Tasmanian devil

Appearance

The Tasmanian devil is the largest surviving carnivorous marsupial. It has a squat, thick build, with a large head and a tail which is about half its body length.

Unusually for a marsupial, its forelegs are slightly longer than its hind legs, and devils can run up to 13 km/h for short distances. The fur is usually black, often with irregular white patches on the chest and rump.

These markings suggest that the devil is most active at dawn and dusk, and they are thought to draw biting attacks toward less important areas of the body, as fighting between devils often leads to a concentration of scars in that region.

Males are usually larger than females, having an average head and body length of 652 mm, a 258 mm tail and an average weight of 8 kg. Females have an average head and body length of 570 mm, a 244 mm tail and an average weight of 6 kg, although devils in western Tasmania tend to be smaller.

Devils have five long toes on their forefeet, four pointing to the front and one coming out from the side, which gives the devil the ability to hold food. The hind feet have four toes, and the devils have non-retractable claws. The stocky devils have a relatively low centre of mass.

Devils are fully grown at two years of age, and few devils live longer than five years in the wild.

The devil stores body fat in its tail, and healthy devils have fat tails. The tail is largely non-prehensile and is important to its physiology, social behaviour and locomotion. It acts as a counterbalance to aid stability when the devil is moving quickly. An ano-genital scent gland at the base of its tail is used to mark the ground behind the animal with its strong, pungent scent.

The male has external testes in a pouch-like structure formed by lateral ventrocrural folds of the abdomen, which partially hides and protects them. The testes are subovoid in shape and the mean dimensions of 30 testes of adult males was 3.17 cm × 2.57 cm. The female's pouch opens backwards, and is present throughout its life, unlike some other dasyurids.

The Tasmanian devil has the most powerful bite relative to body size of any living mammalian carnivore, exerting a force of 553 N. The jaw can open to 75–80 degrees, allowing the devil to generate the large amount of power to tear meat and crush bones—sufficient force to allow it to bite through thick metal wire.

The power of the jaws is in part due to its comparatively large head. The teeth and jaws of Tasmanian devils resemble those of hyenas, an example of convergent evolution. Dasyurid teeth resemble those of primitive marsupials. Like all dasyurids, the devil has prominent canines and cheek teeth. It has three pairs of lower incisors and four pairs of upper incisors. These are located at the top of the front of the devil's mouth. Like dogs, it has 42 teeth, however, unlike dogs, its teeth are not replaced after birth but grow continuously throughout life at a slow rate.

It has a "highly carnivorous dentition and trophic adaptations for bone consumption". The devil has long claws that allow it to dig burrows and seek subterranean food easily and grip prey or mates strongly. The teeth and claw strength allow the devil to attack wombats up to 30 kg in weight. The large neck and forebody that give the devil its strength also cause this strength to be biased towards the front half of the body; the lopsided, awkward, shuffling gait of the devil is attributed to this.

The devil has long whiskers on its face and in clumps on the top of the head. These help the devil locate prey when foraging in the dark, and aid in detecting when other devils are close during feeding. The whiskers can extend from the tip of the chin to the rear of the jaw and can cover the span of its shoulder.

Hearing is its dominant sense, and it also has an excellent sense of smell, which has a range of 1 kilometre. The devil, unlike other marsupials, has a "well-defined, saddle-shaped ectotympanic". Since devils hunt at night, their vision seems to be strongest in black and white. In these conditions they can detect moving objects readily, but have difficulty seeing stationary objects.

Distribution

Devils are found in all habitats on the island of Tasmania, including the outskirts of urban areas, and are distributed throughout the Tasmanian mainland and on Robbins Island. The north-western population is located west of the Forth River and as far south as Macquarie Heads.

Status

Widespread across Australia in the Pleistocene, the Tasmanian devil had declined and become restricted to three relict populations during the mid-Holocene period around 3,000 years ago.

Rock art and a single fossil near Darwin point to a northern population, and remains in the southeast signify a southeastern population ranging from the mouth of the Murray River eastwards to the vicinity of Port Phillip in Victoria. This population had contracted from northern Victoria and New South Wales. The rising sea levels in the Holocene also cut it off from Tasmanian populations.

The cause of their disappearance from the mainland is unclear, but their decline seems to coincide with the expansion across the mainland of indigenous Australians and dingoes. However, whether it was direct hunting by people, competition with dingoes, changes brought about by the increasing human population, who by 3000 years ago were using all habitat types across the continent, or a combination of all three, is unknown; devils had coexisted with dingoes on the mainland for around 3000 years.

