on behavioral sink). A 2007 study found rats to possess metacognition, a mental ability previously only documented in humans and some primates. Domestic rats differ from wild rats in many ways. They are calmer and less likely to bite; they can tolerate greater crowding; they breed earlier and produce more offspring; and their brains, livers, kidneys, adrenal glands, and hearts are smaller (Barnett 2002). Brown rats are often used as model organisms for scientific research. Since the publication of the rat genome sequence, and other advances, such as the creation of a rat SNP chip, and the production of knockout rats, the laboratory rat has become a useful genetic tool, although not as popular as mice. When it comes to conducting tests related to intelligence, learning, and drug abuse, rats are a popular choice due to their high intelligence, ingenuity, aggressiveness, and adaptability. Their psychology seems in many ways similar to that of humans. Entirely new breeds or "lines" of brown rats, such as the Wistar rat, have been bred for use in laboratories. Much of the genome of Rattus norvegicus has been sequenced. General intelligence Early studies found evidence both for and against measurable intelligence using the "g factor" in rats. Part of the difficulty of understanding animal cognition generally, is determining what to measure. One aspect of intelligence is the ability to learn, which can be measured using a maze like the T-maze. Experiments done in the 1920s showed that some rats performed better than others in maze tests, and if these rats were selectively bred, their offspring also performed better, suggesting that in rats an ability to learn was heritable in some way. As food Rat meat is a food that, while taboo in some cultures, is a dietary staple in others. Working rats Rats have been used as working animals. Tasks for working rats include the sniffing of gunpowder residue, demining, acting and animal-assisted therapy. Rats have a keen sense of smell and are easy to train. These characteristics have been employed, for example, by the Belgian non-governmental organization APOPO, which trains rats (specifically African giant pouched rats) to detect landmines and diagnose tuberculosis through smell. As pests Rats have long been considered deadly pests. Once considered a modern myth, the rat flood in India occurs every fifty years, as armies of bamboo rats descend upon rural areas and devour everything in their path. Rats have