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Moloch horridusThorny Devil, Mountain Devil

By Tanya Dewey

Geographic Range

Thorny devils, or mountain devils, are found in the Great Sandy Desert interior of Australia in regions with sandy soils. (Pianka and Pianka, 1970; Pianka, 2003; Pianka, et al., 1998)

Habitat

Thorny devils are found in sand plain and sand ridge deserts and in mallee scrub on sandy soils. They are found only in sandy or sandy loam soils, not in rocky or hard soils. Vegetation in these habitats is characterized by spinifex grasses (Triodea) and acacia scrub ("mulga"). (Pianka and Pianka, 1970; Pianka, 2003)

Physical Description

Thorny devils are well-known for the many, large spines on their bodies. Their large spines are entirely boneless, only two parietal spines are supported by modest bony bosses on the skull. Their bodies are covered with thick shields and spiny cones and they are marked with golden and brown patches that serve to camouflage them in their arid habitats. They are extremely difficult to find because of their camouflage and escape detection even at close quarters and with experts who are actively looking for them. The head is ornamented with a large, spiny boss on the back and the eyes are small. Even the ventral surface is covered in conical, non-overlapping scales. It was long assumed that the spiny armor of thorny devils was exclusively a defensive trait, protecting them from predators. However, the complicated surface texture of thorny devils also helps them to retain and absorb water through body contact, especially condensation of water on body surfaces. Thorny devil skin color varies with temperature; individuals are brown or olive in the mornings and become light yellow as temperatures rise in the afternoon. Observations suggest thorny devils use postural changes to regulate body temperature, either maximizing contact with a warm surface (the ground) or minimizing it via standing with two feet against a shrub. (Gans, et al., 1982; Pianka and Pianka, 1970; Pianka, 2003; Pianka, 2009; Pianka, et al., 1998; Witten, 1993)

Females are larger than males, averaging 45.5 g (range 33 to 88.7 g) and 91 mm snout-vent length (range 80 to 110 mm). Males average 31.2 g and 78.7 mm snout-vent length and never weigh more than 49 g or measure more than 96 mm. Body weight changes substantially throughout the year and can vary quickly, losing or gaining up to 30% of their body mass in a few months. Oviposition in females results in a substantial loss of body mass, which can be regained quickly - in one case a female gained 40% of her body mass in a little over a month. (Pianka, 2009; Pianka, et al., 1998)

Thorny devils have been widely compared with Phrynosoma species, North American horned lizards. Thorny devils and Phrynosoma species are an example of convergent evolution towards a highly armored, slow-moving, thermally labile, ant-specialist lizard niche. (Pianka and Pianka, 1970; Pianka, 2009; Pianka, et al., 1998)

Unlike most Australian agamids, thorny devils have no femoral pores on the ventral surface of their thighs. They also lack a supratemporal bone. There are 21 presacral vertebrae and 22 postsacral vertebrae. Thorny devils have short digits and vary in the number of phalangeal bones, with a reduced formula throughout most of their range. The primitive agamid phalangeal formula is found in portions of the range. (Witten, 1993)

Thorny devil teeth are modified for their ant specialist diet. Ants are relatively hard-bodied insects, with high levels of chitin. The mandibular teeth are modified to fit neatly between two maxillary teeth, creating a shearing apparatus. Thorny devils have also lost the anterior pleurodont teeth, reflecting their use of the tongue to capture prey, rather than the teeth. (Witten, 1993)

  • Sexual Dimorphism
  • female larger
  • Range mass
    28.5 to 57 g
    1.00 to 2.01 oz
  • Range length
    76 to 110 mm
    2.99 to 4.33 in

Development

Male and female thorny devils hatch out of eggs at approximately the same size and grow at similar rates for their first year. Females begin to grow faster after that and growth continues until they reach 5 years old. (Pianka, 2009)

Reproduction

There is not much information on the mating system in Moloch horridus. Anecdotal evidence, as well as their wide-ranging habits during the mating season, suggests thorny devils may walk relatively long distances to converge at landmarks for mating. Limited observations suggest males approach females, bob their heads, and mount the female if she seems receptive. Females fall and roll to throw off males if they are unreceptive. (Pianka, 2009)

Thorny devils mate and lay eggs mainly in the late winter through early summer (August to December). Mating has also been observed in the fall, suggesting there may be a mechanism for sperm storage. Females lay eggs in oviposition burrows up to 15 cm long at depths up to 22 cm below the surface. These burrows are different from their normal burrows and are often dug into southern facing sand ridges. Females lay from 3 to 10 (median 8) eggs that are incubated for 90 to 132 days (mean 118). After laying eggs, females fill in the oviposition burrows and smooth out the surface to cover evidence of their activity. Hatchlings weigh 1.8 grams on average and measure 63 to 65 mm snout to vent length. Young apparently eat their own egg casing before climbing out of the ovipostion nest. (Pianka, 2009; Pianka, et al., 1998)

