Scientific Classification

Taxonomists generally disagree about the most appropriate placement of the subfamilyCricetinae within the superfamily Muroidea. Some place it in a family Cricetidae that also includes voles, lemmings, and New World rats and mice; others group all these into a large family called Muridae. Their evolutionary history is recorded by 15 extinct fossil genera and extends back 11.2 million to 16.4 million years to the Middle Miocene Epoch in Europe and North Africa; in Asia it extends 6 million to 11 million years. Four of the seven living genera include extinct species. One extinct hamster of Cricetus, for example, lived in North Africa during the Middle Miocene, but the only extant member of that genus is the European or common hamster of Eurasia.

• Subfamily Cricetinae
  • Genus Allocricetulus
    • Species A. curtatus—Mongolian hamster
    • Species A. eversmanni—Eversmann's or Kazakh hamster
  • Genus Cansumys
    • Species C. canus—Gansu hamster
  • Genus Cricetulus
    • Species C. alticola—Tibetan dwarf or Ladak hamster
    • Species C. barabensis, including "C. pseudogriseus" and "C. obscurus"—Chinese striped hamster, also called Chinese hamster; striped dwarf hamster
    • Species C. griseus—Chinese (dwarf) hamster, also called rat hamster
    • Species C. kamensis—Kam dwarf hamster or Tibetan hamster
    • Species C. longicaudatus—long-tailed dwarf hamster
    • Species C. migratorius—gray dwarf hamster, Armenian hamster, migratory grey hamster; grey hamster; migratory hamster
    • Species C. sokolovi—Sokolov's dwarf hamster
  • Genus Cricetus
    • Species C. cricetus—European hamster, also called common hamster or black-bellied field hamster
  • Genus Mesocricetus—golden hamsters
    • Species M. auratus—golden or Syrian hamster
    • Species M. brandti—Turkish hamster, also called Brandt's hamster; Azerbaijani hamster
    • Species M. newtoni—Romanian hamster
    • Species M. raddei—Ciscaucasian hamster
  • Genus Phodopus—dwarf hamsters
    • Species P. campbelli—Campbell's dwarf hamster
    • Species P. roborovskii—Roborovski hamster
    • Species P. sungorus—Djungarian hamster or winter-white Russian dwarf hamster
  • Genus Tscherskia
    • Species T. triton—greater long-tailed hamster, also called Korean hamster

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Hamsters are typically stout-bodied, with tails shorter than body length, and have small, furry ears, short, stocky legs, and wide feet. They have thick, silky fur, which can be long or short, colored black, grey, honey, white, brown, yellow, red, or a mix, depending on the species. Two species of hamster belonging to the genus Phodopus, Campbell's dwarf hamster (P. campbelli) and the Djungarian hamster (P. sungorus), and two of the genusCricetulus, the Chinese striped hamster (C. barabensis) and the Chinese hamster (C. griseus) have a dark stripe down their heads to their tails. The species of genus Phodopusare the smallest, with bodies 5.5 to 10.5 cm (2.2 to 4.1 in) long; the largest is the European hamster (Cricetus cricetus), measuring up to 34 cm (13.4 in) long, not including a short tail of up to 6 cm (2.4 in). The Angora hamster, also known as the long-haired or teddy bear hamster, which is a type of the golden hamster is the second-largest hamster breed, measuring up to 18 cm (7.1 in) long.^[3]

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Species

Although the Syrian hamster or golden hamster (Mesocricetus auratus) was first described scientifically in 1839, researchers were not able to successfully breed and domesticate hamsters until 1939.^[3] The entire laboratory and pet populations of Syrian hamsters appear to be descendants of a single brother-sister pairing. These littermates were captured and imported in 1930 from Aleppo [Syria] by Israel Aharoni, a zoologist of the University of Jerusalem.^[4] In Jerusalem, the hamsters bred very successfully. Years later, animals of this original breeding colony were exported to the USA, where Syrian hamsters became one of the most popular pets and laboratory animals. Comparative studies of domestic and wild Syrian hamsters have shown reduced genetic variability in the domestic strain. However, the differences in behavioral, chronobiological, morphometrical, hematological, and biochemical parameters are relatively small and fall into the expected range of interstrain variations in other laboratory animals.^[5]

