Tammar wallaby
The tammar wallaby, also known as the dama wallaby or darma wallaby, is a small macropod native to South and Western Australia. Though its geographical range has been severely reduced since European colonisation, the tammar wallaby remains common within its reduced range and is listed as "Least Concern" by the International Union for Conservation of Nature. It has been introduced to New Zealand and reintroduced to some areas of Australia where it had been previously extirpated. Skull variations differentiate between tammar wallabies from Western Australia, Kangaroo Island, and mainland South Australia, making them distinct population groups.
The tammar wallaby is among the smallest of the wallabies in the genus Notamacropus. Its coat colour is largely grey. The tammar wallaby has several notable adaptations, including the ability to retain energy while hopping, colour vision, and the ability to drink seawater. A nocturnal species, it spends the nighttime in grassland habitat and the daytime in shrubland. It is also very gregarious and has a seasonal, promiscuous mating pattern. A female tammar wallaby can nurse a joey in her pouch while keeping an embryo in her uterus. The tammar wallaby is a model species for research on marsupials, and on mammals in general. Its genome was sequenced in 2011.
Taxonomy and classification
The tammar wallaby was seen in the Houtman Abrolhos off Western Australia by survivors of the 1629 Batavia shipwreck, and recorded by François Pelsaert in his 1629 Ongeluckige Voyagie. It was first described in 1817 by the French naturalist Anselme Gaëtan Desmarest, who gave it the name eugenii based on a specimen found on an island then known as Ile Eugene in the Nuyts Archipelago off South Australia, which is now known as St. Peter Island. The island's French name was given in honour of Eugene Hamelin, caption of the ship Naturaliste; whose name is now the specific name of the tammar wallaby. The common name of the animal is derived from the thickets of the shrub locally known as tamma that sheltered it in Western Australia. It is also known as the dama wallaby or darma wallaby.The tammar wallaby is traditionally classified together with the kangaroos, wallaroos and several other species of wallaby in the genus Macropus, and in the subgenus Notamacropus with the other brush wallabies, all of which have a facial stripe. However, some authors have proposed elevating the three subgenera of Macropus, Macropus, Osphranter, and Notamacropus into distinct genera, making the tammar's specific name Notamacropus eugenii. This has been supported by genetic studies.
Fossil evidence of the tammar wallaby exists from the Late Pleistocene Eraremains were found in the Naracoorte Caves. The mainland and island-dwelling tammar wallabies split from each other 7,000–15,000 years ago, while the South Australian and Western Australian animals diverged around 50,000 years ago. The extirpated tammar wallabies on Flinders Island were greyer in colour with thinner skulls than present-day Kangaroo Island tammars, which are in turn larger than the East and West Wallabi Islands animals. The island tammar wallabies were once thought to be a separate species from the mainland population.
A 1991 examination of tammar wallaby skulls from different parts of the species' range found that the populations can be divided into three distinct groups: one group consisting of the populations from mainland Western Australia, East and West Wallabi Islands, Garden Island and Middle Island; a second group consisting of the populations from Flinders Island, 19th-century mainland Southern Australia and New Zealand; and a third group consisting of the population from Kangaroo Island. The Western Australia Department of Environment and Conservation listed these populations as the subspecies Macropus eugenii derbianus, M. e. eugenii and M. e. decres, respectively.
A 2017 study found many genetic differences between tammars from Western and South Australia and comparably little between the Kangaroo island and introduced New Zealand tammars. The researchers proposed dividing the species into two subspecies; the subspecific name eugenii for South Australian tammars and derbianus for those from Western Australia.
Characteristics and adaptations
One of the smallest wallaby species, the tammar wallaby features a proportionally small head with large ears, and an elongated tail, with a thick base. It has dark greyish upperparts with a paler underside and rufous-coloured sides and limbs. The tammar wallaby exhibits great sexual dimorphism, males reaching in weight compared to for females. Males are long while females are, while both sexes stand tall. The tail has a length of for males and for females.Locomotion
As with most macropods, the tammar wallaby moves around by hopping. This species typically leaps with 3.5 landings per second. Proximal muscles at the knee and hip joints provide the power for each leap, which shifts to the ankle muscles as the animal pushes off. As it lands, the energy of the jump is converted into strain energy made when its leg tendons are stretched. As it leaps back off the ground, the tammar wallaby can recover much of this energy for reuse through elastic recoil. When on the move, animal's respiration is tied to its hopping cycle, inhaling when leaping and exhaling when landing. As it moves faster, its heart rate increases nearly twice as much as its hopping frequency.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, and explains why tammar wallabies can increase their hopping speed without using more energy. The tammar wallaby shares this characteristic with other macropods that move on flat terrain, like the red kangaroo. By comparison, rock-wallabies, such as the yellow-footed rock-wallaby, have traded efficient energy-saving for greater tendon strength: an adaption for rocky cliffs which allows them to leap higher and lowers the risk of their tendons breaking.