  • Breeding interval
    Thorny devils breed once yearly.
  • Breeding season
    Thorny devils mate and lay eggs mainly in the late winter through early summer (August to December).
  • Range number of offspring
    3 to 10
  • Range gestation period
    90 to 132 days
  • Average gestation period
    118 days

Female thorny devils have fat stores that vary seasonally, with lower fat body reserves in summer and more in winter. This is related to the timing of oviposition, when they must mobilize fat reserves to supply eggs with nutrients. Three females lost from 37 to 42% of their body weight during oviposition. (Pianka, 2009; Pianka, et al., 1998)

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

Lifespan/Longevity

Thorny devils live from 6 to 20 years in the wild. (Pianka and Pianka, 1970)

  • Typical lifespan
    Status: wild
    20 (high) years
  • Average lifespan
    Status: wild
    6 years

Behavior

Thorny devils can remain active at a wide range of body temperatures. Average field body temperature was 28.3 degrees Celsius in one study (Withers and Dickman, 1995). Thorny devils have a bimodal pattern of activity. They are active in the fall (March through May) and in the late winter through early summer (August through December). They become almost completely inactive during the hottest (January to February) and coldest parts (June and July) of the year, taking refuge in burrows that they dig themselves. Movement patterns have been described as "semi-nomadic" to take advantage of shifting patterns of ant availability, although this nomadic movement occurs within a relatively small area. During the summer and fall thorny devils are relatively sedentary. Areas include several shrubs, ant trails, and a defecation site. Individuals emerge from their shrub cover in the morning, warm themselves on the sand, walk to the defecation site, and then return to their shrub cover along the same route, sometimes stopping to eat ants along the way. In August and September thorny devils range widely, possibly in search of of mates. Thorny devils take shelter in small burrows, larger enough to fit a single individual at up to 10 cm below the surface and approximately 5X5X12 cm in area. They seem to use defecation areas apart from feeding areas and accumulations of their distinctive fecal pellets are found in these places. (Pianka and Pianka, 1970; Pianka, 2009; Pianka, et al., 1998; Withers and Dickman, 1995)

Thorny devils walk with a characteristic slow, rocking gait. They are deliberate in their movements. (Witten, 1993)

Home Range

Thorny devils are variable in their use of space. Tracked individuals have been recorded remaining within a very limited area for long periods or wandering widely. Thorny devil individuals were found within 100 meters of an originally recorded location over a course of 3 years. They are not territorial and do not seem to have exclusive home ranges, as tracked individuals overlapped widely in their use of areas. Average daily movement was estimated at 77.9 meters in one study, but some individuals moved more than 500 meters (Withers and Dickman, 1995). Males may move more than females, on average. (Pianka, et al., 1998; Withers and Dickman, 1995)

Communication and Perception

Pianka et al. (1998) suggested that thorny devils may use landmarks to navigate visually. (Pianka, et al., 1998)

Food Habits

Thorny devils are obligate myrmecophages; they eat only ants. They primarily eat ants of the genera Iridomyrmex (especially Iridiomyrmex rufoniger) and Crematogaster. Other ants recorded in their diet include Ectatomma, Monomorium, Camponotus, Pheidole, and Polyrhachis species. They are "sit-and-wait" predators, finding a feeding site near cover and waiting for their ant prey to pass. Iridomyrmex species are eaten as they pass in terrestrial trails and Crematogaster species are eaten mainly from their trails on currant bushes (Leptomeria preissiana). Thorny devils select feedings sites near Triodia tussocks or Thryptomeme or Leptomeria shrubs and only actively feed at temperatures above 24 degrees Celsius. In some areas thorny devils were only observed feeding in the morning (before 11 a.m.) or in the late afternoon (3 to 6 p.m.), but not from 11 a.m. to 3 p.m. Because ants contain chitin and formic acid and are relatively low in nutritive value, large amounts must be consumed. Thorny devils have large stomachs to accommodate consuming large numbers of ant prey; it is estimated that they eat 750 ants daily. Feeding rates averaged 2.9 ants per minute, but rates up to 1 ant per second were recorded. Research suggests that thorny devils assimilate about 59% of the metabolizable energy in their ant prey. This compares to an assimilation rate of about 70% for generalized insect diets. Their fecal pellets are distinctive: oval, black, and glossy, they easily crumble to reveal many ant exoskeletons. (Pianka and Pianka, 1970; Pianka, 2003; Pianka, et al., 1998; Withers and Dickman, 1995)

