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Oryzomys galapagoensisGalapagos rice rat

By Coral Wolf

Geographic Range

Oryzomys galapagoensis, also known as the Galápagos rice rat, is endemic to the Galápagos Archipelago, Ecuador. Subspecies O. galapagoensis galapagoensis was endemic to the island of San Cristóbal (area: 558 square kilometers; maximum altitude: 730 m) until its extinction sometime after 1835. Subspecies O. galapagoensis bauri inhabits Santa Fé Island (area: 24 square kilometers; maximum altitude: 259 m). San Cristóbal and Santa Fé Islands are part of the Galápagos National Park. Santa Fé Island is uninhabited by humans. (Swash and Still, 2000)

Habitat

Overall, the Galápagos climate is relatively dry and composed of two distinct seasons. The warm, wet season lasts from December to May. During this time, the monthly average maximum temperature ranges between 25 to 30 degrees Celsius, and the monthly average rainfall ranges between 40 and 80 mm. The dry season occurs from May to December. During this time, the monthly average maximum temperature ranges between 22 and 25 degrees Celsius, and the monthly average rainfall ranges between 0 and 50 mm. (Swash and Still, 2000)

The only island where Oryzomys galapagoensis is currently found is Santa Fé. Santa Fé Island is uninhabited and characterized by desert-type flora. Santa Fé Island has an area of 24 square kilometers and a maximum altitude of 259 m. Oryzomys galapagoensis is common throughout the island of Santa Fé, especially in the arid zone, which is characterized by cacti and deciduous trees. The population density of O. galapagoensis is greater where vegetation density is greater. (Clark, 1980; Swash and Still, 2000)

  • Range elevation
    0 to 259 m
    0.00 to 849.74 ft

Physical Description

Oryzomys galapagoensis is a relatively small, brown rat. It has medium brown pelage with a light underbelly. Oryzomys galapagoensis is sexually dimorphic with the males both heavier (male weight: 74 g vs. female weight: 55 g) and longer (male body length: 118 mm vs. female body length: 108 mm) than females. Oryzomys galapagoensis individuals have large black eyes, pointed noses, and large, almost bare ears. Their thin tails are equivalent in length to their body length and have little hair. (Clark, 1980; Swash and Still, 2000)

Subspecies Oryzomys galapagoensis bauri is likely physically indistinguishable from Oryzomys galapagoensis galapagoensis. The two O. galapagoensis subspecies occurred as two isolated island populations. The only population of O. galapagoensis galapagoensis inhabited San Cristóbal Island and is now extinct. Although three additional endemic rice rats (Nesoryzomys) and two invasive rats (Rattus) inhabit the Galápagos Islands, identification is easy as O. galapagoensis bauri is the only rat to inhabit Santa Fé Island. (Dowler, et al., 2000; Swash and Still, 2000)

  • Sexual Dimorphism
  • male larger
  • Range mass
    55 to 74 g
    1.94 to 2.61 oz
  • Range length
    108 to 118 mm
    4.25 to 4.65 in

Reproduction

There is little available information as to the mating system of O. galapagoensis.

Unaddressed discrepancies exist in the literature as to the timing of the Galápagos rice rat’s reproductive season. Clark described the reproductive season as beginning in the warm, wet season (between January and May). According to Clark, the unpredictable timing of the warm season’s rains affects the start date of the reproductive season for O. galapagoensis. In contradiction with Clark's research, Brosset proposed that Galápagos rice rats breed multiple times in a year. Brosset found O. galapagoensis juveniles (age unspecified) during the months of March, July, and August. Brosset hypothesized that Galápagos rice rat reproduction is more affected by abundance of food than by seasonality. (Brosset, 1963; Clark, 1980)

Galápagos rice rats, although normally nocturnal, change to a diurnal activity pattern during the mating season. Later in the season, pregnant females become intolerant of males. The average number of embryos per pregnant female is influenced by the abundance of previous rainfall and resultant availability of food. During a particularly wet season the average number of embryos per female (n=10) was five. During a wet season with lower rainfall, the average number of embryos per female (n=5) was three. (Brosset, 1963; Clark, 1980)

Few quantitative data describe the gestation period, time to weaning, or age of reproductive maturity for O. galapagoensis. Brosset observed the development of a Galápagos rice rat litter of three at a field station. The mother gave birth nine days after capture. The offspring were described as naked and blind at birth and becoming black on the fourth day. On the ninth day, adult fur began to appear and the young opened their eyes. By the 13th day, the young were walking around the nest, and on the 19th day they ate bananas. After 35 days, the young had attained 75% of the adult female’s length. Brosset described the offspring and adult female as continuing to nest together without aggression. (Brosset, 1963)

