The insect that hears like a human… or a frog

A Copiphora cornuta, from same genus as the katydid Montealegre-Z. et al. studied (photograph taken by me upstairs in the zoology museum here in Cambridge)
A young Copiphora cornuta, from same genus as the katydid Montealegre-Z. et al. studied (photograph taken by me upstairs in the zoology museum here in Cambridge)

Last year a paper appeared in Science entitled Convergent Evolution Between Insect and Mammalian Audition. I, along with my colleagues Gabriela Sobral, Rob Asher and Matthew Mason from Cambridge and Berlin, wrote a comment on the paper and Science‘s reportage of the researchers’ findings. Although Science didn’t publish it (after two months and with a boiler plate ‘no’ response), we made an online comment on the piece. When I’ve got a bit more time I’ll post up a less formal explanation of why we wrote it, but here’s our comment (with references):

The title from Ronald Hoy's comment piece in Science (http://www.sciencemag.org/content/338/6109/894.summary) The similarity to mammals (including humans) made sense, but the ear actually resembled other tetrapods a lot more.
The title from Ronald Hoy’s comment piece in Science (http://www.sciencemag.org/content/338/6109/894.summary) The similarity to mammals (including humans) made sense, but the ear actually resembled other tetrapods a lot more.

The work of Montealegre-Z. et al. (1) on the hearing system of katydid insects is a tour-de-force of state-of-the-art micro-anatomical and experimental research. However, we wish to point out that although the hearing apparatus reported in the katydid does indeed resemble the mammalian ear, similar features have evolved, probably convergently, elsewhere within the tetrapod clade.

Impedance-matching mechanisms found in mammals and katydids rely on an amplification of pressure achieved through a surface-area ratio, together with a lever system to increase force while reducing displacement. Such adaptations are also found among frogs, reptiles and birds. The katydid lever system is shown to be a type 1 lever, like a see-saw, a mechanism documented in ranid frogs (2,3). Mammals, on the other hand, have a type 2 lever system, more like a wheelbarrow (4). Tonotopic organization of the inner ear receptors is well-documented in many vertebrate groups (5). Mechanical travelling waves of the kind described in human and katydid ears are less ubiquitous but have been documented in the basilar membranes of birds (6) and crocodilians (7), and, controversially, have also been proposed to exist in the tectorial membrane of the frog amphibian papilla (8).

Thus, whilst we agree that the hearing mechanisms reported for katydids do indeed converge on those of mammals, certain elements are similar to, and may prove to be more closely comparable with, what is found in other tetrapods.

It is not our wish to detract from the elegant work of Montealegre-Z. et al. but only to highlight that the comparative focus on mammals, emphasized also in at least two comments on this article (9,10), is potentially misleading. Montealegre-Z. et al. have shown that the physical requirements of hearing on land have led insect auditory systems to evolve solutions which resemble those found in mammals: the fact that similar adaptations are found much more widely among tetrapods only adds to the significance of this work.

1. F. Montealegre-Z. et al., Science 338, 6109 (2012)
2. M.B. Jørgensen, M. Kanneworff, J Comp Phys A 182, 59-64 (1998)
3. M.J. Mason, P.M. Narins, J Exp Biol 205, 3153-3165 (2002)
4. M.J. Mason, M.R.B. Farr, J Laryngol Otol Available on CJO 2012 doi:10.1017/S0022215112002496 (2012)
5. Fay & Popper. Hearing Res 149, 1 (2000)
6. A.W. Gummer et al. Hearing Res 29, 63 (1987)
7. J.P. Wilson et al. Hearing Res 18, 1 (1985)
8. C.M. Hillery, P.M. Narins Science 225, 1037 (1984)
9. Hoy, Science 338, 6109 (2012)
10. E. Underwood, “Rainforest Insects Hear Like Humans,” Science Now, 15 November 2012

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: