The mountain degu (Octodontomys gliroides), also known as the Andean degu, is a species of rodent in the family Octodontidae. It is the only species in the genusOctodontomys. It is a medium-sized rodent with long, silky grey-brown fur, large grey ears, and white-furred chin and underparts. It is found in the foothills of the Andes in Argentina, Bolivia and Chile, and closely resembles the mountain viscacha rat and viscacha rats in the genus Tympanoctomys, all of which are found in Argentina. It is an herbivore, and eats grasses, leaves, bark, and cacti. The mountain degu gets most of its water from eating the tissues of prickly pear cactuses and cactuses in the genus Cereus.
The mountain degu is a social animal, though not as much as the related common degu, and lives in family units of two to four individuals that share a burrow, play together, and take dust baths in turn with each other. Mountain degus have no special ability to dig with their claws, and their burrows are often short. They often make their homes in caves and under rocks. Members of a social group communicate through gurgles, twitters and squeaks. The mountain degu's closest relatives are the rodents in the genera Octodon, Spalacopus, and Aconaemys.
The mountain degu was first described in 1844 as a member of the genus Octodon, having been given the scientific nameOctodon gliroides by the French naturalists Paul Gervais and Alcide d'Orbigny. The species name, gliroides, was given due to the animal's similar appearance to that of a dormouse (gliris in Latin).[3] The etymology of the name gliroides comes from a combination of the Latin gliris, 'dormouse' with the Greek suffixes -o and eidos, which indicate "likeness of form".[4] The type locality where it was first identified is the Andes in La Paz, Bolivia.[2]
Octodontomys, a genus created in 1903 by American zoologist Theodore Sherman Palmer, comes from the Ancient Greekὀκτώ (oktṓ), meaning 'eight', ὀδούς (odoús), meaning 'tooth', and μῦς (mûs), meaning 'mouse'.[4] The genus was created to replace the earlier-used name Neoctodon, which was already in use as a genus of beetles from an 1892 work by entomologist Ernest Marie Louis Bedel.[5] The British zoologist Oldfield Thomas had used the name Neoctodon in his 1901 description of the mountain degu (as the new species Neoctodon simonsi, named in honor of the collector Perry O. Simons),[6] which he corrected to the currently used name, Octodontomys gliroides, in his 1913 article On small mammals collected in Jujuy by Señor E. Budin.[7] Thomas noted that the species was "strikingly different" from members of Octodon, and differentiated it based on its "paler colour, whiter belly, and longer, more bushy tail".[6]
The common name "degu" is first recorded from the period between 1835 and 1845, and originates from the Mapuche word deuñ[8] or dewit. Over time, the word changed through its use in Chilean Spanish. Recorded common names for the species include long-tailed octodon, brush-tailed vizcacha rat,[9] and Andean degu.[10]
Phylogeny
The phylogeny and placement of O. gliroides in relation to other octodontids was originally unclear.[11] It was placed as the sole member of Octodontomys in 2005,[12] and a study of mitochondrial DNA across the species' range published in 2016 solidified Octodontomys as a monophyletic group, albeit one with two distinct lineages, one of which is distributed across Bolivia and northern Chile, the other restricted to Argentina.[13] These two lineages diverged roughly 0.69million years ago.[14] Compared to other rodents worldwide, Octodontomys has some similarities to the monophyletic dassie rat.[15] The mountain degu has a diploid chromosome number of 38 and a fundamental number of 64.[16] The 28S and 18S ribosomal RNA of the mountain degu have notably split or eliminated introns, a genetic feature shared only with the tuco-tucos.[17]
The following cladogram is adapted from work done by Rivera and colleagues in 2014.[18] The genus Octodontomys is considered, as of 2018, to be the most primitive among the octodontids. It is thought to have diverged from its sister clade, which includes the genera Octodon, Spalacopus, and Aconaemys, at a point roughly 5to7 million years ago. No fossils of O. gliroides are known, and all estimates of species divergence times are made based on genetic analyses.[19]
The mountain degu is a rodent of medium size with an adult length of 200 to 380mm (8 to 15in), including a tail of 100 to 190mm (4 to 7in), and a body weight in the range of 100 to 200 grams (4 to 7oz). Its hairs are long and silky. The upper surface is greyish-brown, the chin is pure white, and the underparts are white with a grey base to the hairs. There is a tuft of white hair in front of the large ears, which are clad in short grey hair. The slender tail is bi-coloured (dark above and pale below) and has a tuft of brown or ochre hairs at the tip. Juveniles have darker fur above and greyer underparts. Their tails are brownish with a dark brown or black tufted tip.[20] It is similar in appearance to other rodents in the octodontid family, bearing the closest resemblance to the mountain viscacha rat (Octomys mimax) and the viscacha rats in the genus Tympanoctomys. The mountain degu can be reliably distinguished from these rodents in that its molars are asymmetrical when occluded, while other octodontids display more of a fiugre-8 pattern.[21]
