Humpback whales, unlike most mammalian species, learn new songs as adults. Populations of singers
progressively and collectively change the sounds and patterns within their songs throughout their lives
and across generations. In this study, humpback whale songs recorded in Hawaii from 1985 to 1995
were analyzed using self-organizing maps (SOMs) to classify the sounds within songs, and to identify
sound patterns that were present across multiple years. These analyses supported the hypothesis that
recurring, persistent patterns exist within whale songs, and that these patterns are defined at least in part
by acoustic relationships between adjacent sounds within songs. Sound classification based on acoustic
differences between adjacent sounds yielded patterns within songs that were more consistent from year
to year than classifications based on the properties of single sounds. Maintenance of fixed ratios of acoustic
modulation across sounds, despite large variations in individual sounds, suggests intrinsic constraints on
how sounds change within songs. Such acoustically invariant cues may enable whales to recognize and
assess variations in songs despite propagation-related distortion of individual sounds and yearly changes
in songs.