In daily life, we remember faces and voices of several known individuals. Similarly, mammals have been shown to remember calls and faces of known individuals after a number of years. Dr. Markus Boeckle and Dr. Thomas Bugnyar from the Department of Cognitive Biology of the University of Vienna show in a new article, published online on April 19, 2012 in Current Biology, that ravens differentiate individuals based on familiarity. Additionally, they discovered that ravens memorize relationship valence and affiliation. So far it has been unknown whether relationship valence can be remembered based on former positive or negative interactions. As response to calls of formerly known individuals ravens not only increase the number of calls but also change call characteristics dependent on whether they hear former "friends" or "foes." This suggests that ravens remember specific individuals at least for three years. The ability to change call characteristics is especially interesting. In the event they hear a "friendly" individual, they respond with a "friendly" call, whereas when listening to a "foe," they exhibit lower frequencies and rougher characteristics, an effect already described for other animal species. Ravens respond to calls from previously unknown individuals with even lower and rougher calls and thus try to increase their acoustically perceivable body size – also in humans, larger people have lower voices than smaller ones and angry humans rougher voices. While it was known that mammals change their voices based on the relationship they share with others, the researchers here were able to show for the first time that birds also change their calls according to relationship quality.
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