Chimps will bare their teeth in a grin-like facial gesture when frightened.
http://www.lessonsforhope.org/abc/show_description.asp?abc_id=41<snip>
Jane's observation:
When a chimpanzee is suddenly frightened he frequently reaches to touch or embrace a chimpanzee nearby, rather as a girl watching a horror film may seize her companion's hand. Both chimpanzees and humans seem reassured in stressful situations by physical contact with another individual. Once David Greybeard caught sight of his reflection in a mirror. Terrified, he seized Fifi, then only three years old. Even such contact with a very small chimp appeared to reassure him;
gradually he relaxed and the grin of fear left his face.