Extinctions and Loss of Species from Guam: Lizards
The skinks belong to a very diverse and successful family of lizards. They are characterized by large scales covering the head and body that are actually bony plates covered with skin. Such armament of the skin is a useful defense against predation. Considerable variation exists in body form with some species having short bodies and normally proportioned limbs, whereas others have elongated bodies and tails and diminutive fore- and hindlimbs. Skinks with elongated bodies often move using lateral undulation similar to the swimming movements of fishes and snakes. Most skinks have movable eyelids, but it is always the lower eyelid that is best developed for closing over the eye, and many have a clear window in the eyelid allowing vision even when the eyelid is closed. Most skinks are active only during the daytime, but some may be active at night or be seen at night when they are disturbed from the grass or cover where they are sleeping. Skinks lay eggs, but in contrast to geckos, the number of eggs is variable between and within species. The eggs have leathery parchment-like shells. Some skinks retain the eggs in the body of the female and give birth to active young.
Cryptoblepharus poecilopleurus (Snake-eyed skink)
This extremely slender skink has relatively small limbs, and the body has a somewhat flattened appearance when examined closely. It squirms and twists when grasped and is difficult to hold in the hand. The eyelid is fused over the eye, causing it to appear perpetually open. The body is brownish black with three golden stripes fusing into two stripes on the tail. The bottom edges of the stripes are jagged, but the top edges are boldly defined. The area along the middle is a more intense copper color than the bronze dorsolateral stripes. This species climbs readily on rocks and tree trunks but usually stays close to salt water. It has been collected under the rough bark of Australian pines (Cassurina). On islands where other terrestrial skinks are lacking, it may expand its niche to include a broader variety of terrestrial habitats. It is often found in areas of loose sandy soil. The last recorded specimen found on Guam was in 1969, and the status of the species on this island is unknown. It has been reported from Cocos, Guam, Rota, Aguijan, Tinian, Saipan, Anatahan, Sarigan, Guguan, Alamagan, Agrihan, Asuncion, and Maug. A lizard likely to refuge in crevices, under tree bark, and other confined spaces, it may be vulnerable to predation by small foraging snakes.
Emoia caeruleocauda (Blue-tailed skink)
Despite its common name on Guam, only juveniles and females have a conspicuous blue tail. Juveniles and young adults also have three prominent yellow stripes separated by brown or black, but the stripes fade in adults, making it difficult to distinguish this species from the brown curious skink without counting the fingers. Many adults that appear to lack stripes retain a trace of the vertebral or dorsolateral stripes on the head and neck. The blue-tailed skink is difficult to distinguish from the azure-tailed skink, which does not lose its bright stripes. The scales on the underside of the fourth toe number 31-43 in the blue-tailed skink and are much smaller and more numerous in the azure-tailed skink with more than 50 (usually 65-70) scales under the fourth toe. The vertebral stripe of the latter species tends to be somewhat wider on the head, the stripes are slightly whiter, and there is a visible pineal eye spot in the middle of the head behind the eyes. The blue-tailed skink actively forages, noisily, on the ground and on vegetation during daylight hours. It depends on rapid movements in and out of vegetation to protect it from predation by birds and other predators. In most areas where it occurs, it is the most conspicuous lizard on the forest floor, but is increasingly supplanted by the curious skink on Guam and Cocos. It is also known from Rota, Aguijan, Tinian, Saipan, Alamagan, Agrihan, and Asuncion. The abundance of this native species is simultaneously influenced by the curious skink and the brown Treesnake, both introduced species.
Emoia cyanura (Azure-tailed skink)
This small, striped skink currently is present on nearby Cocos Island, but is absent on Guam. No unequivocal records can be substantiated of this species existing on Guam, but its presence on Cocos without occurrence on Guam is difficult to explain, unless it disappeared from Guam without documentation. This species is most likely to be confused with the blue-tailed skink. In other areas of the Pacific where the two exist together, the azure-tailed skink usually occupies the forest edge while the blue-tailed skink is found in the forest interior.
