There are 24 species of hamster and they belong to the family Cricetidae. The Syrian, Russian Dwarf Campbell and Roborovski breeds are the most popular for pets. Hamsters enjoy exploring and use their whiskers to help them sense objects in their environment. A hamster's teeth never stop growing and they have a 'self-sharpening' system where the incisors grind against each other while gnawing, which wears the teeth down. Hamsters are nocturnal, with large eyes and a retina dominated by rods - the part of the eye that can function in lower light. Not all hamsters are sociable - in the wild, Syrian hamster adults generally live on their own in their burrows. Other species, such as the Russian dwarf, naturally live in groups.