Romerolagus diazi is native to the Chichinautzin range of extinct volcanoes 200 miles south of Mexico City. Primarily, they live in a 280 sq. km region spread across the slopes of the mountains Pelado, Tialoc, Popocatepetl and Ixtaccíhuatl. Romerolagus diazi is endemic to the Chichinautzin Mountains.

Biogeographic Regions: nearctic (Native )

Romerolagus diazi has small, short hind legs and feet; small, rounded ears; and a vestigial tail. Dorsal and lateral fur is yellowish brown, and individual hairs are black at the tips and base, resulting in a grizzled appearance. The venter is buff or light grey. Like all members of the family Leporidae, it has large, well positioned eyes that give it a broad viewing range. It is considered the most primitive of extant leporids and is often described as the second smallest leporid behind Brachylagus idahoensis. Romerolagus diazi is sexually dimorphic, with males weighing on average 417 g and females, 536 g. Newborns are altricial and have closed eyes, laid-back ears, and extremely fine brown fur at birth. The vestigial tail is visible in newborns, but not in adults. Romerolagus diazi bears a striking resemblance to members of the family Ochotonidae, and its skull resembles that of Ochotonidae, as both lack an anterior bony projection above the eye socket.

Range mass: 386 to 602.5 g.

Range length: 234 to 321 mm.

Other Physical Features: endothermic ; homoiothermic; bilateral symmetry

Sexual Dimorphism: female larger

• Terrestrial

Romerolagus diazi lives on the upper slopes of an extinct volcanic range south of Mexico City, ranging from 2800 m to 4250 m, and an average elevation of 3252 m. Although it is near the equator and in the tropics, conditions are temperate as a result of high altitude and local weather patterns. Winters constitute the dry season, and summers are exceptionally rainy. Aside from the wet and dry seasons, conditions are relatively stable throughout the year, leaving a long growing season, with an average temperature of 9.6 C. Vegetation throughout consists of tall zacatón bunch grass under sparse pine and alder coverage. Romerolagus diazi relies heavily on these grasses for survival and evasion of predators. It can also be found in dense patches of secondary forest.

Range elevation: 2800 to 4250 m.

Average elevation: 3252 m.

Habitat Regions: temperate ; tropical ; terrestrial

Terrestrial Biomes: forest ; scrub forest ; mountains

Other Habitat Features: agricultural

Romerolagus diazi feeds primarily on zacaton grass, but also consumes young herbs and bark. During the summer rainy season, it sometimes feeds on cultivated plants. In captivity, R. diazi eats zacaton grasses provided in their enclosure, as well as other traditional rabbit foods, including high-protein chinchilla pellets, fruits, grasses, and other vegetable material. Young R. diazi begin eating solid food at 15 to 16 days after birth and are completely weaned by 3 weeks of age. Similar to other lagomorphs, R. diazi sometimes consumes their feces as a method of retaining as much nutrition and water as possible. Specific plant species eaten by R. diazi include aromatic mint plant, numerous species of zacaton grass (Festuca amplissima, Stipa ichu, Epicampes), two genera of spiny grass (Erynigium and Cyrsium), lady's mantle, and Museniopsis arguta.

Plant Foods: leaves; wood, bark, or stems; seeds, grains, and nuts; flowers

Other Foods: dung

Primary Diet: herbivore (Folivore ); coprophage

Little is known of the ecological role that Romerolagus diazi fills in its ecosystem. It is a folivore and may disperse seeds throughout its habitat. This species is prey for bobcats, long-tailed weasels, coyotes, red-tailed hawks and probably a number of other carnivorous mammals and birds. Romerolagus diazi is host to a number of endoparasites including roundworms (Boreostrongylus romerolagi, Thichostrongylus calcaratus, Longistrata dubia, Dermatoxys veligera), whipworms (Trichuris leporis), and flatworms (Anoplocephaloides romerolagi). It is also host to a number of ectoparasites, including various species of flies, ticks, and fleas.

Commensal/Parasitic Species:

• roundworms (Boreostrongylus romerolagi)
• roundworms (Thichostrongylus calcaratus)
• roundworms (Longistrata dubia)
• roundworms (Dermatoxys veligera)
• whipworms (Trichuris leporis)
• flatworms (Anoplocephaloides romerolagi)
• flies (Diptera)
• ticks (Acari)
• fleas (Siphonaptera)

Romerolagus diazi lacks the speed of many of its close relatives. Instead, it relies on finding cover in the grasses and rocks of its habitat. To protect their young, female volcano rabbits create burrows in and around patches of zacaton grass, digging slightly into the ground and reinforcing these burrows with the nearby grasses to offer both shelter and security. Romerolagus diazi has also been observed to make noise vocally when threatened. Major predators of this species include long-tailed weasels, bobcats, coyotes, and red-tailed hawks.

