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Oryzomys albigularisTomes's rice rat

By Caitlyn Gibson

Geographic Range

Oryzomys albigularis, also known as the Tome's Rice Rat, Montane Rice rat, and Nephelomys albigularis, is distributed in the mountains of Costa Rica and Panama in Central America. It is also common in the Andes Mountains in Colombia, ranging east through northern Venezuela and southwest through Ecuador, Peru and northwestern Bolivia. (Eisenberg, 1989; Reid, et al., 2010; Reid, 1997)

Habitat

Oryzomys albigularis lives in high elevation forests within 1,000 and 3,000 m. It is usually found in wet environments, such as mountain top cloud forests or in forests adjacent to streams with high flow rates. This species is strictly terricolous and lives inside and around fallen trees and moss covered rock formations. Oryzomys albigularis are adequate swimmers and use streams to escape predators by diving into fast moving waters and swimming to the opposite bank. (Eisenberg, 1989; Reid, et al., 2010; Reid, 1997)

  • Range elevation
    1,000 to 3,000 m
    to ft

Physical Description

Oryzomys albigularis is a large rat-like rodent with a head and body length ranging between 132 and 174 mm and an average mass of 61.9 g. It has a naked, non-penicillated tail that is usually equal to or slightly shorter than its body length. Like other Oryzomys species, it has a long and pointed snout, and its molars are mainly penta-lophodont. Oryzomyines are distinguished form other genera by the length of their lower second molar, which is twice the size of their lower third molar. Oryzomys albigularis has large feet, with an average length of 31.8 mm, making their hind foot length almost 22% of their head and body length. Finally, members of Oryzomys have a unique toe configuration, as the fifth toe is shorter than the fourth and is used to distinguish them from members of Isthmomys. (Eisenberg, 1989; Moscarella, et al., 2001; Reid, 1997)

Tome’s rice rats have long, thick pelage that covers their entire body, including their hind feet. Fur color is slightly variable, ranging from dark reddish-brown to blackish-brown. Pelage along the sides of the body is light in color, and venter pelage is white to off-white in color. Their ears are scantly covered with blackish hair. Although males and females are similar in appearance, males are slightly larger than females. (Eisenberg, 1989; Moscarella, et al., 2001; Reid, 1997)

  • Sexual Dimorphism
  • male larger
  • Average mass
    61.9 g
    2.18 oz
  • Range length
    132 to 174 mm
    5.20 to 6.85 in

Reproduction

Based on lab studies, Oryzomys albigularis is thought to be monogamous; however, this has not yet been confirmed in the wild. A lack of strong sexual dimorphism and similar developmental patterns are also thought to suggest monogamy. Finally, O. albigularis displays biparental care, a behavioral characteristic common in monogamous species. (Moscarella, et al., 2001)

Breeding season and general mating behavior of Tome’s rice rats in the wild have not been documented. In captivity, however, females exhibit post-partum estrous, and mating occurs frequently throughout the year. Gestation lasts for 26.7 days on average, and litter size ranges between 3 and 5 pups, with an average of 3 pups per litter. On average, pups weigh 4.89 g at birth. Oryzomys albigularis has smaller litters, lactates longer, and has a slower growth rate than other sigmodontine rodents. Pups nurse for approximately 22 days and are about a third of their adult weight when weaning is complete. Most males reach sexual maturity by 47 days old, and most females reach sexual maturity by 52 days old. In laboratory settings, Oryzomys albigularis exhibits biparental care, which is common in monogamous species but not other rodents. Males help provide warmth and protection to offspring and even groom and carry young back into the nest after handling by researchers. (Moscarella T. and Aguilera M., 1999; Moscarella, et al., 2001)

  • Range number of offspring
    3 to 5
  • Average number of offspring
    3
  • Average gestation period
    26.7 days
  • Average weaning age
    22.1 days
  • Range age at sexual or reproductive maturity (female)
    26 to 93 days
  • Average age at sexual or reproductive maturity (female)
    51.94 days
  • Range age at sexual or reproductive maturity (male)
    32 to 64 days
  • Average age at sexual or reproductive maturity (male)
    46.81 days

