On the Occurrence and Significance of Motivation-Structural Rules in Some Bird and Mammal Sounds

Agonistic Screams in Wild Chimpanzees (Pan troglodytes schweinfurthii) Vary as a Function of Social Role.

“…Each data point represents the median value for three successive calls measured at the beginning of a scream bout from a single individual (r 2 refers to the relative amount of variance explained by the respective variable). Morton's (1977) idea of motivational-structural rules, which provide useful clues as to the possible evolutionary mechanisms underlying the diversification of these two scream types. In line with these rules, the screams of aggressive chimpanzees are shorter and contain larger elements of decreasing frequency patterns than do the screams of attacked chimpanzees.…”

‘Protest’ Sounds of a Grasshopper: Predator‐Deterrent Signal?

“…The sound itself has a wide frequency spectrum such as that characterizing alarm calls of birds (Marler 1957;Morton 1977 …”

Auditory Communication in Domestic Dogs

“…As such, it is very likely that the functional decisions of receivers, based on this information, may place additional selection pressure on signals at the level of their production. In an influential paper based on a comparative study of vocalisations used in agonistic displays in a range of mammalian and avian species, Morton (1977) suggested that audible frequency differences in vocalisations reflect ritualised signalling: animals with aggressive motivation produce low-pitched, broadband vocalisations (such as growls and hisses), whereas animals with affiliative or submissive motivation produce high-pitched vocalisations (such as whimpers and whines). This theory, known as Morton's motivation-structural code, is based on the observation across several species that aggressive and dominant animals seek to project (both visually and acoustically) a larger impression of body size and greater threat, whereas friendly or submissive animals seek to project a smaller impression of body size and lesser threat (Morton, 1977;Ohala, 1984;Owings & Morton, 1998).…”

“…While mechanisms for visual maximisation or exaggeration of body size such as piloerection is relatively common among mammals, few acoustic equivalents have been reported (although see the retraction of the larynx in red deer; Reby & McComb, 2003). The use of lower frequency vocalisations could thus be representative of aggressive motivation not just across call types as described by Morton (1977) but within call types as a ritualised function of the acoustic size code (Ohala, 1984(Ohala, , 1996Taylor & Reby, 2010).…”