Gila monster
The Gila monster is a species of venomous lizard native to the Southwestern United States and the northwestern Mexican state of Sonora. It is a heavy, slow-moving reptile, up to long, and it is the only venomous lizard native to the United States. Its venomous close relatives, the four beaded lizards inhabit Mexico and Guatemala. The Gila monster is sluggish in nature, so it is not generally dangerous and very rarely poses a real threat to humans. Nonetheless, it has a fearsome reputation and is sometimes killed despite the species being protected by state law in Arizona.
History
The name "Gila" refers to the Gila River Basin in the U.S. states of Arizona and New Mexico, where the Gila monster was once plentiful. Heloderma means "studded skin", from the Ancient Greek words , "the head of a nail or stud", and , "skin". The species epithet suspectum comes from the describer, paleontologist Edward Drinker Cope. At first, this new specimen of Heloderma was misidentified and considered to be a northern variation of the beaded lizard already known to live in Mexico. He suspected that the lizard might be venomous due to the grooves in the teeth.The Gila monster is the largest extant lizard species native to North America north of the Mexican border. Its snout-to-vent length ranges from. The tail is about 20% of the body size, and the largest specimens may reach in total length. Body mass is typically in the range of. They appear strong in their body structure with a stout snout, massive head, and "little"-appearing eyes, which can be protected by a nictitating membrane.
The Gila monster has four close living relatives, all of which are beaded lizards. There are three species in Mexico: Heloderma exasperatum, Heloderma horridum and Heloderma alvarezi, as well as another species in Guatemala: Heloderma charlesbogerti.
The evolutionary history of the Helodermatidae may be traced back to the Cretaceous period, when Gobiderma pulchrum and Estesia mongolensis were present. The genus Heloderma has existed since the Miocene, when H. texana lived. Fragments of osteoderms from the Gila monster have been found in Late Pleistocene deposits near Las Vegas, Nevada. Because the helodermatids have remained relatively unchanged morphologically, they are occasionally regarded as living fossils. Although the Gila monster appears closely related to the monitor lizards of Africa, Asia, and Australia, their wide geographical separation and distinct features indicate that Heloderma is better placed in a separate family.
Skin
The scales of the head, back, and tail contain little pearl-shaped bones similar to those found in the beaded lizards from farther south. The scales of the belly are free from osteoderms. Female Gila monsters go through a total shed lasting about 2 weeks before depositing their eggs. The dorsal part is often shed in one large piece. Adult males normally shed in smaller segments in August. The young seem to be in constant shed. Adults have more or less yellow to pink colors on a black surface. Hatchlings have a uniform, simple, and less colorful pattern. This drastically changes within the first 6 months of their lives. Hatchlings from the northern area of the species' distribution have a tendency to retain most of their juvenile pattern. Gila monsters in areas with darker rocks and substrate will have darker colorations.The heads of males are very often larger and more triangular-shaped than in females. The length of the tail of the two sexes is statistically very similar, so it does not help in differentiation of the sexes. Individuals with stout tail ends occur in both nature and under human breeding.
Distribution and habitat
The Gila monster is found in the Southwestern United States and Mexico, across a range including Sonora, Arizona, and parts of California, Nevada, Utah, and New Mexico. No records have been given from Baja California. They inhabit scrubland, succulent desert, and oak woodland, seeking shelter in burrows, thickets, and under rocks in locations with a favorable microclimate and adequate humidity. Gila monsters rely heavily on the use of shelters and spend much of their time dwelling there. Often times these shelters are in rocky areas in Navajo Sandstone and basaltic lava flows. Gila monsters depend on water resources and can be observed in puddles of water after a summer rain. They avoid living in open areas, such as flats and open grasslands.Ecology
Gila monsters spend 90% of their lifetime underground in burrows or rocky shelters. They are active in the morning during the dry season. The lizards move to different shelters every 4–5 days up to the beginning of the summer season. By doing so, they optimize for a suitable microhabitat. Later in the summer, they may be active on warm nights or after a thunderstorm. They maintain a surface body temperature of about. Close to, they are able to decrease their body temperature by up to 2 °C by an activated, limited evaporation via the cloaca. One study investigating a population of Gila monsters in southwestern Utah noted that the lizard's activity peaked from late April to mid June. The average distance traveled during their bouts of activity was, but on occasion some lizards would travel distances greater than. During the Gila monster's active season of approximately 90 days, only ten days were spent active. Gila monsters are slow sprinters, but they have relatively high endurance and maximal aerobic capacity compared to other lizards. They are preyed upon by coyotes, badgers and raptors. Hatchlings are preyed on by snakes, such as kingsnakes.Among adaptations to a dry environment is a slow metabolism, allowing them to use less than half the amount of energy expected for lizards of their size. Gila monsters, and possibly also the Mexican beaded lizard, store water in their urinary bladder and reabsorb it across the bladder epithelium. Their tail is used for energy storage in the form of fat.
Diet
The Gila monster's diet consists of a variety of food items – small mammals, small birds, snakes, lizards, frogs, insects, other invertebrates, carrion, and the eggs of birds, lizards, snakes, and tortoises. Three to four extensive meals in spring are claimed to give Gila monsters enough energy for a whole season. They can store fat in their tails and therefore do not need to eat often. Nevertheless, they feed whenever they come across suitable prey. Young Gila monsters can swallow up to half of their body weight in a single meal. Adults may eat up to one third of their body weight in one meal.The Gila monster uses its extremely acute sense of smell to locate prey. The strong, two-ended tipped tongue, which is pigmented in black-blue colors, picks up scent molecules to be transferred to the opening of the Jacobson organ around the middle of the upper mouth cavern.
Prey may be crushed to death if large, or eaten alive, most of the time head first, and helped down by muscular contractions and neck flexing. After food has been swallowed, the Gila monster may immediately resume tongue flicking and search behavior in order to identify further prey such as eggs or young in nests. Gila monsters are able to climb trees, cacti, and even fairly straight, rough-surfaced walls.
Venom
Pioneer beliefs
In the Old West, the pioneers believed a number of myths about the Gila monster, including that the lizard had foul or toxic breath and that its bite was fatal. The Tombstone Epitaph of Tombstone, Arizona, wrote about a Gila monster that a local person caught on May 14, 1881:On May 8, 1890, southeast of Tucson, Arizona Territory, Empire Ranch owner Walter Vail captured and thought he had killed a Gila monster. He tied it to his saddle and it bit the middle finger of his right hand and would not let go. A ranch hand pried open the lizard's mouth with a pocket knife, cut open his finger to stimulate bleeding, and then tied saddle strings around his finger and wrist. They summoned Dr. John C. Handy of Tucson, who took Vail back to Tucson for treatment, but Vail experienced swollen and bleeding glands in his throat for sometime afterward.
Dr. Handy's friend, Dr. George Goodfellow of Tombstone, was among the first to research the actual effects of Gila monster venom. Scientific American reported in 1890, "The breath is very fetid, and its odor can be detected at some little distance from the lizard. It is supposed that this is one way in which the monster catches the insects and small animals which form a part of its food supplythe foul gas overcoming them." Goodfellow offered to pay local residents $5.00 for Gila monster specimens. He bought several and collected more on his own. In 1891, he purposely provoked one of his captive lizards into biting him on his finger. The bite made him ill and he spent the next five days in bed, but he completely recovered. When Scientific American ran another ill-founded report on the lizard's ability to kill people, he wrote in reply and described his own studies and personal experience. He wrote that he knew several people who had been bitten by Gila monsters but had not died from the bite.