Figure 1. A laboratory-bred wild rat.

R. norvegicus is a relatively small rodent with a brown fur and sparsely haired tail. Its head is stout with a pointed muzzle and darkly pigmented, slightly bulging eyes. Characteristic of all rodents, rats have large and continuously growing front teeth. The durable enamel on the front surface of these teeth contains an iron-based pigment, which gives them an orange color. This individual belongs to the Warsaw Wild Captive Pisula Stryjeck (WWCPS) colony in Poland. 

Image credit: Klaudia Modlinska and Rafał Stryjek.