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Sauromalus aterNorthern Chuckwalla

By Tanya Schultz

Geographic Range

Sauromalus ater, the common chuckwalla, inhabits deserts in the western United States and Mexico and 30 known islands in the Gulf of Mexico. Specifically, the Mojave and Sonoran deserts are known to have thriving populations of chuckwallas. They can be found as far west as southeastern California and Nevada, and are abundant in Arizona, New Mexico, Texas, Utah, and parts of Colorado. (Hammerson, 2007; Hollingsworth, 1998; Johnson, 1965)

Habitat

Common chuckwallas are found in deserts, where the air is hot and dry (9 to 45 degrees Celsius). They are found in deserts with rocks and crevices for hiding, such as areas of past lava flows, rocky hillsides, and outcrops. They use underground burrows and crevices for hibernation in the winter. They inhabit island and coastal environments. These lizards require a moderate amount of vegetation and foliage in the habitat in order to sustain themselves. Common chuckwallas can be found between sea level and 1400 m elevation. (Cliff, 1958; Hammerson, 2007; Johnson, 1965; Montanucci, 1997)

  • Range elevation
    1400 (high) m
    4593.18 (high) ft
  • Range depth
    0 (low) m
    0.00 (low) ft

Physical Description

Common chuckwallas are stout lizards with baggy skin folds near the neck, shoulders, and stomach. The skin is covered in hard scales, with the largest scales located between the nose and eyes. They have five digits on each limb and the tail is rounded at the tip. Head color can vary from dark brown and grey to dark yellow. Color variation depends on geographic location, temperature, surroundings, and mood. Tails are thicker near the body and taper toward the tip. Males are slightly larger than females and, in some cases, have different scale patterns and head shapes. Females may have a slightly narrower snout and head. There is also variation in color, young and females have lighter markings than males. Average weight is 245 grams (range from 24 to 315 grams), though males are usually heavier than females. Average head length is 35.5 mm with the greatest head width being 33.2 mm. Average snout-vent length is 162 mm (range from 80 to 197 mm) and length of tail averages 182 mm (Cliff, 1958). Almost half of the length is comprised of the tail. (Cliff, 1958; Friends of the Rosamond Gifford Zoo Education Volunteers, 2008; Hollingsworth, 1998; Johnson, 1965)

  • Sexual Dimorphism
  • male larger
  • sexes colored or patterned differently
  • male more colorful
  • sexes shaped differently
  • Range mass
    24 to 315 g
    0.85 to 11.10 oz
  • Average mass
    245 g
    8.63 oz
  • Range length
    80 to 197 mm
    3.15 to 7.76 in
  • Average length
    162 mm
    6.38 in

Development

After a gestation period of one month, females lay eggs in an underground nest. After common chuckwallas hatch from their egg, they begin growing at a rate of about 17 mm a year. Common chuckwallas leave their young after they hatch and the young are independent at that point. They reach sexual maturity at around ages 2 or 3. Growing to adult size takes about 25 years, after which they continue to grow but at a significantly decreased rate of about 5.5 mm a year. Growth rate is correlated with abundance of food. Common chuckwallas grow more in the spring and summer when the temperature is high and food is plentiful. Molting, or shedding of the epidermis, also occurs in the spring when growth rate is high. (Hammerson, 2007; Johnson, 1965)

Reproduction

Common chuckwalla courtship is comprised of males enticing females by size or persistence. Usually males have multiple females living in their territory and he mates with them. Male chuckwallas use head-bobbing, licking, circling, nudging, jaw-rubbing, and other methods to persuade females to mate. Females allow copulation if they determine the male is desired, males may assert themselves if they are large and forceful. During copulation, males use their jaws to grasp females by the loose skin on their nape. Mating usually takes place in the early spring after hibernation. Both sexes have multiple partners year to year. Males compete for females by biting and head-knocking. (Montanucci, 1997; Nagy, 1973)