Mountain degus have a dental formula of 1.0.1.31.0.1.3 × 2 = 20, with two incisors, no canines, two premolars, and six molars on each side of their jaw for a total of 20 teeth.[19] The mountain degu has some distinctive dental and skeletal traits, as its molariform (premolar and molar) teeth lack as many folds as are found in other octodontids. Additionally, the first molariform tooth is inclined backwards near the socket (in the alveolar portion). The mountain degu's auditory bullae (bone structures that enclose part of the ear canal) are on average 14.6mm (0.57in) in diameter, being of medium size and slightly smaller than those found in other octodontids adapted to arid climates.[21] The incisors constantly grow throughout the mountain degu's life and are hypsodont,[22] adapted to consuming plenty of cellulose.[23]
Distribution and habitat
This species is found in the mountainous areas of southwestern Bolivia, northwestern Argentina and northern Chile.[24] It has a wide altitudinal range, occurring at 200 to 300 metres (660 to 980ft) in Jujuy Province in Argentina, and up to 4,400 metres (14,400ft) in Potosí Department in Bolivia.[20]
As the only species of octodontid found in the higher parts of its range, it inhabits dry rocky areas with tall cacti, shrubs and herbs.[20] It is found in both the Atacama Desert of Chile and the Puna grassland habitats of Bolivia,[10] and is especially well suited for xeric habitats within and east of the Andes mountain range.[16] Populations of mountain degus are separated from each other by natural features such as unvegetated salt flats, rivers, mountains, and floodplains, with the major barriers between Argentinian populations and the populations found in Bolivia and Chile being the San Juan River and high mountain ranges at the Chile-Argentina border.[25]
The mountain degu is a nocturnal,[10]herbivorous rodent, being most active just after sunset.[26] It does not have any special adaptations of skull or limbs for tunnelling, but digs short burrows and lives under rocks and in caves. Its tail can be autotomised, and it can be used as a prop when climbing. It is a sociable animal and communicates by means of a range of low, medium and high-pitched gurgles, twitters and squeaks,[20] similar to the common degu and various species in genus Cavia.[27] Social groups of the mountain degu range from 2 to 4 individuals that share a burrow system together to rest and hide from predators. Female mountain degus have a smaller home range than that of males.[28] The mountain degu is notably less social than the common degu. Family groups engage in play shortly after waking from sleep, with individuals locking incisors with and riding on each other.[29] Like other octodontids,[20] the mountain degu takes dust baths, with each member of a family unit taking turns at the same location.[23]
The mountain degu feeds on the leaves and bark of shrubs, Acacia sheaths, and cactus fruits. It can obtain sufficient moisture from its food, particularly cactus, to satisfy its water requirements.[20] Much of the degu's water comes from the tissues of cacti in the genera Cereus and Opuntia. Cacti are largely preferred during the winter, and in the summer, mountain degus are found to feed upon a mixture of plants that undergo crassulacean acid metabolism and cool season grasses.[23]
Mountain degu young in various stages of development have been found in November, and pregnant females and young in both January and May. A gestation period of 100–109 days has been reported with a litter size of one to three pups. The pups are precocial when born, already having their eyes open and being well-furred.[26]
Relationship with humans
The mountain degu hosts parasites that are infectious to humans, such as the Chagas disease-causing Trypanosoma cruzi. It is also parasitized by bloodsucking fleas in genus Ectinorus and lice in the genera Ferrisella. Otherwise, it has little to no societal impact, similar to other degus.[30]
Conservation
The mountain degu is common within its distribution[31] and has no known threats. The International Union for Conservation of Nature considers it to be a least-concern species because it is widely distributed, occurs in some protected areas, and can tolerate changes in its habitat. The mountain degu population is not declining at an appreciable rate, and there are no conservation actions being taken towards it.[1]
Ojeda, A.A. (2016). "Family Octodontidae (Viscacha Rats, Degus, Rock Rats and Coruro)". In Wilson, D.E.; Lacher, T.E.; Mittermeier, R.A. (eds.). Handbook of the Mammals of the World. Vol.6. Lagomorphs and Rodents I. Barcelona: Lynx Edicions. pp.536–551. ISBN978-84-941892-3-4.
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