Octodontidae and genus Octodon

Octodons, whatever their species, are part of the Octodontidae family. It is a group of rodents, native to South America and located in Chile, Bolivia, Argentina and Peru. This family includes 8 genus⁰¹, 13 species (O. ricardojeda is not taken into account for the moment) and 3 subspecies⁰². Octodons are part of the genus Octodon, and are composed of 5 species⁰³, including one discovered in 2020⁰⁴.

Octodontidae

Taxonomy

Kingdom	Animalia
Phylum	Chordata
Class	Mammalia
Order	Rodentia (rongeur)
Suborder	Hystricomorpha
Infraorder	Hystricognathi
Familly	Octodontidae
Genus (8)	Aconaemys Octodon Octodontomys Octomys Pipanacoctomys Salinoctomys Spalacopus Tympanoctomys
Species (13)	Aconaemys fuscus Aconaemys porteri Aconaemys sagei Octodon degus Octodon bridgesi Octodon lunatus Octodon pacificus Octodon ricardojeda Octodontomys gliroides Octomys mimax Pipanacoctomys aureus Salinoctomys loschalchalerosorum Spalacopus cyanus Tympanoctomys barrerae
Subspecies (3)	Spalacopus cyanus cyanus Spalacopus cyanus maulinus Spalacopus cyanus poeppigii

Common characteristics of Octodontidae

Octodontids are small rodents, which look like rats or mice⁰⁵. They measure up to more than 20cm long and weigh in the 300g. Their tail is often long, and sometimes with a bushy “feather” depending on the species. These feathers tend to be amputated during encounters with predators. Octodontidae have a large head, a pointed nose and rather large and rounded ears. They have large vibrissae, which they use to complement their eyesight. Their legs are relatively short, with front legs with 4 toes and upper legs with 5 toes and with pointed and curved claws. One of the characteristics of octodontidae is their fur which can be quite long. This coat is dense and silky, and has a developed undercoat. Shorter hairs cover their tails, even forming a feather duster in some species. They often have long, stiff hairs that extend beyond the toes of the hind legs. Finally, with the exception of one species, most of them are grayish/brown and have lighter sides and bellies. Only Spalacopus has an almost entirely black fur.

All members of the family are very good burrowers, and live in burrows. But we can notice that some spend more time underground, like Aconaemys and Spalacopus. Most of them live in colonies and show a complex social behavior, as for the octodon. All are mainly herbivorous. Octodons have become “agricultural pests” in some areas, as they can be very destructive. Octodontidae are mainly nocturnal, with some diurnal species.

Presentation of genera and species

Habitat

The family of rodents Octodontidae, includes 8 genera of herbivorous rodents, which present many different behaviors. Present in South America, some members of this family cohabit in the same geographical areas. Octomys (Thomas 1920) is a surface rodent, while Octodontomys (Palmer 1903), Octodon (Bennett 1832) and Tympanoctomys (Yepes 1942) are burrowers. Aconaemys (Ameghino 1891) is semi-subterranean, while Spalacopus (Wagler 1832) is rather subterranean and stays most of the time in its burrow⁰⁶.

Geographical distribution

Octomys and Tympanoctomys inhabit the arid and semi-arid areas of central-western Argentina. Spalacopus is endemic to the scrubland areas of central Chile. Octodons and Aconaemys are found in the grasslands and forests of southwestern Argentina, central and southern Chile. Finally, Octodontomys inhabits open areas of northwestern Argentina, southwestern Bolivia and northern Chile⁰⁶.

Emergence of Octodontidae

We can notice that this family has only few species, 13 of which three Aconaemus and four octodons (without counting O. ricardojeda). All the other genera have only one species. Only the Scalapocus have subspecies, 3 in number.

The evolution of the family Octodontidae, through diversification⁰⁷ of species occurred at the end of the Miocene, about 8.8 million years ago⁰⁸, during the Tortonian period⁰⁹.

Genus Octodon

Etymology

Known under other names, such as the dègue, and named almost exclusively degu in English (even for the other species), the octodon owes its name to the shape of its teeth.  Indeed its premolars and molars are naturally in the shape of “8”. Etymologically, the generic term Octodon derives from the Greek language where : “okto” means “eight” and “odous”, “odontos” is the “tooth”¹⁰.

Taxonomie

Kingdom	Animalia
Phylum	Chordata
Class	Mammalia
Order	Rodentia (rongeur)
Suborder	Hystricomorpha
Infraorder	Hystricognathi
Familly	Octodontidae
Genus	Octodon
Species	Octodon degus Octodon bridgesi Octodon lunatus Octodon pacificus Octodon ricardojeda

History

The first species of Octodontidae described was O. degus, with a type locality in Santiago, central Chile, by Molina in 1782¹¹. It is the only species of the genus that has several names available: Octodon clivorum by Thomas in 1927¹², Octodon cumingii by Bennett in 1832, O. getulinus by Poeppig in 1829, Octodon pallidus by Wagner in 1845 and Octodon peruana by Tschudi in 1845. But no subspecies is currently recognized ^{13 14}, even if two subspecies were described ¹² by Thomas, in 1927, Octodon degus degus and Octodon degus clivorum ¹³. 

It was not until the middle of the 19th century, in 1945, that Octodon bridgesi (or bridgesii) was described, by Waterhouse¹⁵. Its territory was defined by Thomas in 1927, in the center of Chile, with a type locality in Río Teno, Colchagua¹². It was not until 100 years later, in 1943, that the Octodon lunatus was described for the first time by Osgood¹⁶. He defined its type locality in Olmue, Province of Valparaiso. However, this new species of octodon was questioned by Tamayo and Frassinetti in the 1980s¹⁷, because it physically resembles O. bridgesi. However, lunatus and bridgesi have distinct DNA characteristics¹⁸. It was not until 1994 that Octodon pacificus was described and named by Hutterer, which would live only on the island Mocha ¹⁹.

Octodon species

From the end of the XXth century, the octodon is no longer considered as an endemic species in Chile ^{20 12} It is found in Peru ²¹, but especially in Argentina, thanks to archaeological research ^{22 23}. Living populations, as well as traces found via birds of prey ²⁴ show that these O. bridgesi are present in several places in Argentina ²⁵. It is now recognized that there are populations north of Neuquén, Argentina. Questions were already raised 20 years ago about the species of octodon, when the species pacificus was discovered. In his publication, Hutterer ¹⁹ already questioned the Argentine populations of bridgesi, with the possibility that this one was another species. From gene analysis, Octodon pacificus has a specific distinction from O. bridgesi, although they have very similar genes ²⁶).

Towards a revision of the taxonomy?

With the discovery of a new species of octodon, Octodon ricardojeda²⁸), and in view of the numerous studies on this subject, it is important to conduct research on octodon species. Even without considering O. ricardojeda, it appears that Octodon bridgesi currently encompasses more than one species²⁹. Some populations of O. brigesi, from the BioBio and Maule coastal areas are genetically and morphologically different from other populations of bridgesi. Scientists who have described Octodon ricardojeda, consider this variation as coastal, hence the name coastal Octodon bridgesi. Thus, this coastal form cannot be assigned to O. lunatus, its sister species. The two species are genetically and morphologically distinct. In addition, the two species have different chromosomes.

Studying Octodontidae in a global way, as well as doing studies with a large number of individuals would be relevant to (re)define the species of the genus Octodon. With a limited approach, it is still likely to discover one or more new species in this family. It is by integrating morphological and genetic variation within and between populations that reliable results could be obtained.

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