Common chuckwallas breed between April and August when food is most abundant. When females are receptive, their ovaries become enlarged. Males produce sperm each year while females produce eggs every other year, on average. Males reach sexual maturity when they reach a 125 mm snout-vent length, or at about 2 years old. Females reach sexual maturity at the same length, but may take 2 to 3 years to reach that length. Average clutch size per breeding season varies from 5 to 16 eggs and the annual reproductive frequency of each female varies greatly from year to year depending on food availability and rainfall. Some females produce 2 separate broods, others produce none. Females incubate the eggs until they hatch. The incubation period lasts about 35 days (range 33 to 50 days). The birth mass range of the eggs is 6.0 to 9.6 grams. Larger females produce clutches more frequently than smaller females. However, smaller females may reproduce in consecutive years. (Friends of the Rosamond Gifford Zoo Education Volunteers, 2008; Johnson, 1965; Montanucci, 1997)

  • Breeding interval
    Common chuckwallas may not breed every year, but they breed in years of heavy rainfall and abundant food.
  • Breeding season
    Breeding takes place from April to August.
  • Range number of offspring
    5 to 16
  • Average number of offspring
    8
  • Range gestation period
    33 to 50 days
  • Average gestation period
    35 days
  • Average time to independence
    immediate minutes
  • Range age at sexual or reproductive maturity (female)
    2 to 3 years
  • Average age at sexual or reproductive maturity (female)
    2-3 years
  • Average age at sexual or reproductive maturity (male)
    2 years

Females prepare an underground nest in an area with dry soil that is unlikely to be disturbed. Females protect their eggs from predators and other threats while they incubate. However, after hatching, females no longer care for their young. Males do not provide parental care. (Friends of the Rosamond Gifford Zoo Education Volunteers, 2008; Hammerson, 2007; Johnson, 1965)

  • Parental Investment
  • female parental care
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female

Lifespan/Longevity

Common chuckwallas generally live 10 years or more, given suitable conditions, limited predation, and adequate food availability. The average lifespan in the wild is estimated to be about 15 years. Although there is considerable variability from year to year, on average, first year survivorship is estimated at 38%. Egg mortality causes the greatest impact on survivorship less than one year old. Those that survive past one year experience a mean survivorship of 75% per year. The oldest known chuckwalla in the wild was 30 years old. The oldest common chuckwalla in captivity lived to 65 years old. (Abts, 1987; Friends of the Rosamond Gifford Zoo Education Volunteers, 2008; Johnson, 1965; Montanucci, 1997; Sullivan and Sullivan, 2008)

  • Range lifespan
    Status: wild
    30 (high) years
  • Range lifespan
    Status: captivity
    65 (high) years
  • Average lifespan
    Status: wild
    15 years

Behavior

Common chuckwallas are mainly solitary and diurnal. They leave rock shelters during the day to eat foliage, but generally remain close to their shelters. They are active from mid-March to mid-August, but may aestivate in the summer months when food is scarce. From November through March they may go through brumation (similar to hibernation, but observed in ectotherms). They are active for about 8 hours each day, usually between 06:30 and 19:00. Common chuckwallas are most active from 9:15 to 11:30. To prevent overheating, they move in and out of the shade and assume different orientations towards the sun. Most of their time is spent basking on rocks when they are not looking for food. They retreat into crevices and shallow holes during the hottest portion of the day and use these crevices to avoid predators. Males are territorial and male fighting occurs over territory and females. Females are not hostile towards each other but hierarchies for both males and females are based on size. (Johnson, 1965; Montanucci, 1997; Nagy, 1973)

  • Range territory size
    1000 to 7000 m^2
  • Average territory size
    3500 m^2

Home Range

Male home ranges (0.57 ha) are typically larger than female home ranges (0.17 ha) and do not overlap with those of other males. Females have smaller home ranges, so that male home ranges generally overlap with those of several females. Female home ranges may overlap. According to a 7-year study in the Colorado Desert of southwestern California, densities ranged from 15 to 30 individuals per hectare. Home range sizes do not change from spring to summer. (Abts, 1987; Hammerson, 2007; Hollingsworth, 1998)

Communication and Perception

There is little information on communication and perception in common chuckwallas. Because they are solitary, there is little intraspecific communication, except when males compete for access to territory or females for mating. Most communication is limited to mating and courtship. Males fight other males and vigorously bob their heads to impress females. Females assess males based on their movements, nudging, and jaw-rubbing before mating. When looking for food, chuckwallas use the lingual vomeronasal system to decide what is food and what is not based on the chemical signals from the tongue. They have acute eyesight and perceive movement within 30 m. Their sense of hearing is not well-developed. (Cooper and Al-johany, 2002; Johnson, 1965; Montanucci, 1997; Sullivan and Kwiatkowski, 2007)

