We know from how animals behave that they can feel emotions like hunger and pain. They can also experience fear; which we know from seeing animals run away or shoot up a tree when they are faced with their predators. Animals show curiosity, like when they see something new for the first time.
The way animals greet other animals may also suggest some emotion, the noises that they make might suggest that they are happy to see each other. Monkeys have been shown to reconcile or ‘make friends again’ by kissing and hugging after they have fought with each other so it is possible that they can experience forgivenes.
There is some research to suggest that animals display their emotions through facial expressions and this has been seen in mice when they feel pain, joy in apes and in dogs. It is quite difficult to prove that animals feel emotions because we basically cannot know what or how an aminal feels. Scientists try to relate likely displays of emotion to how humans or other animals act under certain circumstances and then link this to common emotions that we know of or feel ourselves.