Western Banded Gecko (Coleonyx variegatus)
Western Banded Gecko, Photo by Young Cage.
The Western Banded Gecko (Figs. 1-5) is one the most frequently encountered nocturnal reptiles in the Sonoran Desert, yet surprisingly few field studies of its ecology and life history have been published. The most extensive is Parker’s (1972) research on a population in South Mountain Park near Phoenix, Maricopa Co., Arizona.
Seasonal Activity and Movement.
The nocturnal abundance of these lizards on Sonoran Desert roads in May is truly remarkable. This May activity peak is reflected by Klauber’s (1945) data for southern California, where he recorded 24 individuals in a 79-km (49-mi) stretch of road on 16 May 1941, and an overall monthly distribution as follows: March (1), April (45), May (340), June (130), July (28), August (24), September (8), and October (12). In contrast, Parker (1972), capturing 262 Western Banded Geckos in pitfall traps, found that the number of captures increased from April to July, after which they declined, in spite of the arrival of the monsoons and the increased frequency of juveniles in late summer. Juveniles (Fig. 2) were active every month of the year except December, whereas adults were recorded from April to October. The conflicting results of the two studies most likely reflect differences in the methods employed. Klauber’s data were based on road-driving surveys, and the May peak probably reflects long-distance activity associated with reproduction, whereas the pitfall traps used by Parker likely were more sensitive to near-home activity such as foraging.
The high numbers of geckos in May probably are the result of mate-seeking activity by males. Compared to females (Fig. 3), males (Figs. 4-5) were more frequently caught in pit-fall traps, had greater surface activity, moved larger distances, and were active later at night (Cooper et al. 1985, Kingsbury 1989, Parker, 1972).
Parker (1972) found that the mean distance moved between capture points was 17.3 m (56 ft) for males, 12.9 m (42 ft) for females, and 17.5 m (57 ft) for hatchlings. He estimated a density of 12 geckos/ha (4.6/acre).
Courtship is initiated by the male approaching the female, usually from behind with his body held close to the ground and tail waving. He makes an initial bite on the female’s tail, flank, or neck. If the female is receptive and remains motionless, he advances forward with jerking movements, eventually mounting and shifting his bite to the neck or shoulder. The female responds by raising her tail, and the male entwines and brings his cloaca beneath hers. The cloacal spurof the male (Figs. 4-5) appears to serve to draw back the female’s posterior cloacal lip on the side where the hemipenis is inserted (Greenberg 1943).
Frequencies of gravid females (Fig. 3) were found to be highest in May and June, simultaneous with (rather than after) the male reproductive peak, suggesting sperm storage may occur (Parker 1975). The species has a nearly fixed clutch size of two eggs, and some females lay two or more clutches per year (Parker 1975, Vitt 1977). The clutch size of these lizards is only slightly larger than the litter size of the Desert Night Lizard (Xantusia vigilis) which averages 1.6 to 1.9 young (Miller 1954, Vitt 1977), the lowest of Southwestern lizards. But, due to its multiple clutches, the species has a significantly higher reproductive rate than the Desert Night Lizard which is not known to produce more than one brood a year and in unfavorable years does not reproduce.
The stalking behavior of the Western Banded Gecko is distinctly feline: the animal fixes its elliptical pupils (Fig. 1) on the prey, holds its undulated body and tail just above the substrate, slowly advances, and vibrates its tail tip just before striking. Foraging mode is intermediate between that of sitand-wait predators and active foragers. The foraging geckos move 0.25-0.50 m (9.8-19.7 in) per minute, licking the substrate and investigating objects encountered (Kingsbury 1989).
Prey detection may occur with or without tongue-flicking. Cooper (1998) found increased tongue-flicking (volmerolfaction) when the lizards were presented with swabs that had been rubbed on the surfaces of crickets. Dial et al. (1989) found increased frequency of biting, but not tongue flicking, when they were presented with swabs having an extract of whole crickets. It appears that for strong stimuli (cricket extracts), the geckos rely primarily on olfaction which avoids tongue flicking movements that could increase the probability of an escape reaction by the prey (Cooper 1998).
Prey and Defecation.