Food Habits

Common chuckwallas are herbivores, feeding on perennial and annual plants. Their diets are generally high in fiber and low in fat and protein. Annuals comprise 60% of their diet in the summer months, while perennials comprise 40%. When annuals die in the winter, perennials become their main source of plant food. They have been reported eating leaves and the fruit of creosote bushes (Larrea tridentata) in the summer months, as well as some insects that reside on the vegetation they eat. Browneyes (Oenothera clavaeformis) and desert ragweed (Franseria dumosa) are the main plants in the chuckwalla diet. In captivity they are fed various greens, vegetables, fruits, and flowers. They obtain water from the plants they eat and produce it metabolically. (Friends of the Rosamond Gifford Zoo Education Volunteers, 2008; Johnson, 1965; Montanucci, 1997)

  • Animal Foods
  • insects
  • Plant Foods
  • leaves
  • fruit
  • flowers

Predation

Common chuckwallas are experts at detecting and evading predators. Their main adaptation is to hide in crevices and rocks when they feel threatened or scared. Tight holes and crevices between rocks are inaccessible to larger predators and offer a defensive advantage against most larger mammals and birds, while snakes and other reptiles may be able to fit into them. Common chuckwallas inflate their lungs in order to wedge themselves even tighter against the rocks and they position the tail towards the crevice opening and against their body for added protection. Their neutral-colored scales serve as camouflage in the warm browns, tans, and greys of the desert landscape. Predators include red-tailed hawks (Buteo jamaicensis), American kestrels (Falco sparverius), coyotes (Canis latrans), and Mojave rattlesnakes (Crotalus scutulatus). (Cooper Jr., et al., 2000; Hollingsworth, 1998)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Common chuckwallas are herbivorous, feeding on plants and occasional insects. Through fecal deposits and plant clearing, they impact diversity in the habitats they occupy. Important predators of common chuckwallas are red-tailed hawks (Buteo jamaicensis), American kestrels (Falco sparverius), coyotes (Canis latrans), and Mojave rattlesnakes (Crotalus scutulatus). Plasmodium mexicanum (Saurian Malarial Parasite) and mite species (Hirstiella pyriformis and Hirstiella trombidiiformis) live in and on common chuckwallas. (Montanucci, 1997; Nagy, 1973)

Commensal/Parasitic Species
  • Saurian malarial parasites (Plasmodium mexicanum)
  • mites (Hirstiella pyriformis)
  • mites (Hirstiella trombidiiformis)

Economic Importance for Humans: Positive

Humans keep chuckwallas as pets. Currently, Nevada is the only state that allows commercial pet collection because of the large sustaining population. Illegal trade of common chuckwallas does occur in states that do not permit reptile collection. A small number of common chuckwallas are collected for universities and biological scientific research. (Brodie, Jr, et al., 2003; Montanucci, 1997; Nagy, 1973)

  • Positive Impacts
  • pet trade
  • research and education

Economic Importance for Humans: Negative

There are no known adverse effects of common chuckwallas on humans. They are solitary animals that are easily intimidated and scared. Generally, they are rarely seen by humans and don't cause any disturbances. (Nagy, 1973)

Conservation Status

Common chuckwalla populations are protected by national parks and naturally protected due to their remote habitat (rugged terrain and harsh climate). Trends towards developing desert areas of of the United States and parts of Mexico are affecting common chuckwalla habitat; however the population status as a whole has been reported as large, and stable.

Commercial sales of the chuckwalla have not shown any major impact on the species as a whole, but some subspecies are locally threatened by excessive collecting and habitat degradation. The South Mountain subpopulation, near Phoenix, Arizona is targeted because they have a unique color pattern that isv desirable for pet trade. The Colorado River dam has greatly reduced common chuckwalla populations in the Glen Canyon area of Utah, and grazing activities of goats, sheep, and burros have shown a small negative impact. Overall, the IUCN categorizes common chuckwallas as “Least Concern” due to large population distribution and no immediate threats. Although total population size is not known, the estimated total adult population size could be 100,000 or greater. (Hollingsworth, 1998)

Other Comments

Sauromalus ater was formally known as Sauromalus obesus. (Brodie, Jr, et al., 2003)

Contributors

Tanya Schultz (author), Radford University, Karen Powers (editor), Radford University, Tanya Dewey (editor), University of Michigan-Ann Arbor.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

diurnal
  1. active during the day, 2. lasting for one day.
ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

female parental care

parental care is carried out by females

fertilization

union of egg and spermatozoan

folivore

an animal that mainly eats leaves.

frugivore

an animal that mainly eats fruit

herbivore

An animal that eats mainly plants or parts of plants.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

indeterminate growth

Animals with indeterminate growth continue to grow throughout their lives.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

pet trade

the business of buying and selling animals for people to keep in their homes as pets.

