image: Yawning displays an adult male's impressive and potentially dangerous canine teeth. Yawns are often used as a threat between males. When males yawn from fatigue or nervousness, they often pull their lips over their teeth as much as they can or avoid facing males that are likely to respond to the yawn as a threat. This image relates to an article that appeared in the July 15, 2011, issue of Science, published by AAAS. The study, by Laurence R. Gesquiere of Princeton University in Princeton, N.J., and colleagues was titled, "Life at the Top: Rank and Stress in Wild Male Baboons." view more
Credit: Image courtesy of Jeanne Altmann