For a more quantitative description of Galápagos rice rat reproduction, it is necessary to examine the reproductive behavior of the rice rat, Oryzomys palustris. Oryzomys palustris is similar in size to O. galapagoensis (weight: 40 to 80 g) and provides a reproductive guideline to follow. Oryzomys palustris has a gestation period of 25 days and the young weigh 3 to 4 g at birth. The weaning period of O. palustris is 2 weeks and the young reach sexual maturity after 2 months. The reproductive cycle of O. galapagoensis is likely similar; however, the two species vary in a number of life history traits, so these numbers should only be used as a guideline. (Niethammer, 1990)

  • Breeding interval
    Oryzomys galapagoensis may breed once or multiple times a year.
  • Breeding season
    The breeding season is likely to occur from January to May, but may extend into August.
  • Range number of offspring
    2 to > 5
  • Average number of offspring
    3 to 5

There is no available information on parental care for Oryzomys galapagoensis. Females provide milk to their offspring until weaning. Females leave the nest at night to forage. (Clark, 1979; Niethammer, 1990)

  • Parental Investment
  • altricial
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female

Lifespan/Longevity

The maximum age an Oryzomys galapagoensis is known to have lived is about 2 years. This information is based on trap-retrap methods, where both a male and female were retrapped 599 days after their original capture. (Clark, 1980)

  • Range lifespan
    Status: wild
    2 (high) years

Behavior

Little is known about the social system and behavior of Oryzomys galapagoensis. Oryzomys galapagoensis is nocturnal and becomes active at dusk. It is unafraid of people. (Clark, 1980; Swash and Still, 2000)

Home Range

There is currently no information regarding the home range of Oryzomys galapagoensis.

Communication and Perception

There is little available information on the communication methods of Oryzomys galapagoensis. Oryzomys galapagoensis, like other Oryzomys species, utilizes vision, hearing, touch, smell, and taste to perceive its environment. Other Sigmodontinae species have been reported to use high-pitched vocalizations and urinary and fecal odors to communicate. Thus, hearing and olfaction may be particularly important aspects of interspecies communication for Oryzomys galapagoensis. (D'Elia, 2004)

Food Habits

Very little is known about the diet of Oryzomys galapagoensis. In the wild, O. galapagoensis was observed eating fishermen’s fish scraps, insects, and shoots of the plant, Cryptocarpus periformis. Based on the diet of other Oryzomys species, O. galapagoensis is likely a generalist, feeding mainly on succulent plant parts, seeds, and insects. Oryzomys galapagoensis is an opportunistic feeder and will capitalize on human food when available. It is described as having a voracious appetite and will chew its way into tents and boxes to access food. Galápagos rice rats have been observed feeding on bread, dried fruit, beans, meat, flies, and hermit crabs. Researchers also found O. galapagoensis to be cannibalistic when food is limited. It is hypothesized that O. galapagoensis is a seed disperser for Bursera graveolens, a tropical tree found on Santa Fé Island. (Brosset, 1963; Clark and Clark, 1981; D'Elia, 2004; Niethammer, 1990)

  • Animal Foods
  • mammals
  • fish
  • carrion
  • insects
  • terrestrial non-insect arthropods
  • aquatic crustaceans
  • Plant Foods
  • leaves
  • wood, bark, or stems
  • seeds, grains, and nuts
  • fruit

Predation

Likely predators of Oryzomys galapagoensis are Galápagos hawks (Buteo galapagoensis), and short-eared owls (Asio flammeus). Asio flammeus may have a greater impact on the population of Oryzomys galapagoensis due to the two species' overlap in nocturnal activity patterns. An instance of a centipede, Scolopendra galapagoensis, preying on a young O. galapagoensis in a nest has also been documented. (Clark, 1979; Clark, 1980)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Oryzomys galapagoensis may play a key ecological role in the dispersal of Bursera graveolens, a tropical tree that dominates Santa Fé's arid environment. Feral goats (Capra hircus) inhabited Santa Fé for at least 66 years. In 1971, the Galápagos National Park Service extirpated the introduced goat population on the island. Since the goat removal, the severely impacted B. graveolens population has shown a considerable increase in the number of juveniles. Dispersers have played a significant role in the successful return of B. graveolens. Bursera graveolens parent plants inhibit germination. Thus, the plant relies on seed dispersal to decrease proximity to parent trees. Land iguanas (Conolophus pallidus), bird species, and O. galapagoensis are likely aiding in the seed dispersal of a number of plant species on Santa Fé. (Clark and Clark, 1981)

  • Ecosystem Impact
  • disperses seeds

Economic Importance for Humans: Positive

There are currently 3 species of endemic rodents in the Galápagos, including Oryzomys galapagoensis. The Galápagos archipelago is renowned and heavily visited for its unique flora and fauna. The tourism industry, which relies completely on the presence of these endemics, benefits the Ecuadorian government and people monetarily.

