Echo-dislocation

19 04 2010

Good tidings and well-wishes!

As a college student who is incessantly knee-deep in half-written essays, it’s fair to say that were Google Books to somehow cease its operations, I’d be up a very odious creek without a paddle.

Most recently, I’ve completed a nine-page diagnosing the morphology and evolution of the famed avian respiratory system for my comparative anatomy & physiology course. To make things somewhat more interesting for myself and my instructor, I’d elected to compare the breathing apparatus of bats to that hailing from their feathered counterparts in order to best illustrate the contrast in efficiency between these two very different setups. Google Books was thus the natural venue through which the search for this information was conducted.

The musculature of a typical bat.

This venture led me to John D. Altringham, Tom McOwat, and Lucy Hammond’s excellent tome entitled “Bats: Biology And Behaviour“. After acquiring all the required data, I couldn’t resist perusing some of the book’s other segments, during which time I managed to stumble across the following passage entitled “How Do Bats Avoid Being Confused By Other Bats’ Sonar?”:

“In real life, there are all sorts of extraneous sounds, including the sonar of other bats, to interfere with a bat’s own sonar processing. There is no doubt that such factors do influence a bat’s performance level, but laboratory studies have shown that echolocation has evolved to overcome many of the problems…

The moustached bat (Pteronotus parnellii) overcomes the interference caused by other bats’ calls by suppressing the first harmonic in its sonar pulse to around 1% of the total energy of the call. It is then so weak that other bats may not even be able to hear it. However, the bat hears its own first harmonic directly through the tissue between vocal chords and cochlea. The first harmonic is used to open a neural gate which enables the bat’s auditory system to receive and process the echo from the call. The bat does not respond to the weak first harmonies of other bats, they do not therefore open the neural gate which initiates processing, and it is not therefore confused by its presence.

Pteronotus parnellii

A similar mechanism is used by the small fisherman bat, Noctilio albiventris. [A pair of researchers] trained bats to discriminate a target range difference of 5 cm, and then attempted to confuse the bats with recorded sounds. These bats emit paired pulses at 7-10 Hz: an 8 [millisecond or ‘ms’] CF (“constant frequency”) pulse followed by a CF/FM (constant frequency/downward frequency) (6 ms/2 ms) pulse. The bats were found to be insensitive to a wide range of potential jamming sounds, including simulations of CF or FM components of their own CF/FM pulses. However, if simulations of the entire CF/FM pulses were used, the bats performed the discrimination tests badly. The researchers went on to perform experiments which suggested that N. albiventris could only receive and process the FM component, to determine target range, if it was preceded by the CF component with the appropriate temporal spacing. The CF component of the CF/FM pulse is presumed to open a neural gate of short duration, enabling the bat to process the echos from its own pulses. The CF/FM pulses or echoes of other bats are not likely to be heard during this brief window, and are therefore ignored. Finally, it is also becoming clear that in many species, each bat has its own personal call frequency.”

N. albiventris

Why bats would need to have evolved such a sophisticated navigational apparatus grows obvious upon viewing the following clip (invading raptors notwithstanding):

(By the way, I’d advise anyone interested in how evolution and biogeography have shaped the diversity of chiropteran sonar to check out this article).

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