The Amphibians and Reptiles of Sinaloa, Mexico - Smithsonian ...

170 UNIVERSITY OF KANSAS PUBLS., MUS. NAT. HIST.
Much of the confusion has involved the apparent occurrence of
intergradation between the two forms in Sinaloa (Smith and Van
Gelder, 1955:146). The population representing the name torquata
is composed of individuals that have the dark nuchal blotch preceded
by a creamy white band about as wide as the nuchal blotch,
whereas individuals having the dark nuchal blotch preceded by
the normal brown or pale brown dorsal coloration are referred to
ochrorhyncha. Specimens having each pattern have been taken
from several localities in Sinaloa. On September 28, two specimens,
representing both patterns, were taken on Mexican highway 15, 74
kilometers north-northwest of Mazatian, at 2210 and 2212 hours.
Two other specimens, representing both patterns, were taken eight
miles northeast of La Cruz at 2020 hours, on July 2, 1962. Snakes
with the ochrorhyncha nuchal pattern have been taken throughout
the lowlands of Sinaloa, at localities that completely overlap the
range of snakes with the torquata nuchal pattern in southern Sonora
and Sinaloa.
Examination of specimens from western Mexico indicates that,
with the exception of nuchal patterns, there is no way to distinguish
the two taxa. Contrary to what Dixon (1965:126) stated, we find
a decrease in the total number of ventral and sub caudal scales of
ochrorhyncha-pattemed snakes from Arizona and Sonora into Sinaloa
and of torquata-patterned snakes from southern Sonora through
Sinaloa and western Mexico to Guerrero. In the area where snakes
with both nuchal patterns occur the ventral-subcaudal scale counts
fall within the same range. The cline in the number of ventralsubcaudal
scales is independent of nuchal pattern. In addition,
snakes with different nuchal patterns from the same area are
virtually identical in other details of coloration and pattern. Our
findings suggest that the presence of two nuchal conditions in
Hypsiglena is a case of pattern dimorphism in a single, otherwise
uniform, species. Therefore, we refer all Sinaloan specimens to
Hypsiglena torquata pending results of a detailed study now in
progress.
Distribution in Sinaloa.—Throughout the lowlands of the state.
See Fig. 67.
Specimens examined.—Ahome (LACM 8650); 2.7 km. NE Chupaderos,
400 m. (KU 78940); 5.7 mi. S Coyotitan (JFC 65:142); 13,1 mi. S Coyotitan
(LACM 7267); 41.2 mi. S Coyotitan (LACM 7258); 11.7 mi. N Culiacan
(LACM 7274); 15.5 mi. N Culiacan (LACM 7284); 16,3 mi. N Culiacan
(CSCLB 1933); 26.3 mi. N Culiacan (LACM 7277); 19.8 mi. S Culiacan
(LACM 7278); 43 km. S Culiacan (CSCLB 1930); 73.7 mi. S Culiacan (LACM
7279); 97.2 mi. S Culiacan (LACM 7280); 29 km. NW Culiacan (by hwy.