Economic Importance for Humans: Negative

Inhabitants and tourists of the Galápagos may be forced to take particular care not to introduce Rattus rattus or Rattus norvegicus to Santa Fé Island. This is due to the observed vulnerability native Galápagos rodent populations have to the presence of these two species. (Dowler, et al., 2000)

Conservation Status

Oryzomys galapagoensis is comprised of two subspecies and their distinct populations. The population of Oryzomys galapagoensis galapagoensis, which once inhabited San Cristóbal, is extinct. Evidence supports the assumption that the most likely cause of extinction was the introduction of black rats (Rattus rattus). It is unclear as to whether the black rats caused the native rice rat population decline through direct competition for resources or through the introduction of a pathogen. Some researchers believe an O. galapagoensis galapagoensis population may exist on the western side of San Cristóbal, where limited rodent surveys have been conducted. (Dowler, et al., 2000; Gippoliti, 2002)

Oryzomys galapagoensis bauri is currently listed as vulnerable by the IUCN. This population of O. galapagoensis on Santa Fé is healthy, and individuals are considered common. This population, however, is extremely vulnerable to extinction if black rats (Rattus rattus) or Norway rats (Rattus norvegicus) are introduced to this island. (Dowler, et al., 2000; Gippoliti, 2002)

Other Comments

Oryzomys (rice rat) species are known to be good swimmers, and are thought to have floated to the Galápagos Islands on rafts of vegetation drifting from the South American mainland. (Niethammer, 1990)

Contributors

Tanya Dewey (editor), Animal Diversity Web.

Coral Wolf (author), University of Michigan-Ann Arbor, Phil Myers (editor, instructor), Museum of Zoology, University of Michigan-Ann Arbor.

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

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

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.

carrion

flesh of dead animals.

chemical

uses smells or other chemicals to communicate

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

ecotourism

humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.

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.

fertilization

union of egg and spermatozoan

island endemic

animals that live only on an island or set of islands.

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).

motile

having the capacity to move from one place to another.

native range

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

nocturnal

active during the night

omnivore

an animal that mainly eats all kinds of things, including plants and animals

scrub forest

scrub forests develop in areas that experience dry seasons.

seasonal breeding

breeding is confined to a particular season

sexual

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

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.

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

Brosset, A. 1963. Satut actuel des mammifères des îles Galapagos. Mammalia, 27: 323-341.

Clark, D. 1979. A centipede preying on a nestling rice rat (Oryzomys bauri). Journal of Mammalogy, 60 (3): 654.

Clark, D. 1980. Population Ecology of an endemic neotropical island rodent: Pryzomys bauri of Santa Fe Island, Galapagos, Ecuador. The Journal of Animal Ecology, 49 (1): 185-198.

Clark, D., D. Clark. 1981. Efffects of seed dispersal by animals on the regeneration of Bursera graveolens (Burseraceae) on Santa Fe Island, Galápagos. Oecologia, 49: 73-75.

D'Elia, G. 2004. Rice, mice, and relatives IV: South American rats and mice (Sigmodontinae). Pp. 263-279 in M Hutchins, D Kleiman, V Geist, M McDade, eds. Grzimek's animal life encyclopedia, Vol. 16: Mammals V, 2nd Edition. Farmington Hills, MI: Gale Group.

Dowler, R., D. Carroll, C. Edwards. 2000. Rediscovery of rodents (Genus Nesoryzomys) considered extinct in the Galápagos Islands. Oryx, 34 (2): 109-117.

Gippoliti, S. 2002. "Oryzomys galapagoensis" (On-line). 2004 IUCN red list of threatened species. Accessed March 18, 2006 at http://www.redlist.org.

Niethammer, J. 1990. Burrowing rodents. Pp. 206-275 in S Parker, ed. Grzimek's Encyclopedia of Mammals, Vol. 3, English Language Edition. United States: McGraw-Hill Publishing Company.

Swash, A., R. Still. 2000. Birds, mammals, and reptiles of the Galápagos Islands. United States: Yale University Press.

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