The New World monkeys or Platyrrhines are the two families of primates that are found in Central and South America, the Callitrichidae and Cebidae. Both families differ from the Old World monkeys and apes in having long, often prehensile tails. The name means "flat nosed", and this is how they are distinguished from Old World monkeys. Platyrrhine noses are flatter, with side facing nostrils, compared to the narrow noses and downward facing nostrils of Old World Monkeys. Many are small, arboreal and nocturnal, so our knowledge of them is less comprehensive than that of the more easily observed Old World monkeys. Unlike most Old World monkeys, many New World monkeys form monogamous pair bonds, and show substantial paternal care of young.