Known Predators:

• long-tailed weasel, (Mustela frenata)
• bobcat, (Lynx rufus)
• coyote, (Canis latrans)
• red-tailed hawk, (Buteo jamaicensis)

Anti-predator Adaptations: cryptic

Romerolagus diazi is the only member of family Leporidae that is known to vocalize, reacting to help their young and making noises themselves when startled, similar to pikas. They make two different types of calls: a short high-pitched bark, and a more subtle, slightly less audible squeak. They also communicate through thumping their hind feet on the ground. Reproductive status is communicated via scent glands located on the chin and groin.

Communication Channels: acoustic ; chemical

Other Communication Modes: pheromones ; scent marks

Perception Channels: visual ; acoustic

While there is no data on the lifespan of Romerolagus diazi, similar species have been observed to live less than a year in the wild. Some lagomorphs, however, may live up to 12 years in the wild.

Only captive Romerolagus diazi have been observed during mating. Thus, no data are available concerning mating systems of wild populations. It communicates with conspecifics via scent glands under the chin and in their groin, and scent glands likely play a significant role in mating and signaling social status to conspecifics. In captivity, R. diazi is serially monogamous (e.g., multiple pair bondings). Mate access is determined by social status, and only the dominant female and dominant male mate. If either individual dies, however, they are replaced by the highest ranking individual in the hierarchy.

Mating System: monogamous

Breeding occurs year round in Romerolagus diazi but peaks during spring. Females have induced ovulation and in captivity reach sexual maturity by 8 months old. Captive males reach sexual maturity by 5 months old. Gestation lasts for 38 to 40 days and results in 1 to 4 offspring per litter, which weigh about 80 g per kitten. Females can have 4 to 5 litters per year. Typically, offspring are weaned by 3 weeks of age.

Breeding interval: Romerolagus diazi can mate 4 to 5 times per year

Breeding season: Breeding in Romerolagus diazi occurs year-round, but peaks in spring and early summer

Range number of offspring: 1 to 4.

Average number of offspring: 4.

Range gestation period: 38 to 42 days.

Average gestation period: 39 days.

Average birth mass: 80 g.

Average weaning age: 3 weeks.

Average age at sexual or reproductive maturity (female): 8 months.

Average age at sexual or reproductive maturity (male): 5 months.

Key Reproductive Features: iteroparous ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); induced ovulation ; fertilization ; viviparous

Little is known of parental care in Romerolagus diazi in the wild. In captivity, mothers nurse semi-altricial young until weaning is complete at around 3 weeks of age. In the wild, R. diazi digs shallow holes in clumps of zacaton bunch grass, which hide nests and protect young. Nests consist primarily of vegetation fragments and fur. In captivity, R. diazi females avoid their nests unless young vocalize distress calls.

Parental Investment: altricial ; female parental care ; pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female)

• 1996
  Endangered
• 1994
  Endangered
  (Groombridge 1994)
• 1990
  Endangered
  (IUCN 1990)
• 1988
  Endangered
  (IUCN Conservation Monitoring Centre 1988)
• 1986
  Endangered
  (IUCN Conservation Monitoring Centre 1986)
• 1982
  Endangered
  (Thornback and Jenkins 1982)

Romerolagus diazi lives immediately south of Mexico City, one of the largest metropolitan areas in the world containing nearly 21 million inhabitants. As a result, while populations are listed as increasing by IUCN's Red List of Threatened Species, growth and sprawl of the city continues to threaten the habitat of R. diazi. In addition to urban sprawl, other major threats include habitat fragmentation and destruction due to wild fires and agriculture. Recently, R. diazi has increased, likely due to protective legislation focused on habitat preservation. Additionally, part of their range is within protected national parks. Currently, around 7000 individuals are estimated to exist in the wild.

US Federal List: endangered

CITES: appendix i

State of Michigan List: no special status

IUCN Red List of Threatened Species: endangered

Romerolagus diazi occasionally feeds on cultivated plants. If it were more abundant, it may have a significant negative affect on local agriculture.

There are no known positive effects of Romerolagus diazi on humans.