These lizards are opportunistic in their food habits, and a wide variety of arthropods were found in the stomachs of 185 specimens examined from the Mojave and Sonoran Deserts by Parker and Pianka (1974). At South Mountain Park there was a significant dietary shift from predominately beetles, orthopterans, and insect larvae in spring to mostly termites, solpugids, and spiders in late summer. Gardner and Mendelson (1999) presented a photo of a 64 mm (2.5 in; SVL) Western Banded Gecko attempting to consume an 82 mm (3.2 in) DOR centipede (Scolopendra sp.) adhering to a road surface in Pima Co., Arizona. These lizards are known to eat their shed skin (Weldon et al. 1993).
The geckos repeatedly deposit their feces at a specific site, known as a defecatorium (Carpenter and Duval 1995). This is placed away from the favored diurnal resting site and may decrease chances of detection by redators. The lizards appear to recognize the odor of their own feces, and defacatoria also may serve as territorial sign posts.
Predators, Defense, and Tail Autotomy.
Several species of snakes frequently are listed as predators of the Western Banded Gecko (e.g., Parker 1972), but many of these appear to be based on observations of captives. Since the original report by Klauber (1935), both the Saddled (Phyllorhynchus browni) and Spotted (P. decurtatus) Leaf-nosed Snakes have been thought to feed largely on geckos and their tails. Gardner and Mendelson (2003) found that these snakes feed primarily on lizard and snake eggs; none of the 242 museum specimens of P. browni and only 2 of 410 P. decurtatus contained geckos, and these were tails. The timing of the seasonal activity peak of leaf-nosed snakes (late May, early June) suggests that the eggs of banded geckos may constitute a significant part of their diet (pers. obs.). Another potential predator is the night snake (Hypsiglena), but only 1 gecko was found among the 397 specimens in which the digestive system was examined by Rodriguez-Robles et al. (1999). For Sidewinders (Crotalus cerastes), Funk (1965) reported finding geckos in only 3 of 226 specimens from the Yuma area. A recently caught Coachwhip (Coluber flagellum) was observed to disgorge a gecko (Klauber 1945). The Desert Hairy Scorpion (Hadrurus arizonensis) was reported by Hadley and Williams (1968) to prey upon banded geckos, but this appears to be based on captives.
Tail autotomy is an important defense strategy in the Western Banded Gecko, with up to 74% of adults having regenerated tails (Parker 1972). When threatened, the lizard raises its tail, sometimes arches it over the back, and/or undulates it (Fig. 5). This behavior primarily serves as a decoy to deflect predator attacks from the body to the easily detached tail. Possibly, it also may represent mimicking of scorpions. When presented with a snake as a potential predator, these postures were exhibited in 46% of the encounters by geckos with original tails, but in only 38% of those with regenerated tails (Johnson and Brodie 1974). From the high frequency of regenerated tails, autotomy appears to be an effective approach to defense, but it is energetically expensive (Vitt et al. 1977) and the regenerated tails may be less effective in courtship (Greenberg 1943) and defense (Johnson and Brodie 1774).
When captured, these geckos often emit a “squeak” vocalization (Greenberg 1943), the function of which remains unclear. Longevity in the wild is unknown, but individuals have been recorded to live over 14 years in captivity (Mohen 1962).
Temperature and Physiology.
Field-active lizards had an average body temperature of 28.4°C (83.1°F; Parker and Pianka 1974). This is supported by the laboratory study of Vance (1973) who found that the average preferred body temperature in a thermal gradient was 28.6°C (83.5°F), with a range of 24° to 33°C (75.2 to 91.4°F). Males are active over a much broader range of environmental temperatures than are females (Kingsbury 1989).
Dial and Grismer (1992) examined the evolution of physiological character states in banded geckos (Coleonyx) based on the phylogenetic relationships of the seven species of the genus (Grismer 1988). Compared with most other members of the genus, the Western Banded Gecko and its near relatives have lower evaporative water loss, higher preferred body temperatures, and higher metabolic rates, derived features associated with their more active foraging modes.
Aggregation under shelter has been noted in the laboratory (Cooper et al. 1985) and field (Burke 1994). This appears to confer physiological (reduced evaporative water loss) rather than social benefits (Lancaster et al. 2006).
Systematics, Geographic Distribution, and Variation.
