Most people have seen examples of Contagious Yawning in humans. If one person in a group yawns (or convincingly feigns a yawn) it's often quite hard for others to resist. Current estimates are that 40-60% of human yawns are 'contagious'.
Contagious yawing has also been observed and scientifically documented in many other animals including :
Chimpanzees Bonobos Orangutans Macaque monkeys Gelada baboons Wolves Domestic dogs Rats Budgerigars Sheep Elephants
A team at Birkbeck College, University of London recently performed the first formal research to document Trans Species Yawn Contagion. (man > dog) (reference here) And another recent study here. (Anim Cogn. 12(6):833-837)
There are theories regarding the possible evolutionary advantages to yawn contagion, for example, that contagious yawning may be part of a neural network involved in empathy. reference
But a full explanation is currently lacking.