<p><b>Copyright information:</b></p><p>Taken from "Phylogenetic review of tonal sound production in whales in relation to sociality"</p><p>http://www.biomedcentral.com/1471-2148/7/136</p><p>BMC Evolutionary Biology 2007;7():136-136.</p><p>Published online 10 Aug 2007</p><p>PMCID:PMC2000896.</p><p></p> One conspicuous outlier (arrow) represents a contrast including the killer whale () which forms relatively small social groups but produces highly modulated whistles. It has been proposed that the killer whale uses whistles in a manner different from any other delphinid to indicate motivational state. That multiple factors are at work shaping tonal sounds in cetaceans may obscure and make difficult to discover true co-evolutinary histories of characters. Accordingly when is removed from the analysis the regression between the two characters becomes stronger
