means of evaluating such disorders. the canadian psychiatrist heinz lehmann claimed that increases in yawning could predict recovery in schizophrenia. the impairment of contagious yawning can provide greater insight into its connection to the underlying causes of empathy. there is still substantial disagreement in the existing literature about whether or not yawn contagion is related to empathy at all. empathy is a notoriously difficult trait to measure, and the literature on the subject is confused, with the same species sometimes displaying a connection between contagious yawning and social closeness, and sometimes apparently not. different experimenters typically use slightly different measures of empathy, making comparisons between studies difficult, and there may be a publication bias, where studies which find a significant correlation between the two tested variables are more likely to be published than studies which do not. by revising in critical way the literature for and against yawn contagion as an empathy-related phenomenon, a 2020 review has shown that the social and emotional relevance of the stimulus (based on who the yawner is) can be related to the levels of yawn contagion, as suggested by neurobiological, ethological and psychological findings. therefore, the discussion over the issue remains open. non-human seeing a dog & horse & man yawn, makes mefeel how much all animals are built on one structure. charles darwin, notebook m (1838), 65 a white tiger yawning mammals, birds, and other vertebrates yawn. in animals, yawning can serve as a warning signal. charles darwin's book, the expression of the emotions in man and animals, mentions that baboons yawn to threaten their enemies, possibly by displaying large canine teeth. similarly, siamese fighting fish yawn only when they see a conspecific (same species) or their own mirror-image, and their yawn often accompanies aggressive attack. guinea pigs also yawn in a display of dominance or anger, displaying their impressive incisor teeth. this is often accompanied by teeth chattering, purring and scent marking. adelie penguins employ yawning as part of their courtship ritual. penguin couples face off and the males engage in what is described as an "ecstatic display", opening their beaks and pointing their faces skyward. this trait has also been seen among emperor penguins. researchers have been attempting to discover why these two different species share this trait, despite not sharing a habitat. snakes yawn, both to realign their jaws after a meal and for respiratory reasons, as their trachea can be seen to expand when they do this. dogs, and occasionally cats, often yawn after seeing people yawn and when they feel uncertain. dogs demonstrate contagious yawning when exposed to human yawning. dogs are very adept at reading human communication actions, so it is unclear if this phenomenon is rooted in evolutionary history or a result of domestication. fish can also yawn, and they will increase this behavior when experiencing a lack of oxygen. socially contagious yawning has been observed in budgerigars, and anecdotally when tired in other parrot species. culture a soldier hides his yawn from his lady companion in this detail from a painting by oscar bluhm titled ermüdende konversation, or "wearisome conversation". some cultures lend yawning moral or spiritual significance. an open mouth has been associated with letting good immaterial things (such as the soul) escape or letting bad ones (evil spirits) enter, and yawning may have been thought to increase these risks. covering the mouth when yawning may have been a way to prevent such transmission. exorcists believe yawning can indicate that a demon or possessive spirit is leaving its human host during the course of an exorcism. thus, covering one's mouth has been conceived as a protective measure against this. yawning has also been described as disrespectful (when done before others) or improper (when done alone). for example, in his commentary on al-bukhari's hadith collection, ibn hajar, an islamic theologian, mentions that yawning, in addition to its risks of letting demons enter or take hold of one's body, is unbefitting for humans as it makes them look and sound like dogs by crooking men's upright posture and making them howl: he likened excessive yawning to the howling of dogs to deter people from it and to make it seem repulsive, for when a dog howls, it raises its head and opens its mouth widely, and the yawner resembles it when he yawns too much. this is why the devil laughs at those who yawn, for he succeeds in playing with them and deforming them. superstitions regarding the act of yawning may have arisen from concerns over public health. polydore vergil (c. 1470–1555), in his de rerum inventoribus, writes that it was customary to make the sign of the cross over one's mouth, since "alike deadly plague was sometime in yawning, wherefore men used to fence themselves with the sign of the cross ... which custom we retain at this day." yawning is often perceived as implying boredom, and yawning conspicuously in another's presence has historically been a faux pas. in 1663 francis hawkins advised, "in yawning howl not, and thou shouldst abstain as much as thou can to yawn, especially when thou speakest". george washington said, "if you cough, sneeze, sigh, or yawn, do it not loud but privately; and speak not in your yawning, but put your handkerchief or hand before your face and turn aside." these customary beliefs persist in the modern age. one of mason cooley's aphorisms is "a yawn is more disconcerting than a contradiction." a loud yawn may even lead to penalties for contempt of court.