Emoia slevini (Slevin's skink)
This relatively large (to 95 mm) skink may be uniform brown or have a tan body with black sides outlined above and below with small white square blotches that highlight the lateral coloration. The posterior two-thirds of the belly may be bright orange. There are 38 or less scales around the middle of the body, and the length of the interparietal is less than 1.5 times the width. Small individuals resemble the snake-eyed skink. Slightly larger Slevin's skinks resemble brown individuals of the blue-tailed skink, but usually show traces of the black lateral field and an interparietal scale between the parietal scales. This species is found on the forest floor, in old fields, and low on tree trunks. It is known from Cocos, Guam, Rota, Tinian, Guguan, Alamagan, Asuncion, and Maug, although it does not seem to be common on any of the southern islands at present. This species was probably never abundant on Guam and has not been recorded since 1945. Recent fieldwork on Guam, Rota, and Tinian failed to document Slevin's skink. The ecology of this poorly known skink warrants further study on the islands where it remains abundant.
Lipinia noctua (Moth skink)
This small skink (to 55 mm) is documented in the Marianas by only a few specimens from Guam. It is characterized by a yellow spot on the top of the head, which may be contiguous with a narrower yellow mid-dorsal stripe continuing onto the body but fading before reaching the base of the tail. It also has an interparietal scale and lacks supranasal scales, small scales between the scale containing the nostril and the internasals on the top of the snout. Overall, the coloration is brown or tan flecked with lighter and darker marks. A thick black line runs from the snout to the eye and onto the lateral body, fragmenting before reaching the base of the hindlimb. The lips are marked with alternating black and white bars. The belly is yellow to orange under the body and legs, fading to a pale bluish green under the tail and chin. This lizard will break its toes as well as its tail to escape the grasp of a potential predator. Unlike the other lizards of the Marianas, this species gives birth to live young. It is found on the ground or low in trees. It was never documented as common on Guam, although four were collected in one day in 1986. Insufficient data are available to accurately evaluate its current status on Guam or the impacts that snakes might have had on it.
The geckos comprise a widespread and successful group of lizards especially numerous in tropical regions of the world. They differ from other lizards in the Marianas in having relatively fine skin with a velvety texture covering the body and lacking enlarged platelike scales on the head and body. They have well-developed eyes with vertically elongated pupils. All species in the Marianas are nocturnal or predominantly active at twilight; they may be active in the daytime primarily inside buildings, in dense shade, or other subdued lighting. Several of the geckos in the Marianas are widespread species dispersed on islands over a wide area of the Pacific Ocean. Geckos typically lay two eggs with hard, calcareous (containing calcium carbonate, calcium, or lime) shells; the durability of the eggs and their resistance to dehydration may be an important factor in allowing geckos to colonize new islands when the eggs are hidden in logs and vegetation that may be transported by ocean currents and major storms. Such eggs probably facilitate dispersal by ocean currents and incidental transport by both early and modern movements of humans.
Gehyra mutilata (Mutilating gecko)
This is one of the most adaptable of the geckos. It is found in a wide variety of habitats, including forested areas in rotten tree trunks, under bark, in termite nests, and in and on buildings. The eggs of this species may be found in any nook or cavity but are especially obvious in cavities under the bark of standing dead tree trunks where many females may nest in the same site and shells from previously hatched eggs accumulate. A typical clutch consists of two eggs, but unlike most geckos, this species sometimes deposits three eggs. Its stocky, more rotund body form, slightly flattened tail, and spotted charcoal gray coloration help to distinguish it from other geckos, but in its unicolor coloration it is easily confused with the house and island geckos. It differs from the former in lacking the rings of enlarged scales on the tail and from the latter in having paired scales on the undersides of the toes and fingers. Males have 32-39 enlarged pores anterior to their vents. This gecko is known from Cocos, Guam, Rota, Tinian, Saipan, Srigan, Guguan, Alamagan, Pagan, and Agrihan. It is now uncommon in many areas of Guam, but may be locally abundant elsewhere on the island. This spotty distribution is uncharacteristic when compared to other islands and likely a result of differential vulnerability to brown Treesnake predation in varying habitats.
