DIALECTS OVER SPACE AND TIME: CULTURAL IDENTITY AND EVOLUTION IN SPERM WHALE CODAS

Abstract

The overarching goal of my thesis is to better understand how sperm whale (Physeter macrocephalus) dialects vary over space and time, and to quantify the implications of these dynamics on cultural identity and evolution in sperm whale vocalizations. Female and immature sperm whales live in matrilineal social units that are stable over time, and different social units only associate if they belong to the same cultural clan. Clans forage, move, dive, associate, and distribute differently, but the primary way we distinguish them is through their unique dialects. These dialects are comprised of stereotyped patterns of clicks, called codas, and my thesis seeks to clarify the interplay between these social vocalizations and cultural identity in sperm whale clans. Inspired by rhythm in codas, I first survey the literature for evidence of rhythm in cetacean vocalizations and use a comparative lens to theorize about the functional roles it plays. I show that rhythm is common in cetacean vocalizations, but that it may be used in different behavioral contexts by mysticetes and odontocetes. In sperm whales, vocal rhythm is apparent across contexts, suggesting that it is a fundamental feature of communication. Next, I introduce a new method (IDcall) that detects putative biological structure in acoustic datasets using characteristic, repeated call types. I show that IDcall’s underlying theory is broadly applicable and can be used to detect sperm whale clans from codas, wren subspecies from songs, and cricket species from songs. Using acoustic data from 25 locations spanning 42 years, I then investigate the spatial and temporal dynamics of sperm whale clan dialects. In the spatial domain, I document the presence and distribution of seven clans in the Pacific Ocean, and provide empirical evidence that certain coda types function as symbolic markers of sperm whale clan identity, like ethnic markers in humans. In the temporal domain, I leverage long-term research efforts in the Mediterranean, eastern Caribbean, and Galápagos Islands to show that the fine-scale structure of coda types can change over decadal timespans within clans. Collectively, these findings illustrate the interplay between vocalizations and sociality in sperm whales, and emphasize the need for a global, multi-cultural approach to study and conserve this global, multi-cultural species.