Figure 5
From: Phylogenetic review of tonal sound production in whales in relation to sociality
Regression analysis between independent contrasts of mean group size and mean number of inflection points. One conspicuous outlier (arrow) represents a contrast including the killer whale (Orcinus orca) 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 O. orca is removed from the analysis the regression between the two characters becomes stronger.