Senses
The tammar wallaby can see at 324° peripheral vision and 50° binocular vision, which gives them a wide field view but still being about to see their hands in front of them. It can discern light gradients better than most other small mammals, such as rabbits. Its vision is, nevertheless, not as good as that of a cat or human. Tammar wallabies appear to have some colour vision: its eyes have only blue sensitive and green sensitive photoreceptor cones, allowing it to see colour in the blue-green band of the colour spectrum, but not the longer wavelengths of the red-yellow band. Nevertheless, in the band where it can see colour, it can differentiate between two monochromatic colours with wavelengths as close as apart.The pinna of the tammar wallaby is mobile, allowing it to track sounds from different parts of its surroundings without moving its head. A tammar wallaby can point its pinna at a sound source and increase its eardrum's sound pressure by 25–30 dB at 5kHz. When the pinna moves away from the sound source, the animal's hearing level quickly drops. When born, a tammar wallaby's sense of smell is already developed; this allows the newborn to find its mother's pouch by scent.
Thermoregulation and water balance
Tammar wallabies lick their forearms and pant to keep cool in hot weather. They breathe more heavily and lose more water when the temperature is over. Tammar wallabies cannot survive in temperatures above and must find cooler surroundings. To prevent dehydration, tammar wallabies urinate less and suck up water from the distal colon, which gives them relatively dry feces. Being able to concentrate more urine in their kidneys allows them to survive on seawater.Ecology and life history
During the day, tammar wallabies stay close to scrub for shade and move out to more open grassland by nightfall.In winter their home ranges are about, but in the dry summers they range further afield to search for quality food, needing about of space. Tammar wallaby home ranges overlap with those of conspecifics. Like all macropods, the tammar wallaby is herbivorous. They are known to both graze and browse, but the latter is less effective, as they commonly drop leaves when chewing on them. When eating large leaves, tammar wallabies handle them with their fingers. Tammar wallabies consume several plant species such as heart-leaved poison, small-flowered wallaby grass, and marri. They survive on several islands that have no fresh water, subsisting on seawater.Tammar wallabies gather into groups which lessens the chance of an individual being taken by a predator. As the group increases in size, tammar wallabies spend more time feeding, grooming, and interacting and less time being vigilant and moving around. They are also more likely to rest on their sides rather than in a more alert posture where their head is held up. Predators of the tammar wallaby include dingoes, feral cats, red foxes and wedge-tailed eagles. They may also have been preyed upon by the extinct thylacine. Tammar wallabies appear to respond more to the sight than the sound of predators. They can also use their acute sense of smell to detect a potential threat. When a predator is detected, a tammar wallaby will alert others by thumping its foot. When lost, young tammar wallabies are known to emit a distress call and adult females may respond with a similar call.
Breeding and development
The tammar wallaby has a polygynandrous mating system, were both males and females mate with multiple partners. It is a seasonal breeder and with many births taking place between late January and early February. During the breeding season, the male's prostate and bulbourethral gland enlarge while the weight of the testes remain the same. Around two weeks prior to the first births, the males start checking the reproductive status of the females by sniffing their urogenital openings and pouches. After giving birth, females enter estrus and allow males to mate with them. However, a male that attempts to mate with an estrous female may risk attacks from other males. 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. Several males may pursue a single female.The female tammar wallaby is receptive shortly after giving birth. Tammar wallabies undergo embryonic diapause and the blastocyst remains dormant for nearly a year. A joey in the pouch prevents the blastocyst from developing for the first six months and experiments have shown that removing the joey within this time period will stimulate the blastocyst's development. However, after this, the blastocyst remains dormant even after the joey has left. It begins to develop by the summer solstice at the end of December. A 2019 study found that more males are born due to a greater amount of Y chromosome sperm in sires. To balance out the sex ratios, tammar mothers are more likely to abandon male joeys and more females survive to weaning periods.
The lactation period of the tammar is divided into phases 2A, 2B, and 3. Phase 2A encompasses the first 100–120 days after birth, and the underdeveloped young is fed diluted milk which is richer in carbohydrates than proteins and lipids. This allows for the rapid growth of important organs and internal systems including the respiratory system, lymphoid system, and nervous system. During this phase, the young remains latched on to a teat. Phase 2B lasts for another 100 days; the young suckles intermittently but still does not leave the pouch. The composition of the milk is similar, though the proteins are different. During phase 3, the joey can leave the pouch and eat plant material. The joey will continue to suckle, the teat having enlarged and the milk having become richer in proteins and lipids over carbohydrates to give the joey more energy. During this time, the joey also experiences rapid development and transitions from ectothermy to endothermy. The joey no longer needs the pouch by 250 days and is fully weaned at 300–350 days. The tammar wallaby has been observed to engage in alloparental care, in which an adult may adopt another's young. Female tammar wallabies may mature at nine months and live to age fourteen, while males mature around two years and live for eleven years.