The tammar wallaby (Macropus eugenii) also known as the dama wallaby or darma wallaby, is a small macropod native to South and Western Australia as well as some nearby islands. It may have been the first macropod to have been seen by a European. Its range was much more extensive in the past but has been severely reduced since European colonisation. Nevertheless, the tammar remains common within its limited range and is listed under Least Concern by the IUCN. It has been introduced to New Zealand and reintroduced to some areas of Australia where it has been previously eradicated. This species is largely gray in colour and is the smallest wallaby. At least three subspecies are known.

The tammar has several notable adaptations, including the ability to retain energy while hopping, colour vision and the ability to drink seawater. It is a nocturnal species and spends nighttime in grassland habitat, and daytime in shrub. It is also very gregarious and has a seasonal, promiscuous mating system. A female tammar can nurse a joey in her pouch while keeping an embryo in her uterus. The tammar is a model species for research on macropods, and on marsupials in general. It is one of many organisms to have had its genome sequenced, which has allowed greater understanding of some immunity-related genes as well as the discovery of new ones.

Taxonomy and classification

Drawing of tammar wallabies by John Gould (1863).

The tammar wallaby was seen in the Houtman Abrolhos off Western Australia by survivors of the 1628 Batavia shipwreck, and recorded by François Pelsaert in his 1629 Ongeluckige Voyagie.^[2]:53 It was first described by French naturalist Anselme Gaëtan Desmarest in 1817, who named it for where it was collected—an island he knew as Ile Eugene in the Nuyts Archipelago off South Australia, but which is now known as St Peter Island. The island's French name was in honour of Eugene Hamelin, Commander of the ship Naturaliste.^[3]:333 Its common name is derived from the thickets of the local shrub known as tamma (Allocasuarina campestris) that sheltered it in Western Australia.^[4] It is classified together with the kangaroos, wallaroos and larger wallabies in the genus Macropus. Within this, it is placed in the subgenus Notomacropus along with several species of wallaby, all of which have a facial stripe.^[5]

Subspecies

The tammar wallabies on Flinders Island were distinguished from Kangaroo Island tammars by their greyer coats and thinner heads. The East and West Wallabi Islands tammars were distinguished from the latter by their smaller size. Island tammars were once thought to be one species.^[3]:333

A 1991 examination of the tammar skulls from different parts of the species' range, found that populations can be divided into three distinct groups; one group made of populations from mainland Western Australia, East and West Wallabi Islands, Garden Island and Middle Island; a second group comprising populations from Flinders Island, 19th century mainland Southern Australia and New Zealand; and a third group consisting solely of the Kangaroo Island population.^[6]

Tammars at Budapest Zoo

As such, three subspecies of tammar wallaby are classified:^[4]

• M. e. derbianus (Western Australia and nearby islands)
• M. e. decres (Kangaroo Island)
• M. e. eugenii (mainland South Australia and New Zealand).

Evolution

Fossil evidence of the tammar wallaby exists from the late Pleistocene era—remains were found in the Naracoorte Caves.^[5] The mainland and island dwelling tammars split from each other 7,000–15,000 years ago,^[3]:332 while South Australian and Western Australian split around 50,000 years ago. It is likely that tammars originated in South Australia and developed a resistance to sodium fluoroacetate when they reached Western Australia.^[3]:334 Mainland Western tammars appear to be the most resistant to the poison, while those on Kangaroo Island are much more vulnerable.^[7] Tammars from New Zealand likely have a similar resistance level as the latter.^[3]:334 Tammars from East and West Wallabi Islands and Garden Island, which do not have plants containing sodium fluoroacetate, are less resistant than mainland Western tammars but are more resistant than those from Kangaroo Island.^[7]

Population dynamics and conservation

Tammar wallaby on North Island in the Houtman Abrolhos

Since European occupation, tammar wallaby populations on both mainland Australia and some islands have been greatly reduced or even eradicated. The mainland population of tammars have been described as "very plentiful in many parts of the south-west, but rapidly disappearing in the cultivated districts, especially towards the northern end of its range."^[8] Clearings made for wheat and sheep caused the population to decline further.^[3]:332 Farming activities led to the extinction of the tammar on Flinders Island and St. Peter's Island. Tammars in the Eyre Peninsula and around Adelaide were decimated by battues protecting crops and pastures. As a result, the tammar became extinct in the areas around Adelaide by the 1920s and in the Eyre Peninsula around the 1970s.^[3]:332-33

