Human yawning elicited 24 yawns from the juvenile chimpanzees and zero yawns from the infants. Chimpanzees appear to develop susceptibility to interspecies contagious yawning as they grow from infant to juvenile, possibly due to their developing ability to empathize with the person yawning.
Credit: Madsen EA, Persson T, Sayehli S, Lenninger S, Sonesson G (2013) Chimpanzees Show a Developmental Increase in Susceptibility to Contagious Yawning: A Test of the Effect of Ontogeny and Emotional Closeness on Yawn Contagion. PLoS ONE 8(10): e76266. doi:10.1371/journal.pone.0076266
[ Read the Article: Human Yawns Are Contagious To Chimps, But Only Older Ones ]