Thylacines preyed on devils, and devils attacked thylacine young; devils may have hastened the thylacine's extinction. While the thylacine was extant, apart from hunting devils, it may also have put pressure on the devil for survival, by competing for scarce food and dens; both animals sought caves and burrows. It has been speculated that devils may have become more predacious and presided over larger home ranges to fill in the vacancy left by the thylacine.

Habitat disruption can expose dens where mothers raise their young. This increases mortality, as the mother leaves the disturbed den with her pups clinging to her back, making them more vulnerable.

Cancer in general is a common cause of death in devils. In 2008, high levels of potentially carcinogenic flame retardant chemicals were found in Tasmanian devils. Preliminary results of tests ordered by the Tasmanian government on chemicals found in fat tissue from 16 devils have revealed high levels of hexabromobiphenyl and "reasonably high" levels of decabromodiphenyl ether. The Save the Tasmanian Devil Appeal is the official fundraising entity for the Save the Tasmanian Devil Program. The priority is to ensure the survival of the Tasmanian devil in the wild.

Behavior

The Tasmanian devil is a nocturnal and crepuscular hunter, spending the days in dense bush or in a hole. It has been speculated that nocturnalism may have been adopted to avoid predation by eagles and humans. Young devils are predominantly crepuscular. There is no evidence of torpor.

Young devils can climb trees, but this becomes more difficult as they grow larger. Devils can scale trees of trunk diameter larger than 40 cm, which tend to have no small side branches to hang onto, up to a height of around 2.5–3 m.

Devils that are yet to reach maturity can climb shrubs to a height of 4 metres, and can climb a tree to 7 m if it is not vertical. Adult devils may eat young devils if they are very hungry, so this climbing behaviour may be an adaptation to allow young devils to escape. Devils can also swim and have been observed crossing rivers that are 50 metres in width, including icy cold waterways, apparently enthusiastically.

Tasmanian devils do not form packs, but rather spend most of their time alone once weaned. Classically considered as solitary animals, their social interactions were poorly understood.

They are considered to be non-territorial in general, but females are territorial around their dens. This allows a higher total mass of devils to occupy a given area than territorial animals, without conflict. Tasmanian devils instead occupy a home range. In a period of between two and four weeks, devils' home ranges are estimated to vary between 4 and 27 km2, with an average of 13 km2. The location and geometry of these areas depend on the distribution of food, particularly wallabies and pademelons nearby.

Devils use three or four dens regularly. Dens formerly owned by wombats are especially prized as maternity dens because of their security. Dense vegetation near creeks, thick grass tussocks, and caves are also used as dens. Adult devils use the same dens for life.

Habitat

The "core habitat" of the devils is considered to be within the "low to moderate annual rainfall zone of eastern and north-western Tasmania". Tasmanian devils particularly like dry sclerophyll forests and coastal woodlands. Although they are not found at the highest altitudes of Tasmania, and their population density is low in the button grass plains in the south-west of the state, their population is high in dry or mixed sclerophyll forests and coastal heaths.

Devils prefer open forest to tall forest, and dry rather than wet forests. They are also found near roads where roadkill is prevalent, although the devils themselves are often killed by vehicles while retrieving the carrion. According to the Threatened Species Scientific Committee, their versatility means that habitat modification from destruction is not seen as a major threat to the species.

Reproduction

Females start to breed when they reach sexual maturity, typically in their second year. At this point, they become fertile once a year, producing multiple ova while in heat.

As prey is most abundant in spring and early summer, the devil's reproductive cycle starts in March or April so that the end of the weaning period coincides with the maximisation of food supplies in the wild for the newly roaming young devils.

Occurring in March, mating takes places in sheltered locations during both day and night. Males fight over females in the breeding season, and female devils will mate with the dominant male. Females can ovulate up to three times in a 21-day period, and copulation can take five days; one instance of a couple being in the mating den for eight days has been recorded.

Devils are not monogamous, and females will mate with several males if not guarded after mating; males also reproduce with several females during a season. Females have been shown to be selective in an attempt to ensure the best genetic offspring, for example, fighting off the advances of smaller males.

Males often keep their mates in custody in the den, or take them along if they need to drink, lest they engage in infidelity. Males can produce up to 16 offspring over their lifetime, while females average four mating seasons and 12 offspring.

Theoretically this means that a devil population can double on an annual basis and make the species insulated against high mortality. The pregnancy rate is high; 80% of two-year-old females were observed with newborns in their pouches during the mating season. More recent studies of breeding place the mating season between February and June, as opposed to between February and March.