However, tammars from these areas were introduced to Kawau Island in New Zealand over 150 years ago by former South Australian Governor George Grey.^[9] Since their introduction to New Zealand, tammars have flourished to the point where their foraging has damaged indigenous plants. Pest control operators have used sodium fluoroacetate pesticides (1080) to control their populations.^[10] However, the use of 1080 has been controversial, due to its possible effect on organisms not targeted, including humans. As such, cyanide pellets have been used as an alternative.^[11] In 1985, tammars were introduced to the North Island of the Houtman Abrolhos and have made similar impacts on native vegetation. Their numbers have grown up to 450 individuals, but culling efforts appear to have reduced their numbers to 25 individuals by 2008.^[12]:82–83 In 2003, the Monarto Zoo temporarily housed 85 tammars from New Zealand before they were reintroduced to the Yorke Peninsula in Innes National Park.^[13] Four releases have been made, and a stable wild population of 100–120 animals now exists.^[9] Tammars have also been successfully translocated to other areas, such as Nambung National Park and Avon Valley National Park.^[1]

The tammar wallaby is listed under Least Concern by the IUCN, due to "its wide distribution, presumed large population, occurrence in a number of protected areas, and because it is unlikely to be declining at nearly the rate required to qualify for listing in a threatened category".^[1] However, the fragmentation of its range has led to at least some populations having "high levels of effective inbreeding and increased frequency of morphological abnormalities".^[14]

Description

The tammar wallaby is the smallest wallaby species, featuring a small head and large ears with a long tail, thick at the base.^[15] In colour, it has dark grey-brown upperparts with paler grey highlights, rufous on the sides of the body and limbs, particularly in males, and pale grey-buff underparts.^[4] The tammar wallaby exhibits significant sexual dimorphism, with the maximum recorded weight in males being 9.1 kg (20 lb) and maximum recorded weight in females is 6.9 kg (15 lb). The body length is 59–68 cm (23–27 in) in males and 52–63 cm (20–25 in) in females. Both males and females are about 45 cm (18 in) in height. The tails of males range from 34–45 cm (13–18 in) and those of females from 33–44 cm (13–17 in).^[15]

Locomotion

Tammar wallaby in motion

As with most macropods, the tammar wallaby moves around by hopping. This species has a hopping frequency of 3.5 strides per second, with a stride length of 0.8–2.4 m (2.5–8 ft).^[16] When hopping, proximal muscles at the knee and hip joints produce most of the power for the jump, which is delivered by the ankle through multi-joint muscles.^[17] As it lands back on the ground, the energies of the jump are converted into the strain energy made when its leg tendons stretch.^[18] As it leaps back off the ground, the tammar can recover much of this energy for reuse though elastic recoil.^[18] As such, a tammar can increase its hopping speed without losing more energy. In fact, the amount of energy stored in the tendons increases with the animal's speed and the weight of the load it is carrying. This is particularly helpful for mothers carrying young.^[19] The tammar shares this characteristic with other plains-dwelling macropods like the red kangaroo. By comparison, rock wallabies, such as the yellow-footed rock-wallaby, have decoupled energy saving for greater tendon thickness, which allows them to withstand the forces of hopping on rocky terrain without their tendons rupturing.^[20]

Senses

The tammar has 324 degree peripheral vision and 50 degree binocular vision.^[3]:312 It can discriminate black/white gratings of different widths and in different light intensities better than most other small mammals, such as rabbits.^[21] It is nevertheless not as good as cat or human. In addition, it appears that the tammar has some colour vision. Its eyes have only the blue sensitive and green sensitive photoreceptor cones; allowing it to see colour in the blue-green band of the colour spectrum, but not higher wavelengths in the red-yellow band. Nevertheless, in the band where it can see colour, it can differentiate between two monochromatic colours as close as 20 nm (2.0×10⁻⁸ m).^[22] The pinna of the tammar is mobile; allowing it to track sounds from different parts of its surroundings without moving its head. A tammar can point its pinna at a sound source and increase its eardrum's sound pressure by 25–30 dB at 5 kHz.^[23] When the pinna moves away from the sound source, the animal's hearing level quickly drops. When born, a tammar's sense of smell is already developed; this allows the newborn to find its mother's pouch by the scent.^[24]

Ecology and life history

The tammar is mostly nocturnal.

On Kangaroo Island, tammar wallabies can be found under divaricating bushes. They are mosly nocturnal and send much of the time foraging.^[3]:335 Tammars live home ranges which are 42 ha (100 acres) in summer and 16 ha (40 acres) in winter; home ranges being larger in summer can be explained by the need for tammars to spend out more to look for better quaility food in his dry period. Tammar home ranges overlap with those of conspecifics. During the day, tammars stay close to scrub, and move out to more open grassland by nightfall.^[3]:335 Tammars have been known to both graze and browse, however it is not as effective doing the latter as it commonly drops leaves when chewing on them.^[25]:116 This species sends more time chewing its food then other similarly sized macropods which likely aids digestation.^{[25]:115–16} Tammars commonly feed on acacia seeds. Other species consumed include heart-leaved poison (Gastrolobium bilobum), small-flowered wallaby grass (Austrodanthonia setacea), and marri (Corymbia calophylla).^[4]

