Dogs were chosen for the experiments at Brigham Young University, reported in the Journal of Developmental Psychology, because they are a species that communicates with body language and with sound - as we do.
Here's a quote from a report at Physorg.com:
In the experiment, the babies first saw two different pictures of the same dog, one in an aggressive posture and the other in a friendly stance. Then the researchers played - in random order - sound clips of a friendly and an aggressive dog bark...While the recordings played, the 6-month-old babies spent most of their time staring at the appropriate picture. Older babies usually made the connection instantly with their very first glance.
I'm glad to have come across this Physorg site. It looks interesting.