There is no information available regarding parental care of Oryzomys albigularis in the wild. In captivity, this species exhibits biparental care, which is common among monogamous species but uncommon among rodents. Males help to provide warmth and protection to offspring and even groom and carry young back into the nest after handling. Weaning is generally complete by 22 days after parturition. There is no information available regarding the average time to independence in this species. (Moscarella T. and Aguilera M., 1999; Moscarella, et al., 2001)

Lifespan/Longevity

Lifespan of Oryzomys albigularis in the wild is unknown, and research under laboratory conditions is limited. Most male and female rats lived 100 days and none live to be older than 150 days. (Moscarella, et al., 2001)

  • Average lifespan
    Status: captivity
    100 days
  • Average lifespan
    Status: captivity
    100 days

Behavior

Like other rice rats, Oryzomys albigularis is highly mobile and social. It is active during the night and day, and spends most of its time foraging for food or building nests. It is a capable swimmer and uses nearby waterways to escape potential predators. (Nowak, 1999)

Home Range

There is no information available regarding home range size in Oryzomys albigularis; however, closely related species have home ranges from 0.33 ha to 1.33 ha. (Nowak, 1999)

Communication and Perception

There is no information available regarding communication and perception in Oryzomys albigularis. However, like most other rodents, it probably uses a combination of vision, hearing, taste and smell to perceive the immediate environment. In the lab studies, an extended period of sniffing and exploratory behaviors were recorded when new individuals were introduced to each other, suggesting that olfaction and touch play an important role in perception. Other sigmodontine rodents communicate intraspecifically via high pitched squeaks. (Moscarella T. and Aguilera M., 1999; Nowak, 1999)

Food Habits

Oryzomys albigularis is primarily herbivores, but occasionally consumes small insects. Primary forage includes grasses and some roots, and an abundance of fruits and seeds. In captivity, a base diet of normal protein rich animal chow was supplemented with fresh fruits, vegetables, and seeds. (Moscarella T. and Aguilera M., 1999; Nowak, 1999)

  • Animal Foods
  • insects
  • Plant Foods
  • wood, bark, or stems
  • seeds, grains, and nuts
  • fruit

Predation

There is no information available regarding potential predators of Oryzomys albigularis. It is likely that numerous predatory vertebrates, such as birds of prey (e.g., Falconiformes and Strigiformes), prey upon this species. (Nowak, 1999)

Ecosystem Roles

As seed predators, Oryzomys albigularis is likely an important seed disperser throughout its geographic range. They probably help maintain the balance of grasses and smaller plants through frequent foraging as well. They also serve as the prey for lager to medium-sized carnivores throughout their geographic range in Central and South America.

Oryzomys albigularis is host for a range of arthropod ectoparasites, including types of fleas, ticks and mites. Extensive research has been done on the relationship between Oryzomys albigularis and the beetle species, Amblyopinus emarginatus and Amblyopinus tiptonis. Though it has been previously reported that these beetles are parasites of O. albigularis, recent evidence suggests that they have a commensal and possibly even mutualistic relationship. The beetles were only found on the rats during the night and do not appear to harm O. albigularis. Although the two beetle species are relatively large, Oryzomys albigularis showed no signs of even noticing the beetles' presence, suggesting coevolution with the beetles. There is strong evidence that both beetle species feed on mites and ticks found in the nesting area of the rats. Oryzomys albigularis is known to be parasitized by mites in the genus Gigantolaelaps. (Ashe and Timm, 1987a; Ashe and Timm, 1987b)

  • Ecosystem Impact
  • disperses seeds
Commensal/Parasitic Species
  • mites (Gigantolaelaps)
  • beetle (Amblyopinus emarginatus)
  • beetle (Amblyopinus tiptonis)

Economic Importance for Humans: Positive

There are no known positive effects of Oryzomys albigularis on humans. Encounters with people are limited since it is common only to high elevation forests in Central and South America. (Nowak, 1999)

Economic Importance for Humans: Negative

Although human contact with Oryzomys albigularis is extremely limited, this species may serve as a vector for a variety of zoonotic diseases. Oryzomys albigularis is host to a number of ectoparasites, including mites, fleas, and ticks that may harbor diseases that can be transmitted to humans or domestic animals. For example, Oryzomys albigularis is a known vector for the Pichinde virus in Colombia. This virus is highly infectious and fatal to rodents and has been spread to humans who have had close contact with this species. The Pinchide virus is asymptomatic in humans and successful treatment of this pathogen has not yet been determined. (Trapido, 1971)