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Reproduction

Hamsters become fertile at different ages depending on their species. Both Syrian and Russian hamsters mature quickly and can begin reproducing at a young age (4–5 wk), whereas Chinese hamsters will usually begin reproducing at two to three months of age, and Roborovskis at three to four months of age. The female’s reproductive life lasts about 18 months, but male hamsters remain fertile much longer. Females are in estrus about every four days, which is indicated by a reddening of genital areas, a musky smell, and a hissing, squeaking vocalisation she will emit if she believes a male is nearby.^[3]

When seen from above, a sexually mature female hamster has a trim tail line; a male's tail line bulges on both sides. This might not be very visible in all species. Male hamsters typically have very large testes in relation to their body size. Before sexual maturity occurs, it is more difficult to determine a young hamster's sex. When examined, female hamsters have their anal and genital openings close together, whereas males have these two holes farther apart (the penis is usually withdrawn into the coat and thus appears as a hole or pink pimple).^[3]

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Diet

Hamsters are omnivores. Although they can survive on a diet of exclusively commercial hamster food, other items, such as vegetables, fruits, seeds, and nuts, can be given, but these should be removed before they become rotten. Hamsters in the Middle East have been known to hunt in packs to find insects for food. Hamsters are hindgut fermenters and must eat their own feces (coprophagy) to recover nutrients digested in the hindgut, but not absorbed.

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BEHAVIOR/CHRACTERISTICS

KILHAM~ found that hamsters inoculated intracerebrally (i.c.) with sub-lethal doses of rat
virus (RV) during the first few weeks of life developed a number of abnormalities. The
symptoms produced by the virus, which was originally isolated from rats bearing spontaneous
or transplantable tumors, included stunted growth, malformed teeth,2 broadened
facial bones, and protrusions of the eyes and tongue. The affected hamsters are somewhat
‘mongoloid’ in appearance (see Fig. 1). TOOLAN~ had previously reported similar abnormalities
resulting from inoculation of hamsters at birth with cell-free filtrates of human tumor
cells as well as certain tissues derived from human beings and rats carrying spontaneous
cancers. The syndrome has been described previously as ‘hamster mongolism’.297 This
syndrome is to be distinguished from other abnormalities which are produced in hamsters
by the same virus when inoculation occurs at developmentally different times. The use of
the term ‘mongolism’ should not be interpreted to imply any relation other than a superficial
resemblance to the human syndrome of the same name.$
We thought it of interest to investigate the behavioral characteristics of hamsters afflicted
with this syndrome. We have carried out a series of experiments in which hamsters with
virus-induced ‘mongolism’ have been compared with normal hamsters on several standard
behavioral tests.
In the first experiment, differences in learning curves for shuttle-box avoidance learning
were found, suggesting that affected hamsters were slow learners as compared to normal
controls. However, further avoidance conditioning experiments found that this difference,
while replicable, is highly variable. In two experiments a similar rate difference was found,
in a third no rate difference appeared, and the overall performance level of the affected
* This investigation was supported by P.H.S. Grant CA 06010-04, N.I.H.; Grant No. MH 0819841, the
Committee on Faculty Research, Dartmouth College; and (in part) by P.H.S. Research Career Program
Award I-K6-CA 22,652-Ol from the National Cancer Institute. We thank D. C. Downie for assistance.
t The authors, in the order listed, are now at Stanford University, Dartmouth Medical College, and
McGill University.
$ We wish to emphasize that we do not claim that the pathological state studied here is the same as
Down’s syndrome. While the term ‘mongolism’ seems to us to be a good descriptive title for the hamster
disease, by analogy with the human syndrome of the same name, the question of whether the hamster
disease resembles this, or any other human disease, in any aspect of its etiology, is completely open at this time.
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