Arvicanthis niloticus

Geographic Range

The range of African grass rats, Arvicanthis niloticus , is traditionally held to extend along the Nile river valley and across most of sub-Saharan Africa, with the exception of the southern and southwestern regions of the continent. However, much debate over the number and range of species within the genus Arvicanthis has yet to be resolved, and the range of A. niloticus may be much more restricted. From genetic analysis, Ducroz, Volobouev, and Granjon (1998) claim this species occurs only in Egypt and northern West Africa, but Musser and Carleton (1993) argue that A. niloticus also inhabits regions including and surrounding Ethiopia.

• Biogeographic Regions
• ethiopian ethiopian
  • native native

Habitat

As A. niloticus lives in colonial burrows, it requires some degree of ground cover, such as short bushes, trees, rocks, or termite mounds, under which it may nest. A variety of African habitats, including dry savanna, sub-desert, coastal scrub, open woodlands, grasslands, and cultivated areas, provide such protection. Exact altitudinal data are not reported, but A. niloticus is not believed to occur at high altitudes.

• Habitat Regions
• tropical tropical
• terrestrial terrestrial

• Terrestrial Biomes
• desert or dune desert or dune
• savanna or grassland savanna or grassland
• scrub forest scrub forest

• Other Habitat Features
• agricultural agricultural

Physical Description

Rosevear (1969) described African grass rats as “medium-sized rats with stoutish bodies.” Adults of this species range in head and body length from 106 mm to 204 mm with an average of about 130 mm. Tail lengths range from 85% to 90% of the head and body length and average around 100 mm. Average mass of A. niloticus is 118 g, with a range of 50 g to 183 g. Males are slightly larger than females with reported average masses of 120 g to 123 g for males and 92 to 114 g for females.

Arvicanthis niloticus has a roundish head with large, round ears that are covered with short, fine fur. Incisors are not grooved; the snout is rather short, and the tail is covered in small, barely visible hairs. The hindfoot is well-developed, and the inner three hind toes are longer than the outer two. In contrast, the forefoot is smaller with a relatively short, though usable, thumb.

Variation in the coat color of this species has been reported; however, ambiguity in the boundaries of this species may have resulted in the misidentification of another species of the genus Arvicanthis as a color variant of A. niloticus . According to Rosevear (1969), the dorsal fur of these rats consists mostly of ringed hairs, which are dark black or brown at the base, lighter yellow, reddish-brown, or buff in the middle, and black at the tip. Short underfur, gutter hairs, and all-black guard hairs are also present and, combined with the ringed hairs, produce a "salt and pepper" effect. The ventral coat is shorter and lighter in appearance.

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

• Sexual Dimorphism
• male larger

Reproduction

Relatively little is known about the mating structure of this species. Packer (1983) studied one colony in Tanzania and reported that the colony averaged 2.6 females and 3.1 males. Only males immigrated into the colony; new females were born in the colony and presumed not to disperse. All females successfully reproduced, and all males had descended testes, indicating the capability of breeding. Therefore, it is most likely that multiple members of an A. niloticus colony are breeding simultaneously.

Senzota (1990) studied two study sites with multiple A. niloticus colonies and indicated that colonies were mainly equally composed of males and females, with females more often outnumbering males than vice-versa. All-female and all-male colonies were also observed, but Senzota found that males were more likely to disperse than females, confirming Packer's findings.

Arvicanthis niloticus is capable of breeding year-round under highly favorable conditions. However, it usually experiences a sexual rest period beginning in March. This is during the hot dry season prior to the rainy season, and the rest period is induced at this time by long days, dry air, and high temperatures, which have an inhibitory effect on the gonads.

During the breeding season, gestation may take 18 to 25 days, averaging 23 days. Females have two equipotential ovaries and a duplex uterus. Birth weights of pups range from 3 g to 6 g, and litter sizes range from a few to 12 pups, averaging around 5 pups. Females experience a post-partum estrus and thus may be consistently pregnant and lactating, giving birth every 23 to 25 days, during the breeding season (October to March).