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

scent marks

communicates by producing scents from special gland(s) and placing them on a surface whether others can smell or taste them

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

territorial

defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

visual

uses sight to communicate

References

Amer. Suppl. 1996. Influences of environmental conditions on life history tactics of herbivorous Mojave Desert lizards. Bull. Ecol. Soc., 77/3: 446 (abstr.).

Abts, M. 1987. Environment and variation in life history traits of the chuckwalla, Sauromalus obesus. Ecological Monographs, 57: 215-232.

Brodie, Jr, E., T. Edwards, Jr, P. Ustach. 2003. "Status of distribution, populations, and habitat relationships of the common chuckwalla, Sauromalus obesus, in Nevada" (On-line pdf). Division of Wildlife, Department of Conservation and Natural Resources, State of Nevada. Accessed December 04, 2013 at http://www.clarkcountynv.gov/Depts/dcp/Documents/Library/other%20reports/collab/ChuckWallaReport030703.pdf.

Cliff, F. 1958. A new species of Sauromalus from Mexico. Copeia, 1958: 259-261.

Cooper Jr., W., J. Van Wyk, P. Mouton, A. Al-Johany, J. Lemos-Espinal, M. Paulissen, M. Flowers. 2000. Lizard antipredatory behaviors preventing extraction from crevices. Herpetologica, 56: 394-401.

Cooper, W., A. Al-johany. 2002. Chemosensory responses to foods by an herbivorous acrodont lizard, Uromastyx aegyptia. Journal of Ethology, 20.2: 95-100.

Friends of the Rosamond Gifford Zoo Education Volunteers, 2008. "Desert Chuckwalla" (On-line). Rosamond Gifford Zoo. Accessed November 15, 2013 at http://rosamondgiffordzoo.org/assets/uploads/animals/pdf/DesertChuckwalla.pdf.

Hammerson, G. 2007. "IUCN Red List of Threatened Species. Version 2013.2" (On-line). Sauromalus ater. Accessed September 11, 2013 at http://www.iucnredlist.org/details/64054/0.

Hollingsworth, B. 1998. The systematics of chuckwallas (Sauromalus) with a phylogenetic analysis of other iguanid lizards. Herpitological Monographs, 12: 38-191.

Johnson, S. 1965. An ecological study of the chuckwalla, Sauromalus obesus Baird, in the western Mojave Desert. Amer. Midl. Nat., 73/1: 1-29.

Lappin, A., P. Hamilton, B. Sullivan. 2006. Bite-force performance and head shape in a sexually dimorphic crevice-dwelling lizard, the common chuckwalla [Sauromalus ater (= obesus)]. Biological Journal of the Linnean Society, 88/2: 215-222.

Macey, J., T. Papenfuss. 1991. Reptiles. Pp. 291-360 in C Hall, ed. Natural History of the White-Inyo Range: Eastern California. Berkeley, California: Univ. Calif. Press.

Montanucci, R. 1997. Captive management, behavior and conservation of chuckwallas, Sauromalus obesus (Lacertilia: Iguanidae). Bull Chicago Herp. Soc., 32/6: 121-127.

Nagy, K. 1973. Behavior, diet and reproduction in a desert lizard, Sauromalus obesus. Copeia, 1973/1: 93-102.

Sullivan, B., M. Kwiatkowski. 2007. Courtship displays in anurans and lizards: theoretical and empirical contributions to our understanding of costs and selection on males due to female choice. Functional Ecology, 21/4: 666–675.

Sullivan, B., K. Sullivan. 2008. Common chuckwalla (Sauromalus ater) populations in the Pheonix metropolitan area: stability in urban preserves. Herpetological Conservation and Biology, 3/2: 149-154.

Zimmerman, L., C. Tracy. 1989. Interactions between the environment and ectothermy and herbivory in reptiles. Physiological Zoology, 62/2: 374-409.

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