# Omilteme cottontail

The Omilteme cottontail (Sylvilagus insonus), or Omiltemi cottontail (Spanish: conejo de Omiltemi), is a species of cottontail rabbit in the family Leporidae found only in the Mexican state of Guerrero in the Sierra Madre del Sur mountain range. It is a nocturnal, large rabbit, rufous to black in fur colour, with long ears and a short tail.
The Omilteme cottontail is considered one of the most endangered mammal species in the world, and is only known from a few specimens. Once listed by the International Union for Conservation of Nature and Mexican authorities as critically endangered, it is now considered a data deficient species. It is threatened by poaching and habitat desctruction, and much about its natural history is unknown.

## Taxonomy and systematics

The Omilteme cottontail was first described from two specimens by Edward William Nelson in 1904 as a new species of Lepus, "Lepus insonus". The type of this species was a female specimen collected from Omilteme, Guerrero, Mexico, stored at the National Museum of Natural History. It was described as "[a] dark, coarse-haired species with small short tail" and noted as "obviously belonging in the same group as [Lepus] gabbi and L. truei", though it had ears that were twice as large as either species. Both L. gabbi and L. truei are now known as subspecies of the Central American tapeti (Sylvilagus gabbi), a cottontail rabbit of wide distribution and unstable taxonomy.
There is no fossil evidence of Sylvilagus insonus, and it is only known from its type locality in the province of Guerrero, Mexico. Phylogenetic analysis of the relationships between S. insonus and other species in the genus Sylvilagus yielded evidence that it is closely related to both the Mexican cottontail (S. cunicularius) and the desert cottontail (S. audubonii). The former species is sympatric with S. insonus. Prior studies indicated a relationship with the common tapeti (S. brasiliensis), and it has been variously placed in the subgenera Tapeti and Sylvilagus, but morphological studies find that the common tapeti is more closely related to Dice's cottontail (S. dicei) than it is to S. insonus.

## Characteristics

The Omilteme cottontail is a large rabbit (head and body length from 398 to 440 millimetres (15.7 to 17.3 in)) with long ears (60–76 millimetres (2.4–3.0 in)), hind feet of medium length (89–104 millimetres (3.5–4.1 in)) and a short tail (40–45 millimetres (1.6–1.8 in)). Around the nose and orbital (eye) area, its fur coat is a dull grey. The external, convex surface of the ears is a dark brown-black colour; the black is concentrated along the border and tips of the ears. The rabbit's back is rufous (a red-brown colour) mixed with black, while the sides are grey-black in colour. The medium-sized hind feet are white on the dorsal side and the soles are a dark brown. The dorsal side of the tail is reddish-black while the ventral side is darkly buff or yellowish.

### Anatomy

The anatomy of Sylvilagus insonus has been described with particular focus towards the skull. The skull is large, approximately 78 mm (3.1 in) in length and 32 mm (1.3 in) in depth, with a large palate and a wide braincase (back and upper part of the skull). The supraorbital process (projecting bone structure above the eyes) is flat, attached to the braincase and has two extensions (anterior and posterior). The anterior extension of the supraorbital process is attached to the skull, while the posterior extension is slender and can be free of the brain case or attached with a slit in between the process and the braincase. The supraoccipital shield (bony structure above the occiput) is square shaped. The tympanic bullae (bone structures that enclose the middle and inner ear, synonymous with but measured differently from the auditory bullae) are small and less than 12.3 mm (0.48 in) in length. They have medium-sized auditory bullae with a length of less than 9.6 mm (0.38 in). The width of the basioccipital is narrow: less than 9 mm (0.35 in), but broad across the carotid canals. The width of the infraorbital canals is very narrow, being less than 18.3 mm (0.72 in). The width across the nasal structures is very narrow, and their length is less than 32.1 mm (1.26 in).
The mouth consists of a mandible whose height is less than 36.3 mm (1.43 in), with a mandible ramus depth of less than 11.3 mm (0.44 in). The incisive foramen and the diastema are short. The premaxillaries have extensions on the dorsal side. Large maxillary and mandibular tooth rows are present. Like other leporids, the Omilteme cottontail is heterodont with a total of 28 teeth; it has incisors, premolars and molars, lacks canines, and its dental formula is 2.0.3.31.0.2.3. The length of the first upper incisor is generally less than 7.5 mm (0.30 in).