Gehyra oceanica (Island gecko)
This gecko can be locally abundant, especially on concrete structures, limestone cliffs, and in screw pine (Pandanus), palms, and cycads. It is often found in association with the mutilating gecko. The island gecko also may prey on house and mourning geckos, excluding them from living in close proximity. Adults of this species are conspicuous and only likely to be confused with the Micronesian gecko, which differs in having a markedly flattened tail (most evident if the tail is not regenerated), a shorter and less flattened head, and the innermost digit reduced in size resulting in the appearance of only four digits on the hand. The island gecko has elongated scales behind the scale on the tip of the chin, which differ markedly from the rounded ones of the Micronesian gecko. The island gecko differs from the house gecko in having smaller, more numerous scales on the underside of the digits, not highlighted with dark coloration, and in lacking the rings of spines on the tail. Males have 26-42 enlarged pores anterior to the vent. On Guam, this gecko is now rare and virtually extinct, even though it was locally abundant as recently as 1985 and several widely scattered individuals were collected in 1989. As the largest of the geckos and a frequent inhabitant of dense clusters of Pandanus (preferred refugium for the snake), intense predation pressure undoubtedly precipitated its decline in many areas of Guam. It is usually common wherever it occurs: Cocos, Guam, Rota, Tinian, Saipan, Guguan, Alamagan, and Asuncion.
Lepidodactylus lugubris (Mourning gecko)
This gecko is widespread in natural habitats and in close association with humans. It is an all-female species. The black bar running along each side of the head from the snout through the eye and onto the neck is distinctive for this species. The wavy chevron markings vary in intensity, but the pair of black spots or blotches on the base of the tail is evident in nearly all color phases. The tail is slightly flattened and has a distinct angular edge along the side. The mourning gecko differs from the Micronesian gecko in having five digits on all limbs, and from the mutilating gecko in having chevrons instead of rounded spots. The mourning gecko often calls with sharp "chiks" that may be given singly or in a series of up to 10 notes. Common in all habitats that furnish cover from direct sunlight, this species is often active during the day in reduced light, inside houses, and at twilight. Its small size allows it to occupy leaves, small plants, and the tips of twigs. Since females are capable of reproducing without males, any adult female can establish a population, making this species a good colonizer of small patches of habitat. This lizard may have benefited from the disturbances of Guam's ecology by the brown Treesnake. By reducing the birds and lizards most likely to prey on the mourning gecko, the snake may have contributed to the dispersal and success of the gecko in habitats previously unavailable to it. Itself a menu item for the brown Treesnake, the mourning gecko is particularly abundant in houses and on power poles where it is inaccessible to the snake. It has been reported from Cocos, Guam, Rota, Tinian, Saipan, Guguan, Alamagan, Pagan, Agrihan, and Ascuncion.
Nactus pelagicus (Rock gecko)
This attractively patterned gecko may be the least tolerant of man. It is alert and runs on the ground when approached at night, and frequently will stop and depend on cryptic coloration against a background of ground litter to hide. If further threatened, the rock gecko will take refuge in rock crevices and holes. It is occasionally found hiding under objects on the ground during the day, but at night it is often seen foraging on the ground and on rough rock substrates. This species is distinguished from other geckos because it lacks widened digital pads on the hands and feet; has a slender, rounded tail; and has conspicuous coloration of dark transverse markings often highlighted with light countershading. This is an all-female species; no males are known from the Marianas. Not much is known about its original distribution on Guam, although it is suspected to have been locally common because it was regularly collected by off-duty soldiers during World War II. It has been collected on Guam only rarely since 1945 and is now rare and locally restricted, perhaps due to predation by the introduced brown Treesnake and the musk shrew (Suncus murinus). It has been reported from Guam, Rota, and Tinian, but probably has gone undetected on other islands due to its unusual habitat and alert escape response.
Perochirus ateles (Micronesian gecko)
This relatively large gecko may be primarily associated with large trees. One specimen was observed on a primary branch of a large Barringtonia (fish poison tree) in shade during daylight hours. It is the only gecko in the Marianas that has a conspicuously reduced toe and finger; the innermost lacks a claw and is partially fused with the adjacent finger. The toes have webbing, and the scales on the toepads are paired only toward the tips of the digits. When compared to other species, the flattened tail with slightly enlarged scales on its lower surface is distinctive. The underside of the chin has three to four rows of enlarged, relatively rounded scales behind the scale on the tip of the lower jaw. There are two to five enlarged pores in front of the vent of males. This lizard has been reported from Cocos, Guam, Tinian, Rota, and Saipan, although it has probably been extirpated on Guam since it has not been collected since 1978 and has not been seen in recent years.
