Pack rat

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Packrats
Temporal range: Late Cenozoic - Recent
Neotoma cinerea
Scientific classification
Kingdom:Animalia
Phylum:Chordata
Class:Mammalia
Order:Rodentia
Family:Cricetidae
Subfamily:Neotominae
Tribe:Neotomini
Genus:Neotoma
Say & Ord, 1825
Species

Neotoma albigula
Neotoma angustapalata
Neotoma anthonyi
Neotoma bryanti
Neotoma bunkeri
Neotoma chrysomelas
Neotoma cinerea
Neotoma devia
Neotoma floridana
Neotoma fuscipes
Neotoma goldmani
Neotoma lepida
Neotoma leucodon
Neotoma macrotis
Neotoma magister
Neotoma martinensis
Neotoma mexicana
Neotoma micropus
Neotoma nelsoni
Neotoma palatina
Neotoma phenax
Neotoma stephensi

 
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Packrats
Temporal range: Late Cenozoic - Recent
Neotoma cinerea
Scientific classification
Kingdom:Animalia
Phylum:Chordata
Class:Mammalia
Order:Rodentia
Family:Cricetidae
Subfamily:Neotominae
Tribe:Neotomini
Genus:Neotoma
Say & Ord, 1825
Species

Neotoma albigula
Neotoma angustapalata
Neotoma anthonyi
Neotoma bryanti
Neotoma bunkeri
Neotoma chrysomelas
Neotoma cinerea
Neotoma devia
Neotoma floridana
Neotoma fuscipes
Neotoma goldmani
Neotoma lepida
Neotoma leucodon
Neotoma macrotis
Neotoma magister
Neotoma martinensis
Neotoma mexicana
Neotoma micropus
Neotoma nelsoni
Neotoma palatina
Neotoma phenax
Neotoma stephensi

A packrat, also called a woodrat, can be any of the species in the rodent genus Neotoma. Packrats have a rat-like appearance with long tails, large ears and large black eyes. Compared to deer mice, harvest mice and grasshopper mice, packrats are noticeably larger and are usually somewhat larger than cotton rats.[1]

Contents

Species

Range and distribution

Woodrats reach their greatest diversity in the deserts of the western United States and northern Mexico. Several species are also found in the deciduous forest of the east coast, juniper woodlands in the southwest, oak woodlands along the coastal western United States and in the Sonoran Desert, and in the forest and rocky habitats of the western United States and western Canada.[2]

Habitat

Each species of packrat is generally restricted to a given type of habitat within its range. Packrats live anywhere from low, hot, dry deserts to cold, rocky slopes above timberline. Packrats build complex houses or dens made of twigs, cactus joints and other materials. These contain several nest chambers, food caches and debris piles. Dens are often built in small caves or rocky crevices, but when close by human habitations, woodrats will opportunistically move into the attics and walls of houses. Some Neotoma species, such as the White-throated Woodrat (N. albigula), use the base of a prickly pear or cholla cactus as the site for their home, utilizing the cactus' spines for protection from predators. Others, like the Desert Woodrat (N. lepida) will build dens around the base of a yucca or cactus, such as Jumping and Teddy-bear Chollas. The largest species, Neotoma cinerea, has a bushy, almost squirrel-like tail. Bushy-tailed woodrats Neotoma cinerea occupy a range of habitats from boreal woodlands to deserts. They are cliff-dwellers and are often found on isolated, high-elevation bouldery exposures under a variety of temperature and moisture regimes. They require adequate shelter inside the rocks, though they are occasionally found inhabiting abandoned buildings as well.

Characteristics

Behavior

Packrats are nest builders. They use plant material like branches, twigs, sticks, and other available debris. Getting into everything from attics to car engines, stealing their ‘treasures’, damaging electrical wiring and creating general noisy havoc can easily cause them to become a nuisance.[3] A peculiar characteristic is that if they find something they want, they will drop what they are currently carrying, for example a piece of cactus, and "trade" it for the new item. They are particularly fond of shiny objects. These two traits have inspired an anecdote about a man finding his dime replaced by two nickels. They can also be quite vocal and boisterous.

Diet

Bushy-tailed woodrats feed primarily on green vegetation, twigs, and shoots. Mexican packrats eat seeds, fruits, acorns, and cactus.[4]

Size

Adult bushy-tailed woodrat males usually weigh 300-600 grams with an average of 405 grams, and adult females usually weigh 250-350 grams with an average of 270 grams. These ranges are relatively large because this species occupies a large geographic range, and its body size is closely correlated with climate.[5] Average males range in size from 310-470 millimeters with the average being 379 millimeters and average females range from 272 millimeters to 410 millimeters with the average being 356 millimeters.

Reproduction and life cycle

Reproductive habits of rodents are variable in the wild and can become more so when domesticated. Most are born naked and helpless and must be cared for in nests. There are some female packrats who have been known to deliver up to 5 litters per year with each litter having as many as 5 young. The offspring may open their eyes between 10 to 12 days after being born and are usually weaned between 14 and 42 days. After around 60 days most become sexually mature.[3]

Midden

A large pack rat midden (center) from the Pleistocene period.

A packrat midden is a debris pile constructed by a woodrat. A packrat midden may preserve the materials incorporated into it for up to 50,000 years. The middens may thus be analyzed to reconstruct their original environment, and comparisons between middens allow a record of vegetative and climate change to be built. Examinations and comparisons of pack rat middens have largely supplanted pollen records as a method of study in the regions where they are available.[6]

Active pack rat midden in northern Nevada

In the absence of rock crevices or caves, the dens are often built under trees or bushes. The packrats will also use plant fragments, animal dung and small rocks in building the den. The vast majority of the materials will be from a radius of several dozen yards of the nest. Woodrats often urinate on the debris piles; sugar and other substances in the urine crystallize as it dries out, creating a material known as amberat, which under some conditions can cement the midden together. The resilience of the middens is aided by three factors. The crystallized urine dramatically slows the decay of the materials in the midden; the dry climate of the American Southwest further slows the decay; and middens that are protected from the elements under rock overhangs or in caves survive longer.

Climate change indicators

Zoologists examine the remains of animals in middens to get a sense of the fauna in the neighborhood of the midden, while paleobotanists can reconstruct the vegetation that grew nearby. Because middens are abandoned after a short period of time, they are uncontaminated "time capsules" of several decades of natural life, centuries and millennia after they occurred. The analysis of middens was key in understanding the fauna around Pueblo Bonito, and thus helping to explain its history.

Bushy-tailed woodrat on midden.

According to Bergmann’s rule, the body size of vertebrates is closely related to the average ambient air temperature in the region in which the vertebrate lives. According to the rule, organisms in warmer regions are typically smaller than members of the same species in colder regions. This is because, all other things being equal, large body size allows for the conservation of heat, while small body size allows for the dissipation of heat. Therefore it is believed that the body sizes of organisms, woodrats in particular, are good indicators of past climate shifts.

Woodrat middens are composed of many things, including plants macrofossils and fecal pellets. The size of the pellet is proportional to the size of the woodrat. By measuring the pellets, the approximate size of the woodrat is determined based on data from a study of field-trapped woodrats. From Bergmann’s rule, differences in climate then can be determined. A study by Smith, Betancourt, and Brown, published in 1995, involved extensive research on this topic.


References

Further reading

External links