### Differences from other members of the genus Sylvilagus

Sylvilagus insonus differs from S. brasiliensis (forest rabbit) and S. dicei (Dice's cottontail) in that it has a larger skull, wider zygomatic bone, deeper rostrum, wider carotid foramina and dorsal extensions of the premaxillaries that extend posterior to the nasal instead. S. insonus also has a narrower basioccipital and narrower post-dental process. In external appearance, the Omilteme cottontail has a longer bicoloured tail (rufous and black) instead of a uni-coloured tail (solely brown); hind feet with white and brown versus hind feet of only brown; and longer ears. These characteristics are also useful in differentiating the species from the Mexican cottontail (S. cunicularius), with whom it shares its habitat. S. insonus is smaller in the length of its upper incisors, its skull length, nasal length, width of the basioccipital, auditory bulla length, the depth of shield bullae, skull depth, width across infraorbital canals, mandible height and mandible ramus depth. The Omilteme cottontail is a rufous-black colour dorsally, whereas the Mexican cottontail is only grey in the same areas.

## Geographic range and habitat

Sylvilagus insonus is endemic to Mexico and is found only in the Sierra Madre del Sur in the state of Guerrero. It is only known from its type locality, Omiltemi Ecological State Park, located in a wooded summit of a semi-isolated mountain range. It occurs in habitats ranging from 2,133 to 3,505 m (6,998 to 11,499 ft) in elevation. Surrounding the wooded area is the village of Omiltemi at 2,332 m (7,651 ft) above sea level (in Municipio Chilpancingo). The Omilteme cottontail is restricted to a region of less than 500 square kilometres (193 square miles).
The Omilteme cottontail lives at the summit of a steep-sloped mountain range with many ravines covered with dense cloud forests. Common tree genera in the forests it inhabits are pine (Pinus), oak (Quercus), and alder (Alnus).

## Behavior and ecology

Sylvilagus insonus shares its habitat with 37 other mammal species, including one cottontail rabbit, the Mexican cottontail. In the dense cloud forests, the rabbit lives amongst the undergrowth where it makes runways and burrows under rocks and other objects. It is a mainly nocturnal mammal. The only known predator of the species is humans through hunting, though its habitat is frequented by predators such as the wolf, jaguar, and cougar. Very little is known regarding its ecology and reproductive habits.

## Status and conservation

The International Union for Conservation of Nature (IUCN), in its Red List of Endangered Species, lists the Omilteme cottontail as "data deficient", though it previously listed the species as critically endangered in 1996 and 2004. A similar evaluation was given by the Mexican government around this time; as of 2019, the same authority describes it as "endangered". The major threats to the survival of this species are poaching and habitat destruction caused by deforestation. This rabbit went unreported in the wild from the early 1900s to the 1990s; however, two specimens were captured in 1998, confirming that the species was still extant. A team of scientists began searching for evidence of the species in 2019, with several specimens received in 2020 from local hunters and additional rabbits discovered from 2020 to 2022 in the Sierra Madre del Sur area within Guerrero, Mexico. In ten regions surveyed by camera traps, seven showed evidence of the species. The expedition was part of a larger effort to rediscover species without documented observations in at least 10 years. Prior to this expedition, the species was only known from five museum specimens and less than 10 total records.
Sylvilagus insonus is considered one of the most endangered mammals in the world, and though it is known to live within a protected area, it is still at risk. Conservation actions have yet to be implemented by local authorities and the National Commission of Natural Protected Areas, as additional studies are needed on the species' natural history to produce useful proposals. Efforts to study the species further have been stymied by the rough terrain of the Sierra Madre del Sur and social issues in the inhabited areas.