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IGUANA  •  VOLUME 11, NUMBER 2  •  JUNE 2004 87SPECIES PROFILE

S P E C I E S  P R O F I L E

Islands in the Gulf of California support a variety of endemic
species that have differentiated from their mainland ancestors

after various periods of isolation. Some have become distinct as
a consequence of adapting to local conditions on their new island
homes, whereas others differ from their continental relatives as a
consequence of the “founder’s effect.” This is basically a random
process that can cause populations that have descended from a
tiny number of founding individuals to vary considerably from
the stem population solely because the small sample of founders
represents but a tiny fraction of the diverse gene pool present in
the larger aggregate of individuals from which they have become
separated. When that tiny fraction of the gene pool is atypical of
the populational “norms,” the descendant populations may begin
their isolated existence already quite different in appearance or
behavior than their ancestors — and, if they remain separated for
a sufficient number of generations, they are quite capable of
achieving full-species status.

The Tortuga Island Rattlesnake (Crotalus tortugensis) is
endemic to Isla Tortuga in the Gulf of California. It is a medium-
sized rattlesnake, smaller than its nearest relative, the widely dis-
tributed Western Diamondback Rattlesnake (C. atrox), with large
males reaching only about one meter in total length. The head
size is relatively small when compared to C. atrox, a characteris-
tic that may indicate dwarfing. Although insular forms some-
times become giants, larger species and especially predatory forms
often become smaller when isolated on small, resource-deficient
islands.

Crotalus tortugensis is not well differentiated from C. atrox,
although our current understanding of their relationships is “ten-
uous.” Differences in the dorsal blotches (diamonds) are the most
noticeable divergence, with those of C. tortugensis less pro-
nounced and less distinctly bordered than those of C. atrox. In
addition, nearly all specimens of C. tortugensis lack an upper (sec-
ond) loreal scale between the nostrils and eyes and most have at
least one intergenial scale on both sides of their jaws. Also, the
squamosal bone of C. tortugensis is relatively shorter than that of
C. atrox.

References
Beaman, K.R. and C.L. Spencer. 2004. Crotalus tortugensis. Catalogue of

American Amphibians and Reptiles (798): 1–5.

Campbell, J.A. and W.W. Lamar. 2004. The Venomous Reptiles of the
Western Hemisphere. 2 volumes. Comstock Publishing Associates,
Cornell University Press, Ithaca, New York.

Case, T.J. 2002. Reptiles: Ecology, pp. 221–270. In: T.J. Case, M.J.
Cody and E. Ezcurra (eds.), A New Island Biogeography of the Sea of
Cortés. Oxford University Press, Oxford, England.

Grismer, L.L. 1994. Geographical origins for the reptiles on islands in
the Gulf of California, Mexico. Herpetological Natural History 2(2):
17–40.

Grismer, L.L. 1999. An evolutionary classification of reptiles on islands
in the Gulf of California, México. Herpetologica 55: 446–469.

Grismer, L.L. 2002. The Amphibians and Reptiles of Baja California, Its
Pacific Islands, and the Islands in the Sea of Cortés, México: Natural
History, Distribution and Identification. University of California
Press, Berkeley.

Klauber, L.M. 1972. Rattlesnakes: Their Habits, Life Histories, and
Influence on Mankind. 2nd ed. 2 volumes. University of California
Press, Berkeley.

Murphy, R.W. 1983a. Paleobiogeography and genetic differentiation of
the Baja California herpetofauna. Occasional Papers of the California
Academy of Sciences (137): 1–48.

Murphy, R.W. 1983b. The reptiles: origin and evolution, pp. 130–158.
In: T.J. Case and M.L. Cody (eds.), Island Biogeography in the Sea
of Cortez. University of California Press, Berkeley.

Murphy, R.W. and G. Aguirre Léon. 2002. Nonavian reptiles: Origin
and evolution, pp. 181–220. In: T.J. Case, M.J. Cody, and E.
Ezcurra (eds.), A New Island Biogeography of the Sea of Cortés.
Oxford University Press, Oxford, England.

Murphy, R.W., J. Fu, A. Lathrop, J.V. Feltham, and V. Kovac. 2002.
Phylogeny of the rattlesnakes (Crotalus and Sistrurus) inferred from
sequences of five mitochondrial DNA genes, pp. 69–92. In: G.W.
Schuett, M. Höggren, M.E. Douglas, and H.W. Greene (eds.),
Biology of the Vipers. Eagle Mountain Publishing, LC, Eagle
Mountain, Utah.

The Tortuga Island Rattlesnake 
(Crotalus tortugensis)

Kent R. Beaman
Section of Herpetology, Natural History Museum of Los Angeles County

and
Carol L. Spencer

Museum of Vertebrate Zoology University of California–Berkeley

Crotalus tortugensis from Isla Tortuga, Baja California Sur,
México. Photograph by L. Lee Grismer.