Grismer, L. Lee

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2 IGUANA  •  VOLUME 11, NUMBER 1  •  MARCH 2004 L. LEE GRISMERGulf of California showing the location of Isla Santa Catalina.Illustration by John Binns.Dipsosaurus catalinensis from Isla Santa Catalina, Baja California Sur, México. Dipsosaurus dorsalis has a circum-Gulf of California distributionand is a popular symbol of southwestern desert ecosystems.The islands in the Gulf of California, México are renownedfor being among the most evolutionarily dynamic regionsof North America. Indeed, studies of the flora and fauna of theseislands have contributed significantly to much of what we cur-rently understand about the evolution of insular ecosystems andinsular biogeography. Reptiles have figured prominently inmany of these studies, largely because they are represented in theGulf of California by a number of endemic forms and adaptivetypes. Many of the iconoclastic species of North America’sSonoran Desert have given rise to insular populations in theGulf of California that, for a variety of reasons, have attainedfull-species status subsequent to their isolation. The processes ofsampling error, genetic drift, and the new selective pressuresattendant with insular systems have driven many of these speciesto evolve into highly adapted, sometimes bizarre forms.One of the most conspicuous species of lizards with a geo-graphic range that coincides with the circum-gulf distributionof the Sonoran Desert is the Desert Iguana, Dipsosaurus dorsalis.Santa Catalina Island Desert Iguanas,Dipsosaurus catalinensis:An Evolutionary Experiment in ProgressL. Lee GrismerDepartment of BiologyLa Sierra UniversityRiverside, CA 92515–8247All photographs by the author.IGUANA  •  VOLUME 11, NUMBER 1  •  MARCH 2004 3SANTA CATALINA ISLAND DESERT IGUANASDipsosaurus dorsalis has a distribution that borders much of the Gulf of California. This individual is from San Diego County, California.Its relatively large size, high density, docile behavior, and generallack of fear of humans has made this lizard a popular symbol ofsouthwestern desert ecosystems and a relatively well-studiedspecies — but, like many of its sympatric desert associates, theevolutionary history of D. dorsalis has been affected by the for-mation of the Gulf of California, and an isolated population ofDipsosaurus on Isla Santa Catalina off the eastern coast of BajaCalifornia Sur just south of Loreto, has attained species status.This species, Dipsosaurus catalinensis, was named by JohnVan Denburgh in 1922 after the island on which it is found. In1954, at a time when the concepts of species and subspecieswere much different than they are today, this lizard was rele-gated to subspecific status within the D. dorsalis complex. Onlyin recent years, with the advent of a more accurate under-standing of species boundaries has its original status beenreestablished. However, unlike its mainland and peninsularcounterpart and despite a considerable amount of herpetolog-ical research in the region, virtually nothing is known about thelife history and ecology of this endemic species, making it oneof the least known of all iguanas. Still, after only a few minuteson the island spent watching these lizards, the differences inmorphology, color pattern, and behavior that separate themfrom D. dorsalis become quite evident.Anyone familiar with D. dorsalis will be struck immedi-ately by the distinctly shaped head of D. catalinensis. Its snoutis shorter and more blunt than that of its mainland counterpart.This gives its head a relatively more square and an almost“stubby” appearance. The reasons for this are unclear. Theselizards may be feeding differently or constructing burrows in amanner that is facilitated by the blunt head. Scientists lack anyreasonable explanation, mainly because this species has neverbeen studied.Many Santa Catalina Island Desert Iguanas have a notice-ably constricted base of the tail. Again, the reasons for this areunclear, and even the most reasonable attempts to explain it arehighly speculative. Constricted tails usually are associated withfracture planes that facilitate tail loss (autotomy) and regenera-tion after attacks by predators or violent intraspecific interac-tions. However, this does not seem to be the case in thisinstance, as tail breakage frequently does not correspond to theconstriction. Hatchlings and juvenile D. catalinensis have anorangish tail. Brightly colored tails in lizards frequently serve todistract predators from the more vulnerable heads and bodies,4 IGUANA  •  VOLUME 11, NUMBER 1  •  MARCH 2004 L. LEE GRISMERDipsosaurus catalinensis is found only on Isla Santa Catalina.Dipsosaurus catalinensis: note the short, blunt head, the constriction near the base of the tail, the colored tail, and the dark throat.IGUANA  •  VOLUME 11, NUMBER 1  •  MARCH 2004 5SANTA CATALINA ISLAND DESERT IGUANASbut the bright tail color in these young iguanas often serves asa target for aggression by other iguanas. What selective advan-tage could be served by the latter? Again, a question with nocertain answer. Maybe the orange color does distract predatorseffectively, maybe even to the extent that the apparently dele-terious responses generated in conspecifics are negligible incomparison. Certainly, colors in the red to orange portion ofthe spectrum are known to elicit strong reactions in manyspecies of iguanas.One of the most curious differences of this species, how-ever, is its solid, chocolate-brown throat. This is most notice-able in males and probably has some function in courtship oraggression. Certainly, many other species of iguanas regularlyuse throat and dewlap colors to signal status or intent.Behavioral differences are profound and very evident aftereven casual observations. Unlike its docile cousin, D. dorsalis, D.catalinensis is difficult to approach and aggressive when cornered.This wariness is somewhat of an anomaly because insular lizardsin the Gulf of California generally exhibit much shorter flightdistances than their adjacent peninsular counterparts — and themost common explanation for this phenomenon is that theisland populations are