Gestation lasts 21 days, and devils give birth to 20–30 young standing up, each weighing approximately 0.18–0.24 grams. At birth, the front limb has well-developed digits with claws; unlike many marsupials, the claws of baby devils are not deciduous. As with most other marsupials, the forelimb is longer than the rear limb , the eyes are spots, and the body is pink. There are no external ears or openings. Unusually, the gender can be determined at birth, with an external scrotum present.

Tasmanian devil young are variously called "pups", "joeys", or "imps". When the young are born, competition is fierce as they move from the vagina in a sticky flow of mucus to the pouch. Once inside the pouch, they each remain attached to a nipple for the next 100 days.

The female Tasmanian devil's pouch, like that of the wombat, opens to the rear, so it is physically difficult for the female to interact with young inside the pouch. Despite the large litter at birth, the female has only four nipples, so there are never more than four babies nursing in the pouch, and the older a female devil gets, the smaller her litters will become. Once the young have made contact with the nipple, it expands, resulting in the oversized nipple being firmly clamped inside the newborn and ensuring that the newborn does not fall out of the pouch. On average, more females survive than males, and up to 60% of young do not survive to maturity.

Inside the pouch, the nourished young develop quickly. In the second week, the rhinarium becomes distinctive and heavily pigmented. At 15 days, the external parts of the ear are visible, although these are attached to the head and do not open out until the devil is around 10 weeks old.

The ear begins blackening after around 40 days, when it is less than 1 cm long, and by the time the ear becomes erect, it is between 1.2 and 1.6 cm. Eyelids are apparent at 16 days, whiskers at 17 days, and the lips at 20 days.

The devils can make squeaking noises after eight weeks, and after around 10–11 weeks, the lips can open. Despite the formation of eyelids, they do not open for three months, although eyelashes form at around 50 days. The young—up to this point they are pink—start to grow fur at 49 days and have a full coat by 90 days.

The fur growing process starts at the snout and proceeds back through the body, although the tail attains fur before the rump, which is the last part of the body to become covered. Just before the start of the furring process, the colour of the bare devil's skin will darken and become black or dark grey in the tail.

Their eyes open shortly after their fur coat develops—between 87 and 93 days - and their mouths can relax their hold of the nipple at 100 days. They leave the pouch 105 days after birth, appearing as small copies of the parent and weighing around 200 grams.

After being ejected, the devils stay outside the pouch, but they remain in the den for around another three months, first venturing outside the den between October and December before becoming independent in January.

During this transitional phase out of the pouch, the young devils are relatively safe from predation as they are generally accompanied. When the mother is hunting they can stay inside a shelter or come along, often riding on their mother's back. During this time they continue to drink their mother's milk.

Female devils are occupied with raising their young for all but approximately six weeks of the year. The milk contains a higher amount of iron than the milk of placental mammals.

After leaving the pouch, the devils grow by around 0.5 kg a month until they are six months old. While most pups will survive to be weaned, Guiler reported that up to three fifths of devils do not reach maturity. As juveniles are more crepuscular than adults, their appearance in the open during summer gives the impression to humans of a population boom. A study into the success of translocated devils that were orphaned and raised in captivity found that young devils who had consistently engaged with new experiences while they were in captivity survived better than young who had not.

Embryonic diapause does not occur.

Food

Tasmanian devils can take prey up to the size of a small kangaroo, but in practice they are opportunistic and eat carrion more often than they hunt live prey.

Although the devil favours wombats because of the ease of predation and high fat content, it will eat all small native mammals such as bettong and potoroos, domestic mammals , birds, fish, fruit, vegetable matter, insects, tadpoles, frogs and reptiles.

Their diet is widely varied and depends on the food available. Before the extinction of the thylacine, the Tasmanian devil ate thylacine joeys left alone in dens when their parents were away. This may have helped to hasten the extinction of the thylacine, which also ate devils.

On average, devils eat about 15% of their body weight each day, although they can eat up to 40% of their body weight in 30 minutes if the opportunity arises. This means they can become very heavy and lethargic after a large meal; in this state they tend to waddle away slowly and lie down, becoming easy to approach. This has led to a belief that such eating habits became possible due to the lack of a predator to attack such bloated individuals.

Tasmanian devils can eliminate all traces of a carcass of a smaller animal, devouring the bones and fur if desired. In this respect, devils have earned the gratitude of Tasmanian farmers, as the speed at which they clean a carcass helps prevent the spread of insects that might otherwise harm livestock. Some of these dead animals are disposed of when the devils haul off the excess feed back to their residence to continue eating at a later time.

Cultural

The devil is an iconic animal within Australia, and particularly associated with Tasmania. The animal is used as the emblem of the Tasmanian National Parks and Wildlife Service, and the former Tasmanian Australian rules football team which played in the Victorian Football League was known as the Devils.