Md. Hauser, THE EVOLUTION OF NONHUMAN PRIMATE VOCALIZATIONS - EFFECTS OF PHYLOGENY, BODY-WEIGHT, AND SOCIAL-CONTEXT, The American naturalist, 142(3), 1993, pp. 528-542
E. S. Morton proposed that, in birds and mammals, individuals tend to
produce low-frequency atonal vocalizations in highly aggressive situat
ions, whereas they typically produce high-frequency tonal vocalization
s during nonaggressive or fearful situations. This hypothesis, referre
d to as the ''motivation-structural (MS) rules,'' is based on two assu
mptions: the frequency of a vocalization is negatively correlated with
body weight, and large animals are dominant over smaller animals, and
thus aggressive vocalizations tend to have a lower pitch than fearful
vocalizations. The relationship between body weight and frequency is
examined using data on 36 nonhuman primate species representing 23 gen
era and 474 vocalizations. Results show that there is a statistically
significant negative correlation between body weight and frequency. la
rger species produce relatively lower-pitched vocalizations than small
er species. A test of Morton's MS rules provided overall support for t
he predicted relationship between motivational state and frequency (i.
e., high-frequency calls were produced by fearful individuals, and low
-frequency calls were produced by aggressive individuals) but no suppo
rt for the expected relationship between motivational state and tonali
ty. However, the motivational state-frequency pairing was confounded b
y the fact that some taxonomic groups (Platyrrhini and Catarrhini) sho
wed a much stronger level of association than other groups (Prosimii a
nd Hominoidea). In summary, therefore, the nonhuman primate data provi
de only partial support for MS rules. At least three factors may have
influenced the outcome of the current test. First, in some species, mo
tivational state may be more closely associated with other acoustic pa
rameters than absolute frequency and tonality. Second, the acoustic st
ructure of nonhuman primate vocalizations is, at least in some cases,
more closely associated with an external referent than with the caller
's internal state. And third, features of the species-typical habitat
have had direct selective effects on signal structure, optimizing for
effective propagation through the environment.