Rat skulls are similar to beaver skulls, since both animals are rodents. Rats are medium-sized, omnivorous rodents. There are dozens of species of rats worldwide, many of which are endemic to New Guinea. Rats have a very short gestation period which makes them ideal for medical and scientific research. Accordingly, domesticated rats have been studied more extensively than any other mammal. Rats are generalist omnivores and feed on a broad variety of food sources. They prey on bird's eggs, insects, fruits, nuts, fungi, and a wide variety of cultivated crops.