Song hybridization events during revolutionary song change provide insights into cultural transmission in humpback whales

Abstract

Cultural processes occur in a wide variety of animal taxa, from insects to cetaceans. The songs of humpback whales are one of the most striking examples of the transmission of a cultural trait and social learning in any nonhuman animal. To understand how songs are learned, we investigate rare cases of song hybridization, where parts of an existing song are spliced with a new one, likely before an individual totally adopts the new song. Song unit sequences were extracted from over 9,300 phrases recorded during two song revolutions across the South Pacific Ocean, allowing fine-scale analysis of composition and sequencing. In hybrid songs the current and new songs were spliced together in two specific ways: () singers placed a single hybrid phrase, in which content from both songs were combined, between the two song types when transitioning from one to the other, and/or () singers spliced complete themes from the revolutionary song into the current song. Sequence analysis indicated that both processes were governed by structural similarity rules. Hybrid phrases or theme substitutions occurred at points in the songs where both songs contained "similar sounds arranged in a similar pattern." Songs appear to be learned as segments (themes/phrase types), akin to birdsong and human language acquisition, and these can be combined in predictable ways if the underlying structural pattern is similar. These snapshots of song change provide insights into the mechanisms underlying song learning in humpback whales, and comparative perspectives on the evolution of human language and culture.

The extension of biology through culture

“…Hundreds of laboratory experimental studies have demonstrated social learning and transmission in a wide variety of animals. Social learning is now extensively documented in mammals (29), with a particular intensity of research studies in primates (30)(31)(32)(33) and cetaceans (34)(35)(36)(37), in birds (38)(39)(40)(41), in fish (42), and in insects (43,44). The fact that social learning has been shown to play important roles spanning multiple functional contexts (25)(26)(27)(28) suggests that many animals are not simply acquiring one or a few behavioral patterns socially, but rather that social learning is central to their acquisition of adaptive behavior.…”

“…For example, archaeological excavations have shown remains of nut-cracking materials dated to 4,300 y below the surface of the Taï Forest, where modern chimpanzees continue this practice, absent in most parts of Africa (31,46); 27 y of observations have documented the spread from a few to over 600 humpback whales of a new form of hunting technique, lob-tail fishing (47); and new humpback songs have been found to emerge, spread quickly to whole populations, and pass in waves across the Pacific in consecutive years (36,37).…”

“…Garland et al (37) focus on the transmission of complex song in humpback whales, presenting a painstaking and highly illuminating analysis of the process of song hybridization during the remarkable periods of "cultural revolution" found in this species. Behavioral hybridization has often been highlighted as a phenomenon differentiating cultural from genetically based evolution, although hybridization is far from unknown at the species level and recent research increasingly suggests that it has been much more common than previously suspected at the level of genetic transfer (51).…”

“…Cultural evolution may display analogs of organic evolution outlined above, but it is also different in many fundamental respects, further extending the scope of the evolutionary processes that shape biological systems (14,17). Notably, transmission is not only vertical, as in genetic inheritance from parent to offspring, but can be horizontal, between unrelated peers, or oblique, from unrelated individuals in the parental generation (14); moreover, because this involves neurally based learning rather than genetic change, such transmission can be quite rapid, as well illustrated by the case of "revolutions" in the songs of humpback whales, which may change annually yet quickly come to be sung by whole populations (37), and in a variety of human and animal cases further explored in this issue.…”

Chipper: Open‐source software for semi‐automated segmentation and analysis of birdsong and other natural sounds

“…Acoustic communication is one of the few natural behaviours that can be easily recorded, digitized and studied (Catchpole & Slater, ; Cocroft & Rodríguez, ; Garland, Rendell, Lamoni, Poole, & Noad, ; Ryan & Guerra, ). Often, behavioural studies involve laboratory observations, which can lead to fundamental insights but may disrupt natural animal behaviour (Fehér, Wang, Saar, Mitra, & Tchernichovski, ; Marler & Peters, ; Searcy, ).…”