The family Iguanidae includes about 700 species of lizards commonly found throughout the Americas and in the Pacific Islands. Most iguanids have small scales, a loose fold of skin under their head and neck, and a short, non-protruding tongue. In many species, they exhibit a vertebral crest of scales along the back and tail. The anoles are primarily diurnal and arboreal. Like geckos, they have enlarged finger and toe pads that enable them to traverse smooth inclined or vertical surfaces with ease.
Anolis carolinensis (Green anole or American anole, Gualig [Guam])
This lizard is native to the Southeastern United States and was introduced in Guam in the mid-1950s. It is locally common in urban habitats on Guam and is rarely found in forested areas away from homes. Active during daylight hours, this species is visually oriented. Its eyes have round pupils visible as black dots in the center of the eye, and they have eyelids. The body is often brightly unicolored, although they can rapidly change from green to brownish black to blend in with the surroundings. The scales on the head are large and platelike, while the scales on the body are usually smooth and do not overlap. They have wide granular scales and enlarged toe pads on the undersides of the toes and fingers. Males have an enlarged reddish fan under the chin that can be expanded to advertise presence in mating rituals or territorial disputes. Females lay single eggs in the leaf litter and ground detritus. Where anoles are present on Guam, they are conspicuous prey in the stomachs of snakes. Anoles sleep at the ends of branches at night. Foraging snakes apparently are able to approach closely without alerting the sleeping lizards.
The skinks belong to a very diverse and widespread family of lizards. They are characterized by large scales covering the head and body that are actually bony plates covered with skin. Such armament of the skin is a useful defense against predation. Considerable variation exists in body form with some species having short bodies and normally proportioned limbs, whereas others have elongated bodies and tails and diminutive fore- and hindlimbs. Skinks with elongated bodies often move using lateral undulation similar to the swimming movements of fishes and snakes. Most skinks have movable eyelids, but it is always the lower eyelid that is best developed for closing over the eye, and some have a clear window in the eyelid allowing vision even when the eyelid is closed. Most skinks are active only during the daytime, but some may be active at night or be seen at night when they are disturbed from the grass or cover where they are sleeping. Skinks lay eggs, but in contrast to geckos, the number of eggs is variable between and within species. The eggs have leathery parchment-like shells. Some skinks retain the eggs in the body of the female and give birth to active young.
Carlia fusca (Curious brown skink)
The taxonomy of the skinks in the genus Carlia is complex. The exact identification and area of origin of the introduced population of lizards that was established on Guam in the 1960s have not yet been determined. Specimens are medium to dark brown on the back and tail and have beige or gray undersides with a variable rose or bronze suffusion prominent in adult males. The species is most readily distinguished from other skinks in the region by having only four fingers on the hands. Adult blue-tailed skinks and Slevin's skinks are similar in having predominantly brown body coloration, but have five fingers on the hands and white or other ventral colorations instead of gray. It actively forages on the ground and will climb onto low vegetation. It is most active on sunny days and may depend on patches of sunlight even in forested areas. It is frequently seen at night presumably when it is disturbed from sleeping in the grass. It is an extremely alert lizard and quickly investigates any movement on the ground that might represent food. It will readily grab and devour other small lizards including blue-tailed skinks, which are less frequently seen in urban areas where Carlia and house geckos are present. The curious skink is extremely common in many localities on Cocos, Guam, Saipan, and Tinian. Its abundance in urban areas makes it a likely candidate for continued dispersal as a passive stowaway in cargo moved by air and ship traffic from Guam and other source populations. In the 1990s, this skink was the most common food item of the brown tree snake on Guam and is largely responsible for the persistence of the snake's high populations there.
