The Mongolian gerbil or Mongolian jird (Meriones unguiculatus) is a small rodent belonging to the subfamily Gerbillinae.Body size is typically 110–135mm, with a 95–120mm tail, and body weight 60–130g, with adult males larger than females.The animal is used in science and kept as a small house pet. Their use in science dates back to the latter half of the 19th century, but they only started to be kept as pets in the English-speaking world after 1954, when they were brought to the United States. However, their use in scientific research has fallen out of favor.
Gerbillines are small to medium-sized rodents. They range in length from 50 to 200 mm, with tails measuring 56 to 245 mm. They weigh between 10 and 227 grams. Gerbillines vary in the amount to which they are sexually dimorphic; even within a species males may be heavier than females in one population and the sexes may be the same size in another population (Sinai et al. 2003). Most gerbillines have well-furred, long tails and are modified for saltatorial locomotion, with long, narrow hind feet. Some species are cursorial. Gerbillines are generally slender animals with long claws. They may have long or short ears. Their pelage is long, thick, and soft or short and harsh. Some have tufted tips on their tails. Fur color varies widely, and may be reddish, mouse gray, yellowish, clay-colored, olive, dark brown, orangish, sandy buff, or pinkish cinnamon on the dorsal surface. The underparts are generally paler shades of gray, cream, or white. Some species have whitish spots on their heads, especially behind the ears.