Aside from being incredibly sad, Koko's story resonates with people because it suggests something that primate researchers have been trying to prove for a while: that empathy isn't just an emotion that humans are capable of feeling. A new study in this week's "PeerJ" examines the relationship between human and ape empathy. Researchers looked at bonobos - man's closest evolutionary relative - and examined them under the context of emotional contagion, which is the ability to transfer emotion from person-to-person using facial expressions.Have you ever seen someone yawn and suddenly felt the desire to yawn? In fact, you might be feeling like yawning right now, just from reading this. This phenomenon, known as the "yawn contagion", is considered one of the simplest examples of an emotional contagion. A team of researchers observed both humans and bonobos over the course of five years and confirmed that these are the only two species where yawn contagion seems more likely to occur between individuals who share a strong emotional bond. Therefore, it the research would suggest, they are the only two species where yawn contagion seems to follow an empathic trend. Have you ever developed an emotional bond with a pet or any other (non-human) animal? Be sure to share you experiences in the comments below. Read More:Yawn Contagion in Humans and Bonobos: Emotional Affinity Matters More Than Species (PeerJ.com):"In humans and apes, yawn contagion echoes emotional contagion, the basal layer of empathy."