Young are weaned at the age of about three weeks and are considered sexually mature at 3 to 4 months. Males were observed to disperse from their natal colonies around 9 to 11 months of age.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
• viviparous viviparous
• post-partum estrous

Comprehensive examination of parental care in this species is lacking. However, mothers have been observed to defend their young prior to weaning. Lactation lasts about 21 days, and it is most likely, given preliminary data, that females rarely disperse from their natal nest, whereas males often disperse. Thus, parental care beyond lactation may occur. However, Senzota (1990) noted that wild A. niloticus did not defend their sub-adult offspring in the presence of predators but instead retreated immediately to their burrows.

Male parental care is not well-documented. Males may be kept in captivity with their mates and offspring throughout lactation but have occasionally been observed to commit infanticide, which is not uncommon in captive rodents. However, given the communal social structure of A. niloticus , it is likely that males are at the least indifferent to and at most actively parenting their offspring.

• Parental Investment
• altricial altricial
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • female
• pre-weaning/fledging
  • provisioning
    • female
  • protecting
    • female
• post-independence association with parents

Lifespan/Longevity

Refinetti (2004) reports an average longevity of 2 years in captivity, with a standard deviation of 1 year for A. niloticus . Nowak (1999) claims that the longest lived individual of this species in captivity died at the age of 6 years and 8 months. Little is known about longevity in the wild; however, Packer (1983) estimates that females in one colony lived for an average of 10.2 months, with a maximum of 20 months.

Behavior

Arvicanthis niloticus is a gregarious species that lives in underground burrows. These burrows have multiple entrances and run about 20 cm deep. They are found at the base of trees, shrubs, rocks, termite mounds, and any other sufficient ground cover. Associations outside of the burrow, defined as individuals foraging, "playing", or otherwise interacting together, are very common with no age or sex bias.

One of the most striking behaviors of A. niloticus is the creation and maintenance of “runways” that extend from the burrow entrances in varying shapes and lengths. Members of this species clip grasses and other herbaceous plants and remove small obstructions to keep the runways clear during the dry season. Both the number of runways radiating from a burrow and the density of clipped grass (runway clarity) about the runways have an inverse relationship with distance from the burrow. During the wet season, A. niloticus does not create new runways and reduces runway maintenance as food is more readily acquired close to the colonial burrow. Senzota (1990) asserts that the primary function of runways is for rapid escape from predators. Detection of predator movement nearby induces A. niloticus to seek the nearest runway and flee to the burrow.

The activity patterns of A. niloticus have been widely debated, with some claiming that the species is diurnal, nocturnal, crepuscular, or some combination of these. The most recent analyses in both the wild and captivity indicate that this species is diurnal with some crepuscular tendencies.

• Key Behaviors
• terricolous
• diurnal diurnal
• crepuscular crepuscular
• motile motile
• sedentary sedentary
• social social
• colonial colonial

Home Range

Home range size varied both by sex and by season. Home ranges vary from 1400 to 2750 square meters for males and from 600 to 950 square meters for females in the dry and rainy seasons, respectively.

Communication and Perception

Arvicanthis niloticus is capable of perceiving touch and scent at birth, and hearing and sight both develop around 6 to 7 days of age. Communication in A. niloticus has not been adequately studied. However, squeaks and distress calls have been observed to begin between 4 and 6 days of age, and vocalization may be involved in its communicative repertoire. Olfaction is a common form of communication in many mammals, including rodents, and may also be utilized by this species. Because these animals are social and diurnal, both visual and tactile communiation likely take place, although details of these forms of communication are not available in current literature.

• Communication Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

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

Food Habits

A. niloticus is primarily herbivorous, feeding on grasses, leaves and stems of flowering plants, seeds, the bark of some woody plants, and cultivated crops. Arthropods are also eaten by this species. As different food types vary in their availability seasonally, A. niloticus will alter the intake ratio of food types. This flexible, generalist approach may improve its competitive ability. Caching does not appear to be a predominant behavior in this species, but has been observed when larger food items were offered to wild individuals.

