Okinawa Spiny Rat
(Tokudaia muenninki)
Resembling a large vole, the spiny rat has grooved spines protruding from its short, thick body fur. Very little is known about the species. It occurs in temperate forests and has an extremely limited distribution. The population of this little-known mammal is believed to be declining. Although individuals were captured in 1978, no specimens were recorded for a further 35 years despite efforts to capture it in 1994. The species was reportedly found to be still extant in 2008. However, it is thought to occur in an area of less than 3 square km, making it extremely vulnerable to localised threats. Loss of its forest habitat and possible predation by feral cats may spell extinction for the Okinawa spiny rat unless urgent conservation action is taken.
Urgent Conservation Actions
Habitat protection and management or control of introduced predators.
Okinawa, Japanese Ryukyu Islands.
Evolutionary Distinctiveness
Order: Rodentia
Family: Muridae
The family Muridae (rats, mice and gerbils) is the largest mammalian family, comprising some 730 species in 150 genera. The family dates back to the Oligocene (26-38 million years ago) of North America. It is thought to have undergone extensive adaptive radiation during the Miocene Epoch (26-7 million years ago). Murid rodents are a highly successful group. They tend to be adapted for early and prolific reproduction ('r-selected') rather than for long individual life spans. Members of the group are found throughout the world, in almost every terrestrial habitat.

Muennink’s spiny rat belongs to the subfamily Murinae (Old World rats and mice). This group includes 561 species, and is among the most successful of all mammals in terms of ability to survive, multiply and adapt quickly. The Murinae probably originated in southeast Asia in the late Oligocne or early Eocene (25-20 million years ago). The earliest fossils are known from Spain about 6-8 million years ago.

The genus Tokudaia is thought to be related to Margaretamys of Sulawesi or to the fossil Parapodemus. It exists as a relic on the Ryukyu archipelago, the most southwestern region of Japan. It contains three species: T. muenninki (Muennink’s spiny rat) on Okinawa Island, T. tokunoshimensis (Tokunoshima spiny rat) on Tokunoshima Island and T. osimensis (Ryukyu spiny rat) on Amami-ohshima Island.

Tokudaia is characterized by its possession of an extremely unusual chromosomal constitution for mammals: T. muenninki has 2n = 44 with an XX/XY sex chromosome system; T. tokunoshimensis has 2n = 45 with an X0/X0 sex chromosome system; and, T. osimensis has 2n = 25 with an X0/X0 sex chromosome system. The three species show differences in mtDNA and rDNA sequences, and their cranial characteristics and external measurements also vary.

T. muennicki is known from late Pleistocene or early Holocene fossils from Okinawa and nearby Le Island in the Ryukyus.
Head and body length: 120-175 mm
Tail length: 100-125 mm
Weight: 43.4 - 200 g
This species resembles a large vole. It has a short thick body and dense fur, consisting of fine hairs and coarse, grooved spines (hence the common name “spiny rat”). The fur is brownish above and greyish white below with a faint orange tinge. The spines on the animal’s back are black throughout while the spines underneath are usually white with a rufous tip. The spines cover the body except for the regions around the mouth, ears, feet and tail. The tail is bicoloured for its entire length.
Very little is known about the ecology of this species.
The species inhabits forest (chinquapins) over 30 years old in the northern part of the island. It prefers forests with high undergrowth, and has also been found in chinquapin forests surrounded by sugarcane fields.
This species is endemic to Okinawa Island, Ryukyu Islands, Japan. It is found only on the northern part (Yanbaru area) of the island, above 300 m asl. Recent research suggests that it may be restricted to an area of less than 3 square km.
Population Estimate
Population density estimates were of 1.8/ha in 1978 and the population is currently considered to be severely declining.
Population Trend
Classified as Critically Endangered (CR B1ab(iii,v)) on the 2010 IUCN Red List of Threatened Species.
The main threat is from deforestation, mainly through government forestry programs. Feral cats in the forest also predate this species, and this may also be contributing to population declines.
Conservation Underway
This species is legally protected, although there do not appear to be any specific conservation measures in place.
Conservation Proposed
Full protection of the Yanbaru region of Okinawa Island is recommended, along with management or control of introduced predators. Ongoing monitoring of the small, recently discovered population in the north of the island, together with further research into the ecology and distribution of the species will help to inform targeted conservation action for the Okinawa spiny rat. Awareness and education programmes would also be beneficial.
Ishii, N. & Kaneko, Y. 2008. Tokudaia muenninki. In: IUCN 2010. IUCN Red List of Threatened Species. Version 2010.4. <www.iucnredlist.org>. Downloaded on 05 November 2010.

Nowak, R.M. 1999. Walker's Mammals of the World. Sixth edition. The Johns Hopkins University Press, Baltimore and London.

Yamada, F., Kawauchi, N., Nakata, K., Abe, S., Kotaka, N., Takashima, A., Murata, C. and Kuroiwa, A. 2010. Rediscovery after thirty years since the last capture of the critically endangered Okinawa spiny rat Tokudaia muenninki in the northern part of Okinawa Island. Mammal Study 35: 243-255.

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