Temporal and geographical variation in songs of humpback whales, Megaptera novaeangliae: synchronous change in Hawaiian and Mexican breeding assemblages

Abstract

Humpback whale song, a male breeding display, shows a remarkable degree of similarity among distant breeding assemblages, despite constant progressive change. It has been hypothesized that whales maintain continuity through cultural transmission via migratory movements of males. We examined songs of whales breeding off Hawaii and Mexico to determine whether they changed similarly in both areas during the course of a breeding season. Songs recorded off Kauai, Hawaii (11 individuals) and Isla Socorro, Mexico (13 individuals) during winter and spring of 1991, were compared qualitatively and quantitatively. We measured 44 acoustic variables describing all known levels of song structure for each singer and we grouped these variables into six categories. We used two-factor analyses of variance to assess change across the season in each area, comparing the two regions and two 3-week periods (January/February and April). Twenty-seven variables changed significantly during the 12-week study in at least one area. Variables within categories displayed similar trends of change. Time and frequency characteristics describing the structure of song elements (units and phrases) changed synchronously in each area, with 21 of 25 variables displaying significant differences between periods and no interaction with region. Structures of song patterns, as defined by frequency of occurrence and number of unit and phrase types, changed differently in each area, with five of 12 variables indicating a significant interaction between region and period. Our results may suggest cultural transmission during the season, since many variables changed in similar manners. We propose an alternative hypothesis, that whales may be predisposed to gradually change certain features of song independently of cultural influences; change of structural elements may be governed by a discrete set of rules, or according to an innate template. Therefore, continuity of song patterns across the ocean basin may be due to a combination of mechanisms, only partially involving cultural transmission. We assess these hypotheses in relation to humpback whale behaviour and population structure, and cultural transmission and evolution of avian song.