Isothrix bistriata

Geographic Range

Yellow-crowned bush-tailed rats, Isothrix bistriata , are found in South America’s western sides of the Amazon Basin in Peru, Bolivia, Venezuela, southeastern Colombia, and Brazil. The species is also thought to reside in Eastern Ecuador where it has been photographed on 2 occasions. I. bistriata is the only known species of Isothrix west of the Negro-Madeira rivers axis.

• Biogeographic Regions
• neotropical neotropical
  • native native

Habitat

The habitat of yellow-crowned brush-tailed rats is composed of lowland forests with seasonal flooding and within river floodplains. Yellow-crowned brush-tailed rats build their nests in the holes of large trees, especially palms, and live among the canopy.

• Habitat Regions
• temperate temperate
• tropical tropical
• terrestrial terrestrial

• Terrestrial Biomes
• savanna or grassland savanna or grassland
• forest forest
• rainforest rainforest

• Wetlands
• swamp swamp

Physical Description

Yellow-crowned brush-tailed rats are medium-sized arboreal rodents with a cream-colored patch on the crown of their head, two dark stripes going from forehead to nape of their neck, and a furry tail. Their dark head stripes do not meet behind the ears, marking the difference between I. bistriata and the other Isothrix species. Their body fur is soft and non-spiny, and mostly yellowish-brown with some black hairs grizzled about. Their abdomen is pale yellow and their tails go from rusty golden on the first two thirds of the tail to a dark brown third tip in adults, or a pale yellow termination in juvenile individuals. Their tail hair curls outward, resembling a bottlebrush, and doesn’t cover all tail scales. The mean head and body length is 249 mm, while the mean tail length is 242 mm. With their tail about the same length as their body, the mean tail:body ratio of yellow-crowned brush-tailed rats is about 97%. The specific body weight of I. bistriata has not been reported. However, the mean body weight of their close relatives in the Isothrix genus is 320-570 grams. In other body part measurements, their mean hind foot length ranges from 45-54 mm, their mean ear length ranges from 15-20 mm, and their mean condylo-incisive length is 51 mm. Yellow-crowned brush-tailed rats are often considered adults when cheek teeth are erupted and somewhat worn down.

Male yellow-crowned brush-tailed rats have a blunt, short, and wide-grooved phallus. Their baculum is long and narrow. Adult males develop 25 mm midventral chest glands and possess relatively large testes (measuring 24 mm length x 12 mm width) that stay in the abdomen and do not descend to the scrotum.

The number of teats in female yellow-crowned brush-tailed rats are three lateral pairs of mammae and one inguinal pair situated by the groin area. Female I. bistriata develop enlarged nipples in adulthood, possibly as a result of sexual maturity and lactation.

The skeletal features most studied on the yellow-crowned brush-tailed are their skull and their teeth. Like other Isothrix species, I. bistriata have a relatively flat, broad cranium with a short, broad rostrum, making them morphologically resemble a squirrel. Their supraorbital ledges extend posteriorly to the parietals, and the interorbital region is broad. Tympanic bullae in I. bistriata are somewhat large, the auditory meatus is near the squamosal bone and is medium-sized, and the auditory tube is short and tilted forward. The mandible of I. bistriata has a strong base composed of well-developed masseter and pterygoid crests. Their masseter and infraorbital arrangement classify I. bistriata as hystricomorphous.

In terms of teeth, like other Isothrix species, I. bistriata has nearly laminate teeth, with maxillary cheekteeth rows being short, small, parallel to the palate, and somewhat tilted back. Dental formula of I. bistriata was not available, but the dental formula of the Echimyidae family is I(1/1), C(0/0), P(1/1), M(3/3) = 20.

• Other Physical Features
• endothermic endothermic
• bilateral symmetry bilateral symmetry

Reproduction

The mating systems of yellow-crowned brush-tailed rats have not been reported, however of the close relatives within the Echimyidae family that have been studied, mostly polygynous mating systems are known to happen. Within Echimyidae , mating systems can be considered facultatively promiscuous, polygynous, or monogamous because they depend on density and home range sizes.

Yellow-crowned brush-tailed rats are known to be sexually mature when all cheek teeth have erupted and have started to wear down, however their specific age of sexual maturity has not been studied. Males and female pairs nest in tree holes, with their breeding season thought to continue year round. The average litter size is thought to be small, around one or two offspring, evidenced by the number of placental scars never being more than two. It is thought I. bistriata breed throughout the year.

