Some have speculated about the origin of this unusual relationship, claiming that the dog witnessed the death of the mother's monkey, but no one knows for sure, according to a local Malaysian newspaper.
"I think the choices animals make in cross-species relationships are the same as they'd make in same-species relationships. Some dogs don't like every other dog. Animals are very selective about the other individuals who they let into their lives," Dr. Marc Bekoff, who studies animal emotions, says in an interview with Slate.
Bekoff points to this video of a dog and dolphin swimming and playing together as a prime example of this type of special bond.