In rats, sighs correlate with relief.

A deep breath, i.e., a sigh, in mammals is a ubiquitous respiratory phenomenon, whose function is to prevent airlessness (atelectasis) in hypoventilated parts of lungs. Sighs are also correlated with emotions, such as anxiety, anger and resentment in humans and obviously, judging from the expression--sigh of relief--in many languages, with relaxation or relief. If sighs are indiscriminately associated with opposite emotions, their role in social communication is doubtful. If, however, there is a selective facilitation of sighs by either fear, anxiety or relief, then they might, in addition to their respiratory role, function also as a social signal of a particular mood. To induce fear a stimulus was paired with a tail shock (5 times in a daily session). To provide a relief, another stimulus, presented before the expected shock (also 5 times per session), was followed by the omission of shock. In 16 rats experiencing fear during a Danger Stimulus (predictor of tail shock) and a relief during the Safety Stimulus (predictor of the non-occurrence of expected shock) the rate of sighing was 7.5 times higher during relief (180/h) than during fear (24/h), and 20 times higher than between trials (9/h), with all differences highly significant (p<0.001). This clear correlation of sighs with relief (from fear of the tail shock) supports our hypothesis that sighs in social mammals may function as signals of safety.