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Herpetology Notes, volume 8: 331-334 (2015) (published online on 16 June 2015)
Temperature Shock as a Mechanism for Color Pattern
Aberrancy in Snakes
Adam G. Clause1,* and Richard N. Becker2
In snake species with a blotched dorsum, blotch
fusion is one of the most commonly observed pattern
aberrancies. This is particularly true for rattlesnakes
(Viperidae: Crotalus and Sistrurus). Complete or
partial blotch fusion has been reported in 47 individual
rattlesnakes, across 16 taxa. Illustrations, descriptions,
or both have been published for C. adamanteus
(Antonio and Barker, 1983; Jensen et al., 2008), C. atrox
(Nickerson and Mays, 1968; Simons, 1986), C. durissus
terrificus (Amaral, 1932), C. horridus (Gloyd, 1935;
Dundee, 1994; Bechtel, 1995), C. oreganus concolor
(Schuett and Kraus, 1982), C. ravus (Rubio, 1998), C.
scutulatus (Gloyd, 1958; Nickerson and Mays, 1968), C.
viridis (Gloyd, 1935; Ashton et al., 1999; Hammerson,
1999), S. catenatus (Oldham, 1985; Bechtel, 1995),
and S. t. tergeminus (Lardie and Lardie, 1976; Irwin,
1979; Buerer and Geluso, 2014). Passing mention of the
morph can be found for C. aquilus, C. lepidus morulus,
C. o. oreganus, C. o. viridis, C. ruber, and C. stephensi
(Klauber, 1972). Several captive-bred C. o. oreganus
X C. scutulatus hybrid siblings also exhibited partial
blotch fusion (Cook, 1955). While at times superficially
similar to the blotch-fusion aberrancy, we consider the
“patternless” or “striped morph” aberrancy reported
in C. atrox, C. adamanteus, C. durissus terrificus, C.
horridus, C. ruber, C. scutulatus, and C. viridis (Amaral,
1932; Gloyd, 1958; Nickerson and Mays, 1968; Shupe,
1977; Irwin, 1979; Karges, 1979; Holmback, 1985;
Yancey et al., 1997; Rubio 1998) to be a different,
unrelated abnormality. The complete lack of lateral
Warnell School of Forestry and Natural Resources, University
of Georgia, Athens, Georgia
2
10711 Gay Road, Wilton, California
* Corresponding author e-mail: adamclause@gmail.com
1
blotches/stripes, coupled with the pristine appearance of
the faint dorsal stripe (when present) distinguishes this
morph from the blotch-fusion aberrancy.
Complete or partial blotch-fusion is not unique to
crotalid snakes. It has also been documented in five
species of Central and South American viperids:
Bothriechis schlegelii, Bothrops alternatus, B. cotiara,
B. jararaca, and B. pauloensis, (Amaral, 1932; Machado,
1945; Lema, 1960; Campbell and Lamar, 2004; BarrioAmorós, 2015). And in four species of North American
colubrids: Lampropeltis c. calligaster (Smith, 1961;
Anderson, 1965; Cox, 1972; Ashton, 1973; Dyrkacz,
1982; Bechtel, 1995), Pantherophis emoryi (Gloyd,
1935; Bechtel, 1995), P. guttatus (Bechtel and Bechtel,
1978), and P. obsoletus (Werler, 1951; Bechtel,
1995). A similar type of pattern aberrancy, the partial
transformation of transverse bands into longitudinal
stripes, has been reported in Agkistrodon contortrix
mokasen (Gloyd, 1958), unidentified subspecies of
A. contortrix (Bechtel, 1995), Cemophora coccinea
(Bechtel, 1995), Lampropeltis alterna (Tryon, 1979),
L. annulata (Williams, 1978), L. triangulum (Gehlbach,
1962; Williams, 1978), and Rhinocheilus l. lecontei
(McCrystal and Ivanyi, 2005; Bechtel, 1995). Two
additional species, Pituophis c. catenifer and L. getula
californiae, have populations that frequently include
completely striped color morphs, instead of the species’
typical blotched or banded pattern, respectively
(Klauber, 1939; Banta, 1964).
Detailed study of the underlying cause of blotch
fusion has been published for two colubrid taxa. In
Pantherophis guttatus, Bechtel and Bechtel (1978)
presented evidence that partial dorsal blotch-fusion is
attributable to a recessive genetic mutation. In Pituophis
c. catenifer, available evidence from controlled captive
breeding also points to a Mendelian recessive-dominant
mechanism—but with striping dominant to blotching
(Bechtel and Whitecar, 1983). The mechanism(s)
332
Adam G. Clause & Richard N. Becker
Figure 1. Aberrant Crotalus o. oreganus from Wilton, California, all presumed littermates. Photo © Adam G. Clause.
behind blotch fusion in other species, however, remains
uncertain (Bechtel, 1978). Here, we present data on a
collection of neonate northern Pacific rattlesnakes, C.
o. oreganus Holbrook 1840, that helps shed light on the
cause of this aberrancy.
