Rat

Rats are medium-sized rodents characterized by their long tails. While the term "rat" is used broadly, most stereotypical rats belong to the genus *Rattus*. There are 56 known species of rats in the world. Other related genera include *Neotoma*, known as pack rats, and *Dipodomys*, known as kangaroo rats. In common language, the word "rat" usually describes a large muroid rodent, while "mouse" describes a smaller one. However, these names are not taxonomically specific.

Many rat species are opportunistic survivors. Some are considered commensals, meaning they live alongside and near humans. While these commensal species are a minority, they can cause significant food losses, particularly in developing countries. Other rats are island endemics, meaning they live only on specific islands. These species often face the threat of becoming endangered due to habitat loss or competition from the brown, black, or Polynesian rat.

Wild rats can carry various zoonotic pathogens, which are diseases that spread from animals to humans. Examples include *Leptospira*, *Toxoplasma gondii*, and *Campylobacter*. Historically, rats played a role in the Black Death during the Middle Ages. It is traditionally believed that the microorganism *Yersinia pestis* caused this epidemic. The disease was carried by the tropical rat flea, *Xenopsylla cheopis*, which preyed on black rats in European cities. In this process, the rats acted as transport hosts.

The tail of a rat is a complex organ with three primary functions. First, it assists with thermoregulation, which is the process of managing body heat. The tail is hairless and thin-skinned but highly vascularized, meaning it has many blood vessels. This structure allows for efficient countercurrent heat exchange with the environment. Second, the tail aids in proprioception, or the sense of body position. High densities of muscle and connective tissue allow the rat to use its tail to shift its center of mass and maintain balance.

Third, the tail serves as a defense mechanism through a process called degloving. This is a nocifensive response, meaning it is triggered by acute pain. If a predator grabs a rat by the tail, the outermost layer of the integument, or skin, can detach. This allows the rat to escape, though it carries risks. Because the tail is so vascular, losing this skin layer can expose interior elements to bacterial or viral infections.

In terms of life history, rats reach sexual maturity at just 6 weeks of age. They reach social maturity between 5 and 6 months. The average lifespan varies by species, but many wild rats live only about one year due to predation. Since the late 19th century, humans have kept domesticated rats as pets. Most pet rats are variants of the brown rat, *Rattus norvegicus*. Selective breeding has created many varieties, including the hairless rex and the popular Dumbo rat, which has low, saucer-shaped ears.

Rats are also vital subjects in scientific research. In 1895, Clark University began using domestic albino brown rats to study diet and physiology. Rats show remarkable anatomical homology to humans, particularly in their cardiovascular systems. For example, the rat's aortic arches branch in ways very similar to human arteries. This makes them valuable models for studying human heart conditions.

Beyond biology, rats are used in psychological and genetic studies. They are known for their high intelligence, ingenuity, and adaptability. A 2007 study even found that rats possess metacognition, which is the ability to think about their own mental processes. Because their genomes have been sequenced, researchers can use them as precise genetic tools. This work has contributed greatly to our understanding of genetics, diseases, and the effects of drugs.