The fat sand rat (Psammomys obesus) is a terrestrial mammal from the gerbil subfamily that is mostly found in North Africa and the Middle East, ranging from Mauritania to the Arabian Peninsula. This species usually lives in sandy deserts, but may also be found in rocky terrain or saline marsh areas. Fat sand rats are very selective in their diet, only eating stems and leaves of plants from the family Amaranthaceae, more commonly known as the amaranth family. In captivity, fat sand rats can become obese and rapidly develop diabetes-like symptoms when fed the diet typically given to other rodents. P. obesus has an average lifespan of 14 months in the wild and 3–4 years in captivity. Its period of activity is shortest in the summer, starting in the early morning and finishing an hour to several hours after.

Individuals with greater body masses have been found to forage for longer periods of time than those with a smaller body mass. When foraging, a fat sand rat will cut pieces away from plants and bring them back to their burrows, where they either eat the plant in the burrow or around the entrance, or store the plant for later consumption, which is called hoarding. As a result of this strategy, fat sand rats do not need to drink free-standing water.

Aside from foraging, the fat sand rat also spends time outside the burrow exploring. It is atypical in its exploratory behaviour since when it encounters a novel environment it exhibits perimeter patrolling behaviours, moving in straight lines along the border of an environment, rather than engaging in looping or home base behaviours usually displayed by rodents. Typically, members of the same sex actively avoid interacting, but when females do encounter each other, they engage in aggressive behaviours, and will attack each other. Aside from copulation, the type of interaction that results between males and females differs depending on which individual approaches the other. As such, their preferred type of habitat, and the distribution of fat sand rats in a particular habitat changes throughout the year in response to the different growing season of plants in the wadi beds or terraces that they inhabit. Wadi beds are typically populated with Amaranthaceae shrubs, and have dense vegetative cover, while terraces are very sparsely vegetated, leaving the habitat exposed. During the winter, fat sand rats prefer the wadi habitat as a result of the vegetative growth during this time, but after a wet autumn, more individuals will burrow in a terrace habitat even when population density is low, as opposed to after a dry autumn, when individuals only burrow in terrace habitats when the population density is high. The fat sand rat also disturbs vegetation cover while burrowing, further altering its environment. Mounds of active burrows have significantly less percent cover from vegetation than abandoned burrows and undisturbed land near abandoned burrows. Depending on the type of habitat they have burrowed in, they are exposed to different risks concerning predation. When mothers start lactating, their body energy increases, but towards the end of the lactation period, they begin to utilize stored energy instead of increasing body energy, resulting in a decrease in body mass. The growth rate of pups is maximized when the litter is small and the mother eats plants with a higher water content. Therefore, they have been used as an animal model for studies on diabetes and obesity. Sequencing of the complete nuclear DNA genome of Psammomys obesus showed that the Pdx1 homeobox gene, a transcriptional activator of insulin, has undergone massive amino acid sequence changes in sand rat and other gerbil species driven by accumulation of GC-biased mutations (changes from A or T nucleotides to G or C nucleotides). However, a causative link has not been proven. Many regions of the sand rat genome, not only the Pdx1 gene, have been affected by accumulation of GC-biased mutations.

Because they are diurnal, fat sand rats are also used as models for human seasonal affective disorder.

The presence of fat sand rats in North Africa and the Middle East is of healthcare importance. Leishmania kDNA has been discovered in this rodent using molecular studies, suggesting the species can host the parasite that causes leishmaniasis in humans.

These animals have been studied extensively for their remarkably efficient kidneys: they can produce very concentrated urine, which enables them to eat halophyte plants and survive extreme heat and lack of water in their desert habitat.

References

  • e-Gerbil
  • Gerbil information page
  • University of Michigan Museum of Zoology