The mustached bat

Parnell’s mustached bat, Pteronotus parnelli, can be easily identified because of its upturned nostrils, which give way to tufts of hair that protrude from the sides of the snout and give this animal its name. The sub species P.p. portoricensis is readily found throughout Puerto Rico, and the genera Mormoops and Pteronotus are the only members of the New World family Mormopidaee. Pteronotus parnelli has one best studied echolocation systems of any mammal (Herd, 1983; Pollack and Henson, 1973). These bats emit ultrasonic pulses generated by the mouth that consist of two brief, frequency-modulated sweeps of 2-3 milliseconds, one at the beginning and another at the end of their pulse, followed by a much longer component, 16-28 milliseconds long of constant frequency (about 62 kilohertz) sandwiched between both FM pulses (Herd 1983). The use of both of these components to locate prey plays an important role in Pteronotus parnelli’s hunting ability, due to the different conditions in which they live and location of prey feeding. The only long constant frequency modulation in the New World that has a unique use will further be explained along with how a relation between acoustical audition, wing morphology, reaction to Doppler shifts and readings in pulses make a bat capable of adapting in dense vegetation.

The downward sweeping FM component reveals an object’s spatial information, which includes everything from range to shape (Simmons and Stein 1980; Simmons 1990). Contrasted to the FM sweeps, the echoes of CF components, mainly used by Pteronotus parnelli are better used for the detection of motion of prey, such as the fluttering wings of insects, which induce rapid FMs in this signal via the Doppler effect. Experiments which were done to test this idea revealed that mustached bats will not attack an insect unless its wings are fluttering (Goldman and Henson 1977), which reinforces the reason why these bats primarily use CF components, preceded and followed by FM sweeps. Thus, FM sweeps primarily occurs in two forms, quasi-linear aperiodic modulations conveying target range and fine feature information via the terminal FM component and periodic modulations transmitting flutter information using the CF component (Oneill and Brimijoin, 2002). Both types of FM are important for identifying, localizing, and capturing insect prey. The constant-frequency portion of the call and its resulting echo may help locate and determine the speed of a flying insect relative to the bat by means of a Doppler shift (a phenomenon produced as objects move toward or away from each other), whereas frequency-modulated segments of the call provide more information about the position and surface details about the prey. This can also be known as high duty cycle bats, which are specialized for exploiting Doppler shifted echoes generated by fluttering targets (Grinell, 1995, Fenton, 1995). Although many bats include short constant-frequency components in their echolocation calls, the long duration of the constant frequency portion in Parnell’s mustached bat calls is unusual among the New World species, being more similar to the horseshoe bat calls in the Old World, which directly relates to their feeding habits and habitat (Gannon and Rodriguez-Duran, 2005).

The reason for the echolocation difference between P. parnellis and other types of bats mainly is strongly related to their wing morphology. Parnell’s bat has broad wings, which enable it to flight in highly