subjected to less intense predation pres-sures. However, during early April 1992, I noted that, of approx-imately 90 lizards observed, 35–40% had freshly regeneratedtails, suggesting that predation on this population may be quitehigh. That, of course, might account for their uncharacteristi-cally long flight distances. As noted above, I also have observeda great deal of aggressive behavior among hatchlings and juve-niles, which are commonly seen biting and chasing each other.The orange tail is the body part most frequently bitten, and thenumber of individuals in those age classes with broken or regen-erated tails seems disproportionately high when compared topopulations of many other lizards. Although tail breakage iscommon in young animals too inexperienced to effectivelyevade predatory attempts, the percentage of broken tails in apopulation usually doesn’t reach the level seen in very youngSanta Catalina Desert Iguanas until the lizards are much older.Dipsosaurus catalinensis occurs both in the arroyo bottoms,where it takes refuge in the thornscrub, and on the rocky hill-sides, where it usually retreats in crevices between or underrocks. These lizards generally are active throughout the year, butthe least amount of activity occurs during fall and wintermonths, when most individuals seen are juveniles. Smallerlizards heat up more quickly and they are presumably able toutilize the relatively limited environmental sources of heat moreeffectively than older, larger animals.As with most diurnal lizards, D. catalinensis emerges in theearly morning to bask and then spends most of the remainderof the day going about other activities like courtship, territorialdefense, and feeding. Dipsosaurus catalinensis is omnivorous andprobably exploits opportunistically the most abundant season-ally available foods. At certain times of the year, finding groupsIsla Santa Catalina, Baja California Sur.6 IGUANA  •  VOLUME 11, NUMBER 1  •  MARCH 2004 L. LEE GRISMERSome populations of Dipsosaurus dorsalis; are established on islands in the Gulf of California. These individuals are from Isla Cerralvo, Baja CaliforniaSur. Note the longer head, the lack of a constriction near the base of the tail, the tail colored as the body, and the light throat.of two to six individuals feeding communally on the fallenfruits of Cardón Cacti (Pachycereus pringlei) is a common occur-rence. This is most obvious where the cacti occur in stands. Ionce found a carcass of an individual with an intestinal tractfilled to capacity with Cardón Cactus seeds. The seeds weremixed with several small rocks of the same approximate size.These appeared to be chipping away at the seeds’ hard outercovering, serving much the same function as pebbles stored inthe gizzards of seed-eating birds or the much larger gastrolithsthat have been found with fossilized remains of herbivorousdinosaurs. In the large intestine, essentially only the shells wereleft, as the fleshy pulps had been digested. I also have seeniguanas as high as one meter above the ground in PitahayaÁgria (Stenocercus gummosus) feeding on ripened fruits.Dipsosaurus catalinensis serves as a good example of howinsular situations can produce sometimes substantive changesin organisms over even relatively short periods of time. Facedwith unique selection pressures, different suites of predators andcompetitors, and reduced genetic variation, populations will dowhatever they are genetically capable of doing to continue thelineage. In the case of Dipsosaurus, nature has taken a common,docile species and sculpted its descendants into wary andaggressive lizards. What selective forces were responsible for thischange? Are they still operative today? Are similar pressuresmolding populations of other insular lizards? If so, are they elic-iting results comparable to those observed in the Santa CatalinaDesert Iguana or are they having profoundly different effectson populations of lizards with different genetic compositionsand lifestyles? These are some of the interesting questions thatevolutionary biologists could be asking about D. catalinensis. Amodicum of fieldwork leading to even tentative answers mightteach us a great deal about the evolution of insular systems ingeneral.ReferenceGrismer, L.L. 2002. Amphibians and Reptiles of Baja California, including itsPacific Islands and the Islands in the Sea of Cortés. University of CaliforniaPress, Berkeley.S P E C I E S  P R O F I L EIGUANA  •  VOLUME 11, NUMBER 1  •  MARCH 2004 7SANTA CATALINA ISLAND DESERT IGUANASSand Snakes in the genus Chilomeniscus have a circum-Gulf ofCalifornia distribution very similar to that of Desert Iguanas(Dipsosaurus). Similar also is the association with sandy areas inopen deserts, washes, and arroyos. Two species of Sand Snakesoccur in the region: C. stramineus is most widely distributed andoccurs on most of the Baja California Peninsula, in desert regionsto 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 variablepatterns that range from distinctly banded to totally unicolored.Not until studies demonstrated that pattern variants coexisted atmany localities was the clarification of relationships within thegenus possible.Sand Snakes are common in areas of loose soil and are animportant 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 obviousin some areas as a consequence of numerous tracks left in the sandby snakes that were actively foraging during the previous night.Sand Snakes apparently feed exclusively on small arthropods.ReferencesGrismer, L.L. 2002. Amphibians and Reptiles of Baja California, includ-ing its Pacific Islands and the Islands in the Sea of Cortés. Universityof 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 PowellAvila University, Kansas City, MissouriChilomeniscus stramineus from near Loreto, Baja California Sur,México. Photograph by Robert Powell.

Santa Catalina Island Desert Iguanas, Dipsosaurus catalinensis: An Evolutionary Experiment in Progress

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Santa Catalina Island Desert Iguanas, Dipsosaurus catalinensis: An Evolutionary Experiment in Progress

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