• Key Reproductive Features
• iteroparous iteroparous
• year-round breeding year-round breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• viviparous viviparous

Parental care in I. bistriata behavior has not been studied. Captured females accompanied by their offspring suggest relatively high parental investment. It is known for some relatives within the Echimyidae family, like genus Kannabateomys to perform biparental care of their young, meaning that both parents cooperate to take care of their young.

• Parental Investment
• precocial precocial
• female parental care female parental care

Lifespan/Longevity

Specific longevity of yellow-crowned brush-tailed rats has not been studied. However, in a Echimyidae family relative, Proechimys semispinosus , lifespan was found to be a minimum of 6.5-10 months and a maximum of 36-53 months. Lifespan limiting factors have not been studied in I. bistriata .

Behavior

Yellow-crowned brush-tailed rats are arboreal and nocturnal. They are solitary individuals. I. bistriata commonly protrude their heads out during daytime, they appear at the entrance of their tree holes in the afternoon, and come out to forage at night. It is common for all members of the Isothrix genus, including I. bistriata , to be allopatric to their range.

• Key Behaviors
• arboreal arboreal
• nocturnal nocturnal
• motile motile
• solitary solitary

Home Range

Home range of yellow-crowned brush-tailed rat individuals has not been documented. However, relatives from the Echimyidae family appear to be minimally territorial, so their home ranges are determined by population density. There also appears to be a difference in territory size between male and female members of the Echimyidae family, with males having relatively larger home ranges, and present fairly extensive home range overlaps within species. I. bistriata is an arboreal species that prefers floodplain areas, so their home range will be limited by the extent of trees located in floodplains.

Communication and Perception

Communication of yellow-crowned brush-tailed rats has not been studied. There is no information on social communication, potential mate communication, social rank communication, or resource communication found in the literature. There is evidence of chemical communication and signaling in close relatives of the Echimyidae family, such as the spiny rat Trinomys yonenagae and Proechimys semispinosus , by using scent glands near the anus. Scent recognition is thought to be used by spiny rats. However, the presence of scent glands in I. bistriata have not been documented. The olfactory function of the snout of I. bistriata might allow for scent recognition.

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

Foraging behaviors of I. bistriata have not been studied. However, the general diet of their family Echimyidae is omnivorous and includes mainly fruits, vegetative plant parts, seeds, fungi, and/or arthropods.

Predation

There is little available information on predation in yellow-crowned brush-tailed rats. Close relatives, such as the members of the Echimyidae family, are often hunted by humans, smaller primates and spectacled owls Pulsatrix perspicillata , however actual capture and feeding by small primates or spectacled owls on yellow-crowned brush-tailed rats has not been documented.

A possible anti-predator mechanism in relatives of I. bistriata within the Echimyidae family is tail autotomy, or breaking off of the tail, by the spiny rats of the Proechimys semispinosus species. However, this mechanism has not been documented in I. bistriata .

Ecosystem Roles

There is little available information on the ecosystem roles of yellow-crowned brush-tailed rats. However, an important ecosystem impact of some of their relatives within the Echimyidae family consist of seed dispersal, like in the case of spiny rats Proechimys semispinosus , due to their frugivorous diet.

Economic Importance for Humans: Positive

There is little available information on the positive economic impacts of yellow-crowned brush-tailed rats. However, within the Echimyidae family, various species are often hunted or used as laboratory animals.

• Positive Impacts
• food food
• research and education

Economic Importance for Humans: Negative

Information regarding potential negative economic impacts of yellow-crowned brush-tailed rats on humans has not been documented. Some relatives of I. bistriata in the Echimyidae family are reservoirs of disease. For example, Tome’s spiny rats Proechimys semispinosus carry cutaneous leishmaniasis ( Leishmania panamensis and Leishmania chagasi ) which can affect humans infected by sand fly hosts. However, parasites or diseases have been studied in I. bistriata .

Other relatives of I. bistriata in the Echimyidae family, under the Rhizomyinae subfamily, affect sugarcane and tapioca crops in Thailand, such as the Atlantic bamboo rat Kannabateomys amblyonyx , however there are no studies on the effects of I. bistriata on crops.

Conservation Status

Assessed in 2016 as Least Concern by the IUCN. Although Isothrix bistriata is uncommon, due to specimens being found along a wide range within various protected areas, populations are thought to be stable and at low risk of decreasing their size at an alarming rate.

Additional Links

Encyclopedia of Life

Contributors

Angela ChappaLarrea (author), University of Washington, Laura Prugh (editor), University of Washington, Tanya Dewey (editor), University of Michigan-Ann Arbor.

References

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