A Study on Whistles — And Only Whistles

There’s a study in the Journal of Comparative Psychology that comes somewhat up my alley.

Quick, Nicola J. and Vincent M. Janik. 2008. Whistle Rates of Wild Bottlenose Dolphins (Tursiops truncatus): Influences of Group Size and Behavior. J. Comp. Psych. 122(3):305-311.

OK, so what are the authors reporting as results? A lot of that is in the abstract:

In large social groups acoustic communication signals are prone to signal masking by conspecific sounds. Bottlenose dolphins (Tursiops truncatus) use highly distinctive signature whistles that counter masking effects. However, they can be found in very large groups where masking by conspecific sounds may become unavoidable. In this study we used passive acoustic localization to investigate how whistle rates of wild bottlenose dolphins change in relation to group size and behavioral context. We found that individual whistle rates decreased when group sizes got larger. Dolphins displayed higher whistle rates in contexts when group members were more dispersed as in socializing and in nonpolarized movement than during coordinated surface travel. Using acoustic localization showed that many whistles were produced by groups nearby and not by our focal group. Thus, previous studies based on single hydrophone recordings may have been overestimating whistle rates. Our results show that although bottlenose dolphins whistle more in social situations they also decrease vocal output in large groups where the potential for signal masking by other dolphin whistles increases.

I was right with them up to the part bolded in the above. Why am I not convinced that their paper delivers on what the abstract promises? Let’s have a look at the methods of the paper.

The distributed array consisted of three HTI–94–SSQ hydrophones and one HTI–96–MIN hydrophone (High Tech, Inc., Gulfport, MS) all with a frequency response of 2 Hz to 30 kHz +/- 1 dB, attached to tensioned 2m pieces of chain with waterproof tape. The four elements were then distributed around the boat in a box array to allow passive acoustic localization. Hydrophones were positioned at 2m depth and were placed between 160cm and 280cm apart. Recordings were made onto a Fostex D824 multitrack digital recorder (Fostex, Tokyo, Japan) during 2003 and an Alesis adat HD24 multitrack digital recorder (Alesis, Cumberland, RI) during 2004 (sampling frequency 48 kHz, 24 bit for the Fostex, 32 bit for the Alesis). Spoken tracks of the two observers, one detailing the surface behavior of the animals in the focal group and one the positions and behavior of nonfocal groups were also recorded on the multitrack recorder.

Anybody spot the trouble yet?

Sure you did.

First off, one simply isn’t going to get higher frequency response out of a system than that of the least capable component. Starting with the hydrophones, 30 kHz is near the top frequency one might be getting. There is the issue of roll-off, but generally there is pretty steep roll-off at the high end of a hydrophone frequency response curve. I didn’t find an accessible calibration curve for the Hi Tech hydrophones to find exactly what the roll-off would be. But even that is going to be truncated sharply by the recording gear. The Fostek D824 recorder is said in the methods to have a “sampling frequency [of] 48 kHz”, but that is ambiguous. One can report a sampling rate of, say, 48 kilosamples per second, or a Nyquist frequency of 24 kHz. The upshot is that the Fostek recording six channels, as stated in the methods, is capable of 48 kilosamples per second per channel, giving a Nyquist frequency of 24 kHz as the highest frequency that the recorder might manage to represent. So just from that, we know that no frequency data over 24 kHz was part of the set analyzed in this study. (We won’t go into the lack of specification of anti-alias filters in the equipment as it isn’t really relevant to my critique, but any serious acoustic analysis would need to take aliasing into account.)

Here’s something from the discussion…

In our study, whistle rates during socializing in groups of 6 to 10 animals was 0.53 whistles per minute per dolphin, for nonpolarized movement it was 0.27. Dolphins in Sarasota only whistled half as often in similar group sizes during these behavior patterns (Jones & Sayigh, 2002). In Wilmington, whistle rates during milling were the same as in Sarasota, but for socializing they were Increased whistle rates during socializing may be due to animals communicating information to social associates or using calls to maintain contact. According to our definition animals were socializing when they were within very close proximity, often rubbing body parts and touching (see definition in Table 1). Rates may be dependent on social bonds between the individuals present or may be a consequence of increased arousal due to contact with individuals and not be dependent on social relationships. Cook et al. (2004) showed higher signature whistle rates during socializing and suggest that this may function to maintain contact as other group members get more dispersed while individuals are engaged in socializing.

OK, this gets to the stuff that I just can’t handle. Remember that 24 kHz maximum possible frequency in the data set? (Due to practical considerations, it could be even lower.) Bottlenose dolphins have an upper hearing response over eight times as high as that. Bottlenose dolphins have peak frequencies in clicks over six times as high as that. The sweeping speculations about communication inherent in the abstract and the quote just above are made in complete ignorance of over 7/8ths of the acoustic sensitivity of the subject species, and over 5/6ths of the peak frequencies within the vocal repertoire of the species. We know that click-based sounds are used by dolphins in communication; the only experimental work on obligate acoustic communication between individual dolphins to perform a task revealed that the signals used by the subjects were click-based and not whistles. We know that recording lower frequencies does not necessarily secure any vestige of click-based vocalizations. There is absolutely no consideration in the study here given to any of those issues or the fact that click-based sounds could quite plausibly be used for some of the functions being discussed. In fact, the paper does not even contain the word “click” or the word “pulse”. As a result, the claim that dolphins “decrease vocal output” within this study is something that the authors cannot possibly support; they have no clue whatever what dolphin vocal output over 24 kHz might be. They can, at best, report that dolphins decrease whistle rates with increasing group size, but they need to leave the “vocal output” claim out of it, since there is so much more to bottlenose dolphin vocal output(*) than the puny amount of bandwidth that they actually measured.

Whistles do seem to have some importance in dolphin behavior and sociality. But just because whistles have traditionally been relatively simple to acquire by comparison to high frequency, broad bandwidth, narrow beam clicks does not mean that clicks in general or those higher frequencies can be ignored with impunity. Studies like the present one should make their speculations that take cognizance of this technological gap between what we humans can readily find out and what the dolphins are actually using.

(*) What to call dolphin sound emissions is a semantic issue. Personally, I don’t see the problem with “vocal” that some others do; the roots of the words were non-technical in the Latin, and one need not treat “vocal” as pertaining only to laryngeal sound production. Some offer “phonation” as an alternative, which so far as I can see simply grabs another non-technical Latin root that has as many issues and baggage from prior usage as the other. I went so far to avoid even getting into this terminological morass in my dissertation as to invent a neologism for emitted sound, and called them ensonds. For the purposes of this post, I’m simply accepting “vocal output” as sufficiently clear to move along with.