• Primary Diet
• herbivore herbivore
  • folivore folivore

• Animal Foods
• insects

• Plant Foods
• leaves
• wood, bark, or stems
• seeds, grains, and nuts

• Foraging Behavior
• stores or caches food stores or caches food

Predation

Some anti-predatory behaviors have been documented for A. niloticus . Individuals typically retreat immediately down runways back to the communal burrow, possibly stopping to hide under other ground cover from avian predators. Senzota (1990) noted that when no conspecifics were present outside the burrow, individuals of this species spent a substantial amount of time in vigilant behavior at the entrances of the burrow prior to emerging. Movement by predators resulted in retreat into the burrow by A. niloticus , although mere stationary presence would not. If conspecifics were present (foraging, “playing”, or maintaining runways), individuals would readily leave the burrow.

Given the widespread range of this species and the prevalence of predation upon small mammals, particularly rodents, A. niloticus may be preyed upon by a number of carnivorous African animals. It was the primary prey of barn owls , in the Nigerian savanna and accounted for 26.5% of the biomass of these barn owls' diet in one study. Direct predation on A. niloticus by dwarf mongooses , black-backed jackals , spitting cobras , long-crested hawk-eagles , black-shouldered kites , and black-headed herons has been observed in the field.

Ecosystem Roles

Aside from serving as prey to some African carnivores, A. niloticus also serves other important, though perhaps less desirable, roles in its ecosystem. It is an agricultural pest and competes with other African rodents, primarily natal multimammate mice , and savanna gerbils , for both natural and cultivated resources. Arvicanthis niloticus thus has a strong impact on plant diversity. Senzota (1983) also proposed resource partitioning of grasses by A. niloticus and some African ungulates, including blue wildebeest and Thomson’s gazelles , to reduce competition between the rodents and ungulates.

Arvicanthis niloticus also serves as a host and/or vector for a variety of organisms, including fleas, parasitic worms, and viruses. It has been implicated in many plant and human disease outbreaks as a carrier of a multitude of diseases, such as bubonic plague in ancient Egypt, Rice yellow mottle virus in parts of Africa, and Schistosoma mansoni , which causes intestinal schistosomiasis, a disease that sometimes occurs in severe outbreaks in parts of Africa.

Economic Importance for Humans: Positive

Given its rapid breeding capabilities, diurnal activity patterns, and small size, A. niloticus has value in laboratory research in medicine, physiology, behavior, and other related fields. Most rodent-based research in these disciplines utilizes either Norway rats , or house mice . However, both of these species are nocturnal, and captive A. niloticus colonies have been validated as diurnal and are thus more similar in certain respects to humans and other diurnal mammals than typical lab rats or mice.

• Positive Impacts
• research and education

Economic Importance for Humans: Negative

Arvicanthis niloticus is considered an agricultural pest throughout much of Africa, and active pest management programs are currently in effect. Also, this species has been implicated in the transmission of multiple human and crop diseases, including bubonic plague in Egypt, intestinal schistosomiasis, and Rice yellow mottle virus .

• Negative Impacts
• injures humans
  • causes disease in humans causes disease in humans
  • carries human disease
• crop pest

Conservation Status

This species does not appear to be in any danger. The IUCN red list does not have a data entry for A. niloticus ; however, it does list its congener, Arvicanthis blicki as near-threatened. The US Federal list and CITES have no information on A. niloticus .

Other Comments

As noted above, there has been considerable argument over the taxonomy of the genus Arvicanthis . Much of the research conducted prior to the late 1990s on A. niloticus assumed it was the only species of Arvicanthis in existence. This may explain discrepencies in behavioral, circadian, and physical descriptions. Further genetic and morphometric research into the diversity of the genus Arvicanthis may alter the validity of this account.

Additional Links

Encyclopedia of Life

Contributors

Barbara Lundrigan (editor, instructor), Michigan State University, Jessica St. John (author), Michigan State University, Nancy Shefferly (editor), Animal Diversity Web.

References

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