Thorny devils, or mountain devils, are found in the Great Sandy Desert interior of Australia in regions with sandy soils.
Biogeographic Regions: australian (Native )
Distribution: C Australia (North Territory, Queensland, South Australia, West Australia)
Type locality: Western Australia
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.
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.
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.
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.
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.
Range mass: 28.5 to 57 g.
Range length: 76 to 110 mm.
Other Physical Features: heterothermic
Sexual Dimorphism: female larger
Bio-inspiring Capillary Action
Mrs. Nic's students of Saturday Course, Milton, ages 9-14, declare that the way thorny devils drink is an "ultra cool" adaptation. A system of tiny channels lead to the corners of its mouth, absorbing water by capillary action. Not only can this lizard absorb condensation, but it can gulp to suck water towards its mouth. Noah and other "biomimetics detectives" for Mrs. Nicolson have learned that architects and materials scientists are developing ways to mimic this capillary action for green building construction. (source: www.asknature.org)
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").
Habitat Regions: tropical ; terrestrial
Terrestrial Biomes: desert or dune
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.
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.
Animal Foods: insects
Primary Diet: carnivore (Insectivore )
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.
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.
- tapeworms (Oochoristica piankai)
- nematode worms (Parapharyngodon kartana)
- nematode worms (Abbreviata species)
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).
- Australian bustards (Ardeotis australis)
- black-breasted buzzards (Hamirostra melanosternon)
- sand goannas (Varanus gouldii)
- racehorse monitor lizard (Varanus tristis)
- humans (Homo sapiens)
Anti-predator Adaptations: cryptic
Life History and Behavior
Pianka et al. (1998) suggested that thorny devils may use landmarks to navigate visually.
Perception Channels: visual ; tactile ; vibrations
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.
Thorny devils live from 6 to 20 years in the wild.
Status: wild: 20 (high) years.
Status: wild: 6 years.
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.
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.
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.
Key Reproductive Features: iteroparous ; seasonal breeding ; sexual ; oviparous
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.
Parental Investment: pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female)
Evolution and Systematics
Grooves on spikes of thorny devil lizard provide drinking water by drawing condensed dew to mouth by capillary action.
Learn more about this functional adaptation.
Thorny devils have not been evaluated by the IUCN red list and are not listed in the CITES appendices.
US Federal List: no special status
CITES: no special status
State of Michigan List: no special status
Relevance to Humans and Ecosystems
There are no adverse impacts of thorny devils on humans.
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
The thorny dragon or thorny devil (Moloch horridus) is an Australian lizard, also known as the mountain devil, the thorny lizard, or the moloch. This is the sole species of genus Moloch. The thorny devil grows up to 20 cm (8.0 in) in length, and it can live for up to 20 years. Most of these lizards are coloured in camouflaging shades of desert browns and tans. These colours change from pale colours during warm weather and to darker colours during cold weather. These animals are covered entirely with conical spines that are mostly uncalcified.
The thorny dragon also features a spiny "false head" on the back of its neck, and the lizard presents this to potential predators by dipping its real head. The "false head" is made of soft tissue. The females are larger than the males. The thorny dragon's body is ridged in structure, and this enables the animal to collect water from any part of its body. That water is then conveyed to its mouth.
An intimidating array of spikes cover the entire upper side of the body of the thorny dragon. These thorny scales also help to defend it from predators. Camouflage and deception may also be used to evade predation. This lizard's unusual gait involves freezing and rocking as it moves about slowly in search of food, water, and mates.
Taxonomy and naming
The names given to this lizard reflect its appearance; the two large horned scales on its head complete the illusion of a dragon. Although the name Moloch was formerly used for a deity of the ancient Near East, this name began to be used later in demonology to refer to the fallen angel and Prince of Hell. The Thorny Dragon also has other nicknames people have given it. such as the "Devil Lizard", "Thorny Devil", "Horned Lizard", and the "Thorny Toad". 
The thorny dragon was first described in writing by the biologist John Edward Gray in 1841. While it is the only one contained in the genus Moloch, many taxonomists suspect another species might remain to be found in the wild. The thorny dragon is only distantly related to the similar (morphologically speaking) North American horned lizards of the genus Phrynosoma. This similarity is usually thought of as an example of convergent evolution.
The thorny dragon usually lives in the arid scrubland and desert that covers most of central Australia. For example, it inhabits the Spinifex (Triodia) sandplain and sandridge desert in the deep interior and the mallee belt.
The thorny dragon is covered in hard, rather sharp spines that dissuade attacks by predators by making it difficult to swallow. It also has a false head on its back. When it feels threatened by other animals, it lowers its head between its front legs, and then presents its false head.
The diets of thorny devil mainly subsists on ant, especially Ochetellus flavipes and other species in the Iridomyrmex or Ochetellus genera. Thorny dragons often eat thousands of ant's in one day.
Thorny dragons collect moisture in the dry desert by the condensation of dew on their bodies at night. This dew forms on its skin, and then it is channelled to its mouth in hygroscopic grooves between its spines. During rainfalls, capillary action allows the thorny dragon to suck in water from all over its body.
Breeding and reproduction/survival
The females lay clutch of three to ten eggs during the September–December season (spring-summer). They put these in a nesting burrow about 30 cm underground. The eggs hatch after about three to four months.
The popular appeal of the thorny dragon is the basis of an anecdotal petty scam. American servicemen stationed in Southwest Australia decades ago (such as during World War II) were supposedly sold the thorny fruits of a species of weeds, the so-called "double gee" (Emex australis), but those were called "thorny devil eggs" as a part of the scam. Thorny devils have been kept in captivity.
- Browne-Cooper, Robert; Brian Bush; Brad Maryan; David Robinson (2007). Reptiles and Frogs in the Bush: Southwestern Australia. University of Western Australia Press. pp. 46, 65, 158. ISBN 978-1-920694-74-6.
- Pianka, E. R.; Pianka, H. D. (1970). "The ecology of Moloch horridus (Lacertilia: Agamidae) in Western Australia". Copeia 1970: 90–103. doi:10.2307/1441978.
- Australia's Thorny Devil, retrieved 2007-10-31
- Bentley, P. J.; Blumer, F. C. (1962). "Uptake of water by the lizard, Moloch horridus". Nature 194: 699–700. doi:10.1038/194699a0.
- Pianka, E. R. (1997). "Australia's thorny devil". Reptiles 5 (11): 14–23.
- Science Podcast on belly-water-suction mechanism, 24 August 2007
- Moloch horridus, The Reptile Database
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