Tammar resting

The tammar rests in two different positions: firstly, the "sitting tail forward" position, where the hind legs are outstreached with tail brought forward in between, the body prone and the head above ground; secondly the "lying" position, where animal lays on its side with the head touching the ground. The former is of long duration, and done mostly during the day, while the latter is more brief and done mostly at night.^{[25]:217–19} Tammars are known to live in stable groups.^[26] Gathering in groups lessens the chance of an individual being taken by a predator. As the group increases in size, tammars spend more time foraging, grooming and interacting and less time being vigilant and moving around.^[27] Predators of the tammar include dingos, feral cats, red foxes and wedge-tailed eagles. It may also have been preyed upon by the now extinct thylacine. Tammars appear to respond more to the sights than the sounds of predators.^[28] When a predator is detected, a tammar will alert conspecifics by thumping its foot.^[28] When lost, young tammars are known to emit a coarse screeching call and adult females may respond with a similar call.^[26]

Tammars lick their forearms and pant to keep cool in hot winter. They breathe more heavily and lose more water when the temperature is over 30 °C (86 °F). Tammars can't survive in temperatures above 40 °C (104 °F) and must get away from the heat.^[29] To prevent themselves from dehydrating, tammars urinate less and reabsorb water from the distal colon, which gives them relatively dry feces. Tammars live on several islands that have no fresh water and must subsist on seawater or the moisture in plants.^[3]:335 Their ability to concentrate urine allows them to survive drinking seawater.^[30]

Breeding and development

Mother wallaby with young in pouch

The tammar wallaby has a promiscuous mating system.^[31] It is a seasonal breeder and births largely occur during late January and early February.^[32]:77 After giving birth, females enter estrous and mating ends within four hours.^[31] When the breeding season arrives, the male's prostate and bulbourethral gland enlarge and its peripheral testosterone is more concentrated.^[33] However, there does not appear to be any seasonal difference in the weight of the testes. Around two weeks before the first birth, the males will begin sniffing the uro-gential openings and pouches of the females to determine their reproductive status.^[32]:78 Females allow males to mate with them soon after giving birth. However, a male that attempts to mate with an estrous female may face aggression from other males, which can delay ejaculations.^[31] A male can achieve reproductive success by mate-guarding. During the estrous period, males establish a dominance hierarchy and the higher ranking males will try to prevent subordinates from mating with estrous females.^[32]:83 Several males have been observed to pursue a single female.^[31]

The female tammar conceives a few hours after giving birth and suckles her young in her pouch for the next seven months. Tammars undergo embryonic diapause. For the first five months, the dormant embryo in the uterus can't develop while the young or joey is in the pouch. However, after the antipodean winter solstice in June, the embryo's development does not depend on the absence of the joey and will remain dormant in the uterus until after the summer solstice in December; months after the joey leaves the pouch.^[3]:338 Gestation in the tammar lasts 26.5 days. At this time, the female's four mammary glands begin to lactate.^[34] The newborn joey climbs into the pouch and suckles the teat of one mammary gland and the lactation of the other glands decline. The joey remains attached to the teat for 100–125 days. After which, it ceases its attachment, but remains in the pouch and continues to suckle at less regular intervals. The joey begins to leave the pouch and eat grass by 200 days. It suckles more forcefully but still less regularly. The joey leaves the pouch permanently after 250 days and is fully weaned at around 300–350 days.^[34] Male tammars may live around 11 years while females live for 14 years.^[4]

Sudden deaths

In late 1998, about 120–130 tammar wallabies died suddenly in research facilities and zoos in New South Wales and Queensland, perishing less than twelve hours after exhibiting the first symptoms, with most exhibiting no symptoms at all. Post mortem examination revealed haemorrhage of muscle, and abdominal and thoracic organs. The syndrome is known as Tammar Sudden Death Syndrome (TSDS) and the pathogen is an orbivirus of the family Reoviridae. It does not occur south of Sydney. Captive populations of tammar wallabies in New South Wales have subsequently suffered infections in summer months.^[35]

Genome

Tammars are easy to keep in captivity.

The genome of a marsupial is of great interest to scientists studying comparative genomics. Marsupials are at a convenient degree of evolutionary divergence from humans; mice are too close and have not developed many different functions, while birds are genetically too remote.^[36] The tammar wallaby is a model organism as it is cheap to keep in captivity and easy to handle.^[36]

The tammar wallaby's genome consists of 3.6 gigabases and has a short genetic map length of 1172 centimorgans.^[37] In 2009, BACs containing T cell receptor (TCR) and immunoglobulin (Ig) genes were physically mapped in tammar wallaby chromosomes by fluorescence in situ hybridisation (FISH).^[38] The tammar bacterial artificial chromosome (BAC) library yielded (BACs) containing key immune genes.^[38] This research illustrates that the genomic context of the immune genes has been conserved in marsupials, thus underscoring their importance.^[38] In 2011, the tammar wallaby had its full genome sequenced.^[39]

References