Over a five-day period in early March 2012, we
collected a series of seven neonate C. o. oreganus from a
semi-rural private residence in Wilton, Sacramento Co.,
CA (38.403°N, -121.286°W; WGS84), elevation 20 m.
We found each snake under the same sheet of discarded
tin. Surrounding habitat was a mix of ornamental trees
and shrubs imbedded in a matrix of lawn/pasture, less
than 200 m from native riparian vegetation along the
Cosumnes River. We deposited all snakes at the Los
Angeles County Museum of Natural History (LACM
183232–183238 [whole-body specimens], LACM
TC 1079–1085 [liver tissues], LACM PC 1604–1610
[digital color photos]). Immediately after euthanasia,
the snakes measured 241–269 mm snout-to-vent length,
and 257–289 mm total length. Dissection revealed both
males and females among the seven snakes, although
poor gonad development made exact determination
very challenging. Given their similar size and discovery
under the same cover object, we deem it very likely that
all seven snakes were littermates. All snakes exhibited
some degree of dorsal blotch fusion (Figure 1). Fusion
occurred both in the large vertebral blotch series, and
the two lateral series of blotches. One snake showed
complete fusion of the vertebral blotches, presenting as
an uninterrupted dorsal stripe extending from the nape of
the neck to the base of the tail. At the other extreme, one
snake displayed fusion of only about 6 dorsal blotches,
with the rest normal. Between these two extremes, the
degree of blotch fusion among litter-mates varied along
a gradient. In snakes with partial fusion, the abnormal
pattern concentrated on the anterior and posterior third
of the body, consistent with past reports of snakes with
this aberrancy.
Partial dorsal blotch fusion in C. o. oreganus seems
relatively common in this region of northern California.
We have personally observed two other C. o. oreganus
with blotch fusion along the neck at the Stebbins Cold
Temperature Shock as a Mechanism for Color Pattern Aberrancy in Snakes
Canyon Reserve, Solano Co., approx. 72 km WNW
of Wilton. Additionally, Klauber (1956, 1972) briefly
mentions a clutch of 12 C. o. oreganus from Arbuckle,
Colusa Co., approx. 95 km NW of Wilton, all with some
degree of dorsal blotch fusion.
Our new series of presumed siblings, coupled with
existing data on the rattlesnake litters of Cook (1955),
Klauber (1972), and Simons (1986) casts doubt on a
genetic cause for this abnormality. All three authors
describe variation (sometimes dramatic) in trait
expression across complete litters, with a range of
intermediate phenotypes. The clutch reported by Cook
(1955) also included individuals with completely
normal dorsal pattern, like the parents. This spectrum
of within-litter variation suggests that the aberration,
if genetically based, is more complex than a simple
Mendelian recessive-dominant mutation. We argue,
instead, that temperature shock to the developing
embryos is a simpler and more likely explanation, as
first suggested by Simons (1986). This mechanism is
not novel in snake pattern aberrancy. Captive clutches
of Python molurus are known to produce neonates with
similar blotch-fusion aberrancies when embryos are
incubated just 3° C below normal (Vinegar, 1974). We
can easily imagine situations where the body of a large,
gravid rattlesnake, if exposed to unusually cold ambient
temperatures, could establish a temperature/aberrancy
gradient that fits our observed phenotypes.
The hypothesis of temperature shock also fits observed
examples of aberrancy in colubrids. Anderson (1965),
Cox (1972), and Ashton (1973) report three separate
captive-reared L. c. calligaster clutches with variable
expression of blotch fusion. Similarly, Werler (1951)
describes a captive clutch of five viable Pantherophis
obsoletus offspring with just one aberrant individual.
This suggests that temperature shock might explain
blotch-fusion aberrancy across a broad phylogenetic
range in snakes. Controlled experimentation with captive
rattlesnake litters and colubrid clutches is necessary to
confirm this hypothesis, however.
Acknowledgements. Special thanks to Gregory B. Pauly for
reviewing an earlier version of this note, and to Stephen R.
Goldberg for dissection assistance. We also thank Robert C.
Thomson, H. Bradley Shaffer, and Rick Becker for logistical
support. Collection work authorized under California Department
of Fish & Wildlife Scientific Collection Permit #011663,
and University of California-Davis IACUC Protocol #16936.
Financial support provided by a University of Georgia Presidential
Fellowship.
333
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