  • Negative Impacts
  • injures humans
    • carries human disease

Conservation Status

Oryzomys albigularis is classified as a species of least concern on the IUCN's Red List of Threatened Species. It appears that little is known of this species populations trends in the wild, however, they are thought to be common throughout their geographic range and may be slowly declining. Although there are no major threats to the long-term persistence of this species, deforestation may prove detrimental in the future. (Reid, et al., 2010)

Other Comments

Oryzomys albigularis is also known as Nephelomys albigularis and was recently assigned to a new genus, Nephelomys. (Weksler, 2006)

Contributors

Caitlyn Gibson (author), The College of New Jersey, Matthew Wund (editor), The College of New Jersey, John Berini (editor), Animal Diversity Web Staff.

Glossary

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

diurnal
  1. active during the day, 2. lasting for one day.
endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

female parental care

parental care is carried out by females

fertilization

union of egg and spermatozoan

folivore

an animal that mainly eats leaves.

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

granivore

an animal that mainly eats seeds

herbivore

An animal that eats mainly plants or parts of plants.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

male parental care

parental care is carried out by males

monogamous

Having one mate at a time.

motile

having the capacity to move from one place to another.

mountains

This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

tactile

uses touch to communicate

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

vibrations

movements of a hard surface that are produced by animals as signals to others

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

Ashe, J., R. Timm. 1987. Predation by and activity patterns of 'parasitic' beetles of the genus Amblyopinus (Coleoptera: Staphylinidae. Journal of Zoology, 212/3: 429-437. Accessed February 23, 2011 at http://onlinelibrary.wiley.com/doi/10.1111/j.1469-7998.1987.tb02914.x/abstract.

Ashe, J., R. Timm. 1987. Probable Mutualistic Association between Staphylinid Beetles (Amblyopinus) and Their Rodent Hosts. Journal of Tropical Ecology, 3/2: 177-181. Accessed February 23, 2011 at http://www.jstor.org/stable/2559795.

Eisenberg, J. 1989. Mammals of The Neotropics: The Northern Tropics, Vol. 1. Chicago and London: The University of Chicago Press.

Moscarella T., R., M. Aguilera M.. 1999. Growth and reproduction of Oryzomys albigularis (Rodentia: Sigmodontinae) under laboratory conditions. Mammalia, 63/3: 349-362. Accessed February 23, 2011 at http://www.reference-global.com/doi/abs/10.1515/mamm.1999.63.3.349.

Moscarella, ., M. Benado, M. Aguilera. 2001. A Comparative Assessment of Growth Curves as Estimators of Male and Female Ontogeny in Oryzomys albigularis. Journal of Mammology, 82/2: 520-526. Accessed February 23, 2011 at http://www.jstor.org/stable/1383731.

Nowak, R. 1999. Walker's Mamals of the World. Balitmore: Johns Hopkins Univ. Press..

Reid, F., R. Samudio, D. Tirira, M. Weksler, R. Anderson, B. Rivas, C. Delgado, M. Gomez-Laverde. 2010. "ICUN Red List of Threatened Species" (On-line). Nephelomys albigularis. Accessed February 23, 2011 at http://www.iucnredlist.org/apps/redlist/details/15584/0.

Reid, F. 1997. A Field Guide to the Mammals of Central America and Sountheast Mexico. New York and Oxford: Oxford University Press.

Trapido, H. 1971. Pichinde virus: a New virus of the Tacaribe group from Colombia. The American Journal of Tropical Medicine and Hygiene, 20/4: 631. Accessed February 23, 2011 at http://www.ajtmh.org/cgi/content/abstract/20/4/631.

Weksler, M. 2006. Phylogenetic relationships of oryzomine rodents (Muroidea, Sigmodontinae) : separate and combined analyses of morphological and molecular data.. Bulletin of the American Museum of Natural History, 296: 1-149. Accessed February 24, 2011 at http://digitallibrary.amnh.org/dspace/bitstream/2246/5777/1/B296.pdf.

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