Coleonyx variegatus is a member of the family Eublepharidae, geckos with movable eyelids (Fig. 1). Its nearest relatives (Grismer 1988) are the Texas Banded Gecko (C. brevis) and the Black Banded Gecko (C. fasciatus), both of which have sometimes been considered to be subspecies of C. variegatus (e.g., Kluge 1975, Schmidt 1953). Putative natural hybrids between C. variegatus and C. fasciatus have been reported (e.g., Conant 1965, Grismer 1988, Hardy and McDiarmid 1969), and the biogeographic and genetic relationships of these two taxa remain unclear.
The Western Banded Gecko ranges (Fig. 6) from the Cape of Baja California Sur, north through Baja California to southern California, northeast through the Mohave Desert to southern Nevada and southwestern Utah, south through the Sonoran Desert of Arizona and Sonora, and east to southwestern New Mexico. It is found on Islas Ángel del la Guardia, Tiburón, San Marcos, Santa Inez, Coronados, Danzante, San José, Partida Sur, and Espiritu Santo in the Gulf of California and on Islas Cedros and Santa Marguarita in the Pacific (Grismer 2002). Isolated localities in Coconino Co., Arizona, have been reported from Wupatki National Monument (Persons and Nowak 2004) and Lake Powell (Brennan et al. 2002). The species appears to be absent from the grasslands of southeastern Arizona including the southern-most Santa Cruz, San Pedro, and Sulfur Springs valleys (Figs. 6, 7, 8; Table 1).
Seven subspecies have been recognized (Dixon 1970b, Klauber 1945), three of which are found in Arizona (Fig. 8). Klauber (1945) described the Tucson Banded Gecko (C. v. bogerti) from southeastern Arizona and diagnosed the subspecies as having 8 or more preanal pores compared to 7 or fewer in the Desert Banded Gecko (C. v. variegatus) occurring in the western part of state. No range-wide studies of morphological variation in the Western Banded Gecko have been published in the 65 years since Klauber’s (1945) work, but the two subspecies continue to be mapped in southern Arizona (e.g., Dixon 1970b, Espinoza 2009, Grismer 1988, Stebbins 2003).
To appraise geographical variation in Arizona, preanal pores were counted on 133 male specimens of C. variegatus from the state in the University of Arizona (UAZ) collection (Bezy, unpub. data). For simplicity the results are tabulated by county (Table 2, Fig. 8). The data confirm the higher average numbers of preanal pores in Pinal, Pima, Cochise, and Graham Cos., suggestive of C. v. bogerti. When analyzed at the county level, there is considerable overlap in the ranges of variation for the subspecies, and the mean for the sample from Gila Co. appears to be intermediate between the two. Few specimens were examined from western Pima Co., but one from near Wahak Hotrontk (UAZ 4248) has 11 preanal pores, suggesting that populations assignable to the Tucson Banded Gecko may occur west to at least 112°20’W. Larger samples from throughout the range, additional characters, and multivariate analyses are needed to fully examine geographic variation in morphology within the Western Banded Gecko. A detailed study of phylogeography of the species using DNA sequences is being conducted by Dean Leavitt (2008).
In the original description of Stenodactylus variegatus, Baird (1859) listed the type specimen as having been collected in the “Colorado Desert” by A. Schott on the U.S. – Mexican boundary survey. The type locality was later restricted by Smith and Taylor (1950) to “Winterhaven, Calif.” Cope (1866) placed the species in the genus Coleonyx which was described by Gray in 1845.
Coleonyx is derived from the Greek words koleos meaning “sheath” and onyx meaning “claw;” the name variegatus is from the Latin meaning “of different sorts,” referring to the contrasting elements of the color pattern (Brown 1956, Dixon 1970a, 1970b). The patronym bogerti is named for Charles M. Bogert, former curator at the American Museum of Natural History.
I thank George Bradley for access to specimens in the amphibian and reptile collections of University of Arizona Museum of Natural History (UAZ), and Kit Bezy and Kate Bolles for photography, participation in field work, advice on production of the maps, and helpful suggestions on an earlier version of this paper.
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Author: Robert Bezy
Originally published in the Sonoran Herpetologist 2010 23(7):97-102.
For additional information see Sonoran Herpetologist 2017 30(4):72-80.