Thorny devils can employ their complicated surface textures to direct water to their mouths for drinking. The base of each spine is surrounded by a deep interscalar groove that effectively collects water and these grooves interconnect to enable capillary movement of water along the body, even against the pull of gravity, as on the legs. These grooves continue onto the head and empty into the angle of the mouth for drinking. These lizards live in extremely arid regions and water condensation onto their bodies may be the most reliable and abundant form of water typically encountered. (Gans, et al., 1982)

  • Animal Foods
  • insects

Predation

Thorny devils are preyed on by Australian bustards (Ardeotis australis), humans (Homo sapiens), black-breasted buzzards (Hamirostra melanosternon) and goannas (Varanus), although they are protected from predation by their extremely armored bodies and camouflaged coloration. Thorny devils have numerous thorny spines along their bodies, including a prominent spiny "false head" on the top of their skull. When threatened by a predator they tuck their head between their forelegs and present the "false head," making it difficult for anything to swallow them. Their cryptic coloration and elaborate spiny decoration make thorny lizards difficult to see, especially when they remain motionless. They move with slow, jerky movements and will freeze when approached by a threat, often in mid-step. They will also puff themselves up with air when threatened, making themselves more difficult to handle. Other possible predators include snakes and introduced dingos (Canis lupus familiaris) and red foxes (Vulpes vulpes). (Pianka and Pianka, 1970; Pianka, 2003; Pianka, 2009; Witten, 1993)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Thorny devils are ant-specialists and impact ant populations in the regions they inhabit. They co-occur with other insectivorous agamid lizards, including Ctenophorus isolepis and Ctenophorus inermis. However, these lizards eat a more generalized insect diet. (Pianka, et al., 1998)

Thorny devils are usually heavily parasitized by nematode worms (Parapharyngodon kartana and Abbreviata species). These worms may use ants or termites as intermediate hosts to get to their lizard definitive hosts. A new species of tapeworm (Oochoristica piankai) was described from the guts of thorny devils. (Pianka, 2009)

Commensal/Parasitic Species
  • tapeworms (Oochoristica piankai)
  • nematode worms (Parapharyngodon kartana)
  • nematode worms (Abbreviata species)

Economic Importance for Humans: Positive

Thorny devils are popular in zoos because of their extreme morphology. They are a unique example of adaptation to specialized diets in a harsh landscape.

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

There are no adverse impacts of thorny devils on humans.

Conservation Status

Thorny devils have not been evaluated by the IUCN red list and are not listed in the CITES appendices.

Other Comments

Molecular research suggests that, despite their extreme morphological changes from the primitive agamid morphology, thorny devils are relatively recently derived from the rest of the Australian, arid-adapted agamids. (Witten, 1993)

Moloch horridus is named for the terrible Canaanite god called Moloch and horridus, meaning thorny or dreadful. They are so named because of their bizarre and imposing appearance. (Pianka, 2009)

Contributors

Tanya Dewey (author), Animal Diversity Web.

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map

carnivore

an animal that mainly eats meat

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.
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.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

insectivore

An animal that eats mainly insects or spiders.

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).

native range

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

nomadic

generally wanders from place to place, usually within a well-defined range.

oviparous

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

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

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

vibrations

movements of a hard surface that are produced by animals as signals to others

visual

uses sight to communicate

References

Gans, C., R. Merlin, W. Blumer. 1982. The water-collecting mechanism of Moloch horridus re-examined. Amphibia-Reptilia, 3: 57-64.

Pianka, E. 2009. "Australia's thorny devil" (On-line). University of Texas. Accessed January 29, 2009 at http://uts.cc.utexas.edu/~varanus/moloch.html.

Pianka, E. 2003. "Moloch horridus" (On-line). Digimorph. Accessed January 25, 2009 at http://digimorph.org/specimens/Moloch_horridus/whole/.

Pianka, E., H. Pianka. 1970. The ecology of Moloch horridus (Lacertilia: Agamidae) in Western Australia. Copeia, 1: 90-103. Accessed January 23, 2009 at http://www.jstor.org/stable/1441978.

Pianka, G., E. Pianka, G. Thompson. 1998. Natural history of thorny devils Moloch horridus (Lacertilia: Agamidae) in the Great Victoria Desert. Journal of the Royal Society of Western Australia, 81: 183-190.

Withers, P., C. Dickman. 1995. The role of diet in determining water, energy, and salt intake in the thorny devil Moloch horridus (Lacertilia: Agamidae). Journal of the Royal Society of Western Australia, 78: 3-11.

Witten, G. 1993. "Agamidae" (On-line). Fauna of Australia series. Accessed January 28, 2009 at http://www.environment.gov.au/biodiversity/abrs/publications/fauna-of-australia/pubs/volume2a/29-fauna-2a-squamata-agamidae.pdf.

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