, those on the tail tending to be rhomboidal and wider than long. In the region of the anus the number of scale rows diminishes rapidly, leaving only 12 to 14 rows at the base of the tail and only three rows immediately ahead of the tail tip. The tail ends in a spine composed of two scales: one scale covers the bottom, lower parts of the sides, and tip of the spine; and a shorter dorsal scale covers the top and upper parts of the sides of the basal two-thirds of the spine. The spine of embryos and young cottonmouths is blunt, but is pointed in most adults.

TABLE 3.—Variation in Numbers of Supralabials and Infralabials in a Brood

of Seven Cottonmouths.

Number of individuals	Number of supralabials	Number of infralabials
1	7	9
1	7	9-10
2	7-8	8-9
1	7-8	9
1	8	9-10
1	8-9	10

TABLE 4.—Analysis of Number of Scale Rows at Three Parts of the Body

in 81 Cottonmouths.

Number of scales per row	Neck	Mid-body	Anterior to anus
Number of individuals	Percentage	Number of individuals	Percentage	Number of individuals	Percentage
29	1	1.2
28	3	3.7
27	52	64.2
26	16	18.0	2	2.5
25	8	9.9	67	82.7
24	1	1.2	4	4.9
23			8	9.9	4	4.9
22					4	4.9
21					68	84.0
20					5	6.2

The number of scale rows on the neck, at mid-body, and just anterior to the anus is relatively constant at 27-25-21, respectively; but some individual variation is evident (Table 4). Since the rows are diagonally arranged, it is necessary in counting scales to proceed either anteriorly or posteriorly across the back; or the row may be counted in either direction up to the center of the back and then reversed on the other side of the snake. In order to count the scale rows in a position where no scale reduction or addition was occurring and to avoid as much error as possible, I counted from anterior to center and back on the neck, in any direction at mid-body, and from posterior to center and back near the anus. Because females generally are the larger in circumference posteriorly, they could have more scale rows than males just anterior to the anus. The few snakes having more than 21 scale rows in the posterior region offer no conclusive evidence as to tendencies, but in both instances in which this occurred the females outnumbered the males three to one. An odd, rather than an even, number of scale rows occurs on most of the length of the snakes examined, because there is a mid-dorsal row and scale rows tend to be lost on both sides at about the same level. An example of scale reduction of one snake was as follows: