Cultural turnover among Galápagos sperm whales

Abstract

While populations may wax and wane, it is rare for an entire population to be replaced by a completely different set of individuals. We document the large-scale relocation of cultural groups of sperm whale off the Galápagos Islands, in which two sympatric vocal clans were entirely replaced by two different ones. Between 1985 and 1999, whales from two clans (called Regular and Plus-One) defined by cultural dialects in coda vocalizations were repeatedly photo-identified off Galápagos. Their occurrence in the area declined through the 1990s; by 2000, none remained. We reassessed Galápagos sperm whales in 2013-2014, identifying 463 new females. However, re-sighting rates were low, with no matches with the Galápagos 1985-1999 population, suggesting an eastward shift to coastal areas. Their vocal repertoires matched those of two other clans (called Short and Four-Plus) found across the Pacific but previously rare or absent around Galápagos. The mechanisms behind this cultural turnover may include large-scale environmental regime shifts favouring clan-specific foraging strategies, and a response to heavy whaling in the region involving redistribution of surviving whales into high-quality habitats. The fall and rise of sperm whale cultures off Galápagos reflect the structuring of the Pacific population into large, enduring clans with dynamic ranges. Long-lasting clan membership illustrates how culture can be bound up in the structure and dynamics of animal populations and so how tracking cultural traits can reveal large-scale population shifts.

Keywords: Physeter macrocephalus; culture; demographic change; dialect; population ecology; social structure.

Figure 1.

Re-sightings of individual female and immature sperm whales between (a) 1985 and 2004 in the eastern Tropical Pacific, and (b) 2013 and 2014 off Galápagos. Dashed circles loosely indicate study areas, with numbers indicating total of photo-identified individuals. Numbers by arrows indicate match of individuals between areas.

Figure 2.

Encounter rates of sperm whale off the Galápagos Islands over 30 years across clans. Rates were higher in early 1980s, started declining during the 1990s and, after a hiatus in 2000s, started rising again. Colour code indicates clan membership (figure 3) of whales for which both photo-identification and acoustic data were available in that year. Whiskers represent standard errors (s.e.). Asterisks indicate years with no dedicated surveys off Galápagos (the larger gap in the 2000s was mainly motivated by a lack of opportunistic sightings in the area), but in some of these years there were surveys in surrounding areas (electronic supplementary material, tables S1–S3).

Figure 3.

Coda repertoire similarity and clan structure of sperm whale groups from the Pacific between 1985 and 2014. (a) Hierarchical clustering dendrogram (CCC = 0.896) depicts the multivariate similarity (Euclidean distances on absolute inter-click intervals) among coda repertoires of photo-identified groups of sperm whales (branches). Colour code and clan names follow original results [14]; ‘g’s indicate groups observed off Galápagos; dashed branches indicate new groups observed in 2013–2014; arrows indicate mean similarity between and within clans; numbers besides nodes indicate the number of replications (out of 100) in bootstrap analysis. (b) Frequency of coda types containing up to 12 clicks (rows) classified into discrete types for each photo-identified group (columns). Shades of grey indicate the frequency of occurrence of coda types in a given group repertoire; coda type labels represent rhythm (electronic supplementary material, figure S2); numbers under columns indicate total number of recorded codas from each group of whale used in the continuous analysis; numbers on the right indicate the total codas per type used in the categorical analysis.