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of two to six individuals feeding communally on the fallen
fruits of Cardón Cacti (Pachycereus pringlei) is a common occur-
rence. This is most obvious where the cacti occur in stands. I
once found a carcass of an individual with an intestinal tract
filled to capacity with Cardón Cactus seeds. The seeds were
mixed with several small rocks of the same approximate size.
These appeared to be chipping away at the seeds’ hard outer
covering, serving much the same function as pebbles stored in
the gizzards of seed-eating birds or the much larger gastroliths
that have been found with fossilized remains of herbivorous
dinosaurs. In the large intestine, essentially only the shells were
left, as the fleshy pulps had been digested. I also have seen
iguanas as high as one meter above the ground in Pitahaya
Ágria (Stenocercus gummosus) feeding on ripened fruits.

Dipsosaurus catalinensis serves as a good example of how
insular situations can produce sometimes substantive changes
in organisms over even relatively short periods of time. Faced
with unique selection pressures, different suites of predators and
competitors, and reduced genetic variation, populations will do

whatever they are genetically capable of doing to continue the
lineage. In the case of Dipsosaurus, nature has taken a common,
docile species and sculpted its descendants into wary and
aggressive lizards. What selective forces were responsible for this
change? Are they still operative today? Are similar pressures
molding populations of other insular lizards? If so, are they elic-
iting results comparable to those observed in the Santa Catalina
Desert Iguana or are they having profoundly different effects
on populations of lizards with different genetic compositions
and lifestyles? These are some of the interesting questions that
evolutionary biologists could be asking about D. catalinensis. A
modicum of fieldwork leading to even tentative answers might
teach us a great deal about the evolution of insular systems in
general.

Reference
Grismer, L.L. 2002. Amphibians and Reptiles of Baja California, including its

Pacific Islands and the Islands in the Sea of Cortés. University of California
Press, Berkeley.

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

IGUANA  •  VOLUME 11, NUMBER 1  •  MARCH 2004 7SANTA CATALINA ISLAND DESERT IGUANAS

Sand Snakes in the genus Chilomeniscus have a circum-Gulf of
California distribution very similar to that of Desert Iguanas

(Dipsosaurus). Similar also is the association with sandy areas in
open deserts, washes, and arroyos. Two species of Sand Snakes
occur in the region: C. stramineus is most widely distributed and
occurs on most of the Baja California Peninsula, in desert regions
to the north and east of the Gulf, and on several Gulf Islands,
whereas C. savagei occurs only on Isla Cerralvo.

Considerable confusion has clouded taxonomic relation-
ships within the genus, most caused by the extremely variable
patterns that range from distinctly banded to totally unicolored.
Not until studies demonstrated that pattern variants coexisted at
many localities was the clarification of relationships within the
genus possible.

Sand Snakes are common in areas of loose soil and are an
important component of many dune ecosystems. They are spe-
cialized for burrowing. A countersunk lower jaw and valved nos-
trils exclude small particles as these snakes “swim” through sand.
Very smooth scales reduce friction both above and below the sur-
face. Sand Snakes are nocturnal, but their abundance is obvious
in some areas as a consequence of numerous tracks left in the sand
by snakes that were actively foraging during the previous night.
Sand Snakes apparently feed exclusively on small arthropods.

References
Grismer, L.L. 2002. Amphibians and Reptiles of Baja California, includ-

ing its Pacific Islands and the Islands in the Sea of Cortés. University
of California Press, Berkeley.

Grismer, L.L., H. Wong, and P. Galina-Tessaro. 2002. Geographic vari-
ation and taxonomy in the Sand Snakes, Chilomeniscus (Squamata:
Colubridae). Herpetologica 58:18–31.

Banded Sand Snakes 
(Chilomeniscus stramineus)

Robert Powell

Avila University, Kansas City, Missouri

Chilomeniscus stramineus from near Loreto, Baja California Sur,
México. Photograph by Robert Powell.