The geckos comprise a widespread and successful group of lizards especially numerous in tropical regions of the world. They differ from other lizards in the Marianas in having relatively fine skin with a velvety texture covering the body and lacking enlarged platelike scales on the head and body. They have well-developed eyes with vertically elongated pupils. All species in the Marianas are nocturnal or predominantly active at twilight; they may be active in the daytime but primarily inside buildings, in dense shade, or other subdued lighting. Several of the geckos in the Marianas are widespread species distributed on islands over a wide area of the Pacific Ocean. Geckos typically lay two eggs with hard, calcareous (containing calcium carbonate, calcium, or lime) shells; the durability of the eggs and their resistance to dehydration may be an important factor in allowing geckos to colonize new islands when the eggs are hidden in logs and vegetation that may be transported by ocean currents or major storms. Such eggs probably facilitated dispersal by ocean currents and incidental transport by both early and modern movements of humans.
Hemidactylus frenatus (House gecko)
This gecko is extremely successful around man and is a common inhabitant of houses, urban areas, and Guam's forested habitats. It is tolerant of sunlight as evidenced by its occurrence on wooden fence posts in open pastures and its occasional discovery in shade during daylight hours. Although primarily feeding on insects and other arthropods, house geckos probably include the mourning gecko in their diet. Both occur in similar situations, but the distribution of the mourning gecko may be negatively influenced by the presence of the house gecko. This is the only gecko with enlarged spine-like scales arranged in rings on the tail, although these scales are difficult to see in juveniles. This species is often heard, each call lasting two to three seconds, consisting of seven to eight chirps. It may also give a short, soft trill. It frequently carries tiny red parasitic mites on the toes near the claws. Males have 20-40 pores anterior to the vent. On Guam, it is now common throughout the island and particularly in forested areas where it has replaced the native gecko species. In most habitats, it is the most common gecko, but on other Marianas Islands, it is common only in disturbed habitats. The house gecko is known from Cocos, Guam, Rota, Tinian, Saipan, Alamagan, Pagan, and Agrihan. House geckos are frequent prey for brown Treesnakes, which encounter the lizards in trees, on fences, and around buildings.
The monitors are medium to large-sized lizards that are easily recognized by their well-developed limbs, long necks, and long tails. They are distributed in Africa, Asia, Australia, and intervening island areas, but reach their maximal diversity in Australia. They have an extrudable, snake-like bifurcate tongue that is used to chemically sample the air, which makes them efficient predators and scavengers. They are active during the daytime and use the available sunshine to reach their optimal body temperature by basking. Females lay eggs.
Varanus indicus (Monitor lizard or Guana, Hilatai [Guam])
This monitor lizard is distributed widely, from the Solomon Islands and Australia to the Mariana Islands far to the north. Some evidence suggests that the species has been present on Guam since the arrival of the island's original inhabitants, but a more recent introduction to Guam or other islands cannot be excluded. The large size of this lizard made it a potential source of meat for early Pacific islanders. Some introductions of this lizard by Japanese may have resulted from the desire to develop biological controls for rats that damaged rice and other agricultural crops. In recent times, many islanders have come to view the monitor as a pest because it frequently invades populated areas where it preys on poultry and eggs. It is also known to feed on native birds and their eggs. It preys on the brown Treesnake on Guam; however, this is more opportunistic than a dietary specialty. Including insects, crabs, and snails in its diet, it will also readily consume garbage and carrion as well. It is a good climber and frequently seeks refuge in treetops when threatened by man or other predators. The monitor is black to greenish-black with small cream or yellow spots. It may have pale orange lips and an off-white belly. Monitors may have declined in abundance on Guam in the last two decades, but it is unclear whether this is due to predation of the brown Treesnake on eggs and young, a consequence of mortality on Guam's expanding roadways, or to mortality resulting from monitors attacking and attempting to eat the introduced toxic marine toad, Bufo marinus. As a large predator capable of killing brown Treesnakes up to 2.3 m in length, the monitor may be a potential deterrent to the snake colonizing other islands where the monitor occurs. The monitor is found on Guam, Rota, Aguijan, Saipan, Anatahan, Sarigan, Pagan, and probably occurs on Alamagan and Agrihan.