Gila monster

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.^[7] Heloderma means "studded skin", from the Ancient Greek words helos (ἧλος), "the head of a nail or stud", and derma (δέρμα), "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.^[8]

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 26 to 36 cm (10 to 14 in). The tail is about 20% of the body size, and the largest specimens may reach 51 to 56 cm (20 to 22 in) in total length. Body mass is typically in the range of 550 to 800 g (1.21 to 1.76 lb). 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.^{[9][10][11][3]}

The Gila monster has four close living relatives, all of which are beaded lizards. There are three species in Mexico: Heloderma exasperatum, H. horridum and H. alvarezi, as well as another species in Guatemala: H. charlesbogerti.^[3][4]

The evolutionary history of the Helodermatidae may be traced back to the Cretaceous period (145 to 166 million years ago), 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 (10,000 to 8,000 years ago) 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 (varanids) of Africa, Asia, and Australia, their wide geographical separation and distinct features indicate that Heloderma is better placed in a separate family.^[12]

The scales of the head, back, and tail contain little pearl-shaped bones (osteoderms) 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.^[9] This drastically changes within the first 6 months of their lives.^[9] 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 might have darker colorations.^[13]

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.^[14] Sexing requires experience. The sizes of osteodermes on the heads and the different cloacal scales give hints to the sexes.^[9]

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.^[3] 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.^[15] Often times these shelters are in rocky areas in Navajo Sandstone and basaltic lava flows.^[13] 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.^[3]

Gila monsters spend 90% of their lifetime underground in burrows or rocky shelters.^[3] They are active in the morning during the dry season (spring and early summer). 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.^[3][16][17] Later in the summer, they may be active on warm nights or after a thunderstorm. They maintain a surface body temperature of about 30 °C (86 °F).^[3] Close to 37 °C (99 °F), they are able to decrease their body temperature by up to 2 °C (3.6 °F) by an activated, limited evaporation via the cloaca.^[18] 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 210 metres (690 ft), but on occasion some lizards would travel distances greater than 1 kilometre (0.62 mi). During the Gila monster's active season of approximately 90 days, only ten days were spent active.^[13] Gila monsters are slow sprinters, but they have relatively high endurance and maximal aerobic capacity (VO₂ max) compared to other lizards.^[19] They are preyed upon by coyotes, badgers, and raptors. Eggs and young are more vulnerable to predation than adults.^[20]

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.^[21] 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.^[22]

The Gila monster's diet consists of a wide range of small animals – mammals (such as young rabbits, hares, mice, ground squirrels, and other rodents), small birds, snakes, lizards, frogs, insects, other invertebrates, carrion, and the eggs of birds, lizards, snakes, and tortoises.^{[23][24][25][26][27]} Three to four extensive meals in spring are claimed to give Gila monsters enough energy for a whole season.^[28][29][13] They can store fat in their tails and therefore do not need to eat often.^[30] 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.^[18] Adults may eat up to one third of their body weight in one meal.^[10]

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,^[9] picks up scent, enabling the animal to hunt by day or night.^[20]

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.^[3][31]

Gila monsters emerges from brumation in early March. They become sexually mature at 4–5 years old. Mating takes place in April and May.^[3] The male initiates courtship by flicking his tongue to search for the female's scent. If the female rejects his advances, she will bite him and chase him away. When successful, copulation has been observed in captivity to last from 15 minutes to two and a half hours. There is only a single record of attempted mating outside of a shelter.^[25] The female lays eggs at the end of May into June. A clutch may consist of up to eight (rarely more than six) eggs.^[3][9] Hatching occurs near the end of October and immediately proceeds into hibernation without surfacing.^[16] Hatchlings are some 6.5 inches (17 cm) long and weigh around 0.075 pounds (34 g).^[20]

In summer, Gila monsters gradually spend less time on the surface to avoid the hottest part of the season; occasionally, they may be active at night. Females that have laid eggs are exhausted and thin, fighting for survival, and have to spend extra effort to "reconstitute". The brumation of Gila monsters begins in October.^[3]

Little is known about the social behavior of the Gila monster, but it has been observed engaging in male to male combat, in which the dominant male lies on top of the subordinate one and pins it with its front and hind limbs. While fighting, both lizards arch their bodies, pushing against each other and twisting around in an effort to gain the dominant position. A "wrestling match" ends when the pressure exerts their forces, although bouts may be repeated.^[13] These bouts are typically observed in the mating season. Males with greater strength and endurance are thought to enjoy greater reproductive success.^[3]

Gila monsters are listed as near threatened by the IUCN.^[1] They are listed as "Apparently Secure" by NatureServe.^[48]

In 1952, the Gila monster became the first venomous animal to be given legal protection.^[49]

Though the Gila monster is venomous, it poses little threat to humans due to its sluggish nature. Nevertheless, it has a fearsome reputation and is often killed by humans. Myths that have formed about the Gila monster include that the animal's breath is toxic enough to kill humans, that it can spit venom like a spitting cobra, that it can leap several feet in the air to attack,^[51] and that it did not have an anus and therefore expelled waste from its mouth, the source of its venom and "fetid breath".^[9]

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  With a common chuckwalla and a giant horned lizard, Bronx Zoo
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  Illustration in Animaux venimeux et venins, 1922
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  Tank setup with hiding bowl
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  Pattern variations of female H. suspectum
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  Egg deposit in captivity
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  End of hatching process
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  Absorbed yolk in the abdomen
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  Group of Gila monster hatchlings
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  Gila monster twins hatching