Primates are, without doubt, the most intelligent mammals there are. Their brain, especially in chimps, orangutans and gorillas, is big and really developed. They also have a wide range of facial expressions, which are quite similar to human expressions, and they use the five senses in their totality.
The two main communication forms that primates use though are: visual communication and vocal communication.
In regard to visual communication, we should mention that primates in general have good sight, and that they use it to communicate to each other in the distance. They do this through postures, gestures, and movements. For example, a gorilla can tell another he is comfortable with his presence just by lying down, or can press his lips to show he's uncomfortable and in an aggressive mood. This posture or gesture will be noticed through sight.
Regarding vocal communication, we should mention that, even though their sight is good, primates usually communicate through distance by auditive signs. They do this through different sounds, which vary from species to species. They can make warning shouts, or scream for help, when a predator is near, or even sounds of joy or invitations to play (among the youngsters).
Needless to say, these non-verbal ways of communicating that primates possess, follow certain functions that are parallel to humans' - according to British linguistic Deirdre Wilson - such as recognizing an individual's status, inquiring about aliment, imitating, controlling behavior, keeping contact, recognizing an individual, inviting to hunt or to play, or threatening someone, among others.
Because of their communicative skills and their complex social organization forms, we could say that primates are really intelligent creatures in the animal kingdom. They are social, skillful, and imitative beings, and they're really close to man in the phylogenetic scale. All these facts have made many scientists wonder if such smart animals can learn the human language.
Well, the answer is no. At least regarding the verbal language, as their oral physiology doesn't allow them to produce speech sounds. Men's attempts at teaching chimps, orangutans, and gorillas to talk have all failed, then, but they have also been scientific achievements - since now we know their larynx is really different from humans'.
Another reason why primates cannot talk is their brain; it doesn't contain the Broca and Wernicke areas, which are essential to speak. They would need even more elaborate brains to perform this human skill then.
As an alternative, many researchers have tried to teach primates to use sign language. Primates are plantigrades, and have prehensile hands, with five fingers in each, including opposable thumbs. In short, they are the perfect subjects for the experiment.