The active space of sperm whale codas allows for communication within and between social units

Jacobs et al. (2024) recently published a powerful analysis of apparent source levels from acoustically localised sperm whale codas, using it to estimate that the active space of these vocalisations is around 4 km. This is an important step forward in our understanding of how these vocalisations may function, and we congratulate the authors on the rigour of their analysis. However, we have disagreements with the framing and interpretation of these results that we felt were best addressed in a letter.Our motivations for writing are twofold. The first is to correct the record regarding how Jacobs et al. (2024) frame their work. The second is to critically evaluate their apparent claim that a 4 km active space means that codas do not function to communicate between units.First, Jacobs et al. (2024) motivate their hypothesis that the active space of codas is ‘on a par’ with the acoustic footprint of a social unit by asserting that we have made specific claims about communication over a ‘long range’ or ‘larger distances’ (neither are specified) [Introduction, para 5: ‘Such a small active space is, in turn, incompatible with the proposed long-range coda communication for broadcasting clan identity beyond the unit (Gero et al., 2016a,b; Rendell and Whitehead, 2003).’ and Introduction, para 6: ‘Rendell and Whitehead (2003) suggest that identity codas could mediate the social segregation observed between clans by broadcasting a unit's clan identity to other units over larger distances before engaging in a close-proximity social interaction.’], citing a paper of ours (Rendell and Whitehead, 2003). We did not in fact make any claims about long range communication using codas in that paper. If there are passages that could give the impression of the reported claims, we hope Jacobs et al. (2024) can identify them so we can be clearer in our expression going forward. The one field-derived estimate they quote of 600 m (Weilgart and Whitehead, 1997) is <1/6 the active space estimates they present, while in a book published the same year as the cited paper, one of us inferred an active space for codas of ∼2 km (Whitehead, 2003, p. 135), and we were part of a collaboration that documented short range (<400 m) coda exchange interactions and argued that these support a primary functional hypothesis of social bonding within units (Schulz et al., 2008). We have never suggested that codas are used over long (presumably tens of kilometre or more) ranges.Second, a 4 km active space does not preclude codas functioning to communicate between social units or groups, a claim by Jacobs et al. (2024) that we contest [Title: ‘…information for intra-unit communication’; Abstract: ‘We conclude that while sperm whale codas may contain information about clan affiliation, their moderate active space shows that codas are not used for long range acoustic communication between units and clans’; Introduction, para 6: ‘Such a small active space is, in turn, incompatible with the proposed long-range coda communication for broadcasting clan identity beyond the unit’; Discussion, para 1: ‘Our estimate … strongly supports our hypothesis that codas are used primarily for within-unit communication’]. Clearly it matters here what is meant by ‘long range’ in this context – without specifying, Jacobs et al. (2024) present it as synonymous with communication between units. While we agree that within-unit communication is likely a major function of codas, we cannot set aside another important feature of sperm whale social structure – units of different clan dialect almost never form coordinated groups in our primary study areas in the Southeast Pacific (Rendell and Whitehead, 2003), as well as in the Caribbean, where habitat use is organised on vocal clan lines (Vachon et al., 2022). This is despite the presence of units of different clans in the same area (Whitehead and Rendell, 2004; Vachon et al., 2022), and that, in the Southeast Pacific, groups consist of a temporary accretion of about 2–3 social units over time scales of 2 to a few days, so that units are likely regularly making decisions as to which other units to group with (Whitehead, 2003). How do they do this if they are not using coda dialect information specifically to make decisions about which units to associate with?A likely scenario, based on the way that we ourselves encounter sperm whale groups acoustically in the field, is that the first cue received between units when they come into proximity is the louder echolocation clicks. This tells listeners that sperm whales are present, but only when they approach closer do they come within the active space of their codas. While a unit footprint might reach 4 km across, codas produced by animals on the periphery will be detectable up to 4 km away from the edge of the unit. At that point, both units can decide whether to approach or avoid while they are still up to 4 km from each other, obviating the need for any close interaction. We have used the same equipment and methods in both the Caribbean and Southeast Pacific study areas, and our field experience is of similar active space in both.While codas might have first evolved primarily for bonding within social units, once the vocal signals are emitted, they become public information identity cues. The natural selection consequences of other social units using this information would make them subject to processes of signal evolution in the between-unit context also (for example, by increasing the chances that units of the same dialect group can find each other and benefit from increased protection). There is therefore no reason to approach the question of whether codas function within or between units from an either/or perspective – evolution can operate in both contexts. Pluripotent signals, where one signal structure has multiple functions, are a known feature in animal communication (Hebets et al., 2016) and nothing in the evidence Jacobs et al. (2024) present precludes this. Duets and choruses in birds, for example, have flexible, context-dependent function (Mennill and Vehrencamp, 2008) and we see no reason the same should not be true for sperm whales, where coda exchanges can often be characterised as duets or choruses (Schulz et al., 2008).We therefore conclude that the active space of sperm whale codas allows for communication both within and between social units.

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