Response to ‘Biosonar sources in odontocetes: considering structure and function’

Field: Neuroscience (Life Sciences)

If we knew how all 73 species produce sound, it would be a conclusion and not a hypothesis: a hypothesis is a fundamental part of experimental biology where a general mechanism is proposed from a limited data set at hand, and then subsequently tested through falsification attempts in future experiments. Cranford seems to use the terms ‘hypothesis’ and ‘conclusion’ interchangeably, as he later writes: ‘Madsen... omitted... studies that appear to contradict their primary conclusion: that all odontocetes click with only one set of lips at a time – preferably the right pair’. That is our hypothesis, not the conclusion. Additionally, Cranford incorrectly states that we base our hypothesis on a single species and that we ignore pertinent literature. In our paper, we carefully cite six anatomical and modelling papers, all reporting that toothed whales primarily click with their right pair of phonic lips, and here we also point to recent studies by Au and colleagues (Au et al., 2010) and Dubrovskiy and Giro (Dubrovskiy and Giro, 2004) showing the same. Our hypothesis is thus not based on a single species/study.As missed references in which the findings are purported be inconsistent with our hypothesis, Cranford points to Evans (Evans, 1973) and Lilly (Lilly, 1962; Lilly, 1978). He states that Evans recorded ‘click formation on either or both sides of the blowhole’. Here Cranford selects a few words, which in isolation convey a message that is at odds with the text from which they were extracted: ‘More recent work... provides convincing evidence that the nasal plugs may well be involved in the production of whistles as well as clicks, the whistles being produced on the left side and the clicks on the right side. This does, however, not account for the omni-directional characteristics of the whistles and the variable directivity pattern of the echoranging clicks which seems to favour click formation on either or both sides of the blow hole’. Here Evans introduces the speculation, re-proposed by Cranford and colleagues (Cranford et al., 1996), that dynamic beam formation may be achieved by simultaneous activation on both sides, but Evans absolutely never recorded what Cranford quotes him for.The studies by Lilly (Lilly, 1962; Lilly, 1978) do claim clicking on both sides. Cranford quotes Lilly (Lilly, 1962) this way: ‘our animals tend to click only on the left side and whistle only on the right side and can do so simultaneously or separately’. The correct quote reads: ‘One of our animals tends...’. Again Cranford applies his unique quotation technique in omitting the first two words and the ‘s’ to convey that all the animals did this. While Cranford's rendition of the text could cast doubt on the general validity of our hypothesis of predominant right-side bias, the correct quote does not falsify our hypothesis.Also, Lilly's studies suffer from poorly documented methodology, which contrasts with the certainty with which his bold and speculative conclusions are presented. He states, for example, that high frequency clicks are generated from bursting slime bubbles in the larynx and are radiated via resonances of the dolphins' teeth to form a 3 deg beam – claims that have all been falsified since. The data in Lilly [(Lilly, 1978) see his fig. 7] do not demonstrate simultaneous click production: the two hydrophones will pick up the same click, no matter whether it is the result of one or two sources. Hence, in our view, no experimental data demonstrate that the two pairs of phonic lips produce two pulses to form a single click.On the basis that we did not conclusively demonstrate active echolocation, Cranford questions the validity of our clear-cut results. However, we fail to see how this falsifies the single source hypothesis. Does this mean, for example, that studies on bird vocal production must be discounted if the experimenters fail to prove that the bird was actually communicating while vocalizing in the laboratory? On p. 3109 in our paper (Madsen et al., 2010) we do state that we cannot exclude the possibility that behavioural settings might exist in which the animals produced clicks with the left or both pairs of phonic lips. But why? If something is particularly easy to read, would you then close one eye, because it would be overkill to read it using both? Many animals use one of two bilaterally symmetric body parts more than the other. Could it be that toothed whales are simply ‘right handed’ in their click production?Cranford argues that toothed whales with two pairs of phonic lips must use both, as otherwise the inactive one would degenerate. Actually, most toothed whales do have a hypertrophied right pair of phonic lips (Cranford et al., 1996), which makes porpoises, not dolphins, the obvious choice for testing Cranford's hypothesis that both phonic lips pairs are actuated simultaneously to produce a single click. No data have shown that to occur and, as outlined in our paper, the advantages are small and in some cases mutually exclusive. Given that simultaneous actuation of the phonic lip pairs would require sub-microsecond motor-neuronal timing, such a system, if at all possible, would have to be the result of a heavy selection pressure to provide real survival value. But double pulses, unless the animal had perfect timing from the evolutionary onset, would pose problems: imperfectly controlled they would create beams either pointing in random directions or with dips in the spectra at random frequencies, rendering the echoes unfit for classification of targets. Double pulses with delays longer than the ear's integration window would impede the sonar function by creating range ambiguities. So unless toothed whale ancestors got it exactly right the first time, it would impede an already functioning system. Such complexity is hard to reconcile with the principle of parsimony. Thus, from our results, the peer-reviewed literature, the severe functional problems of simultaneous actuation of two sources and the limited advantages of doing so, we maintain that the simultaneous click production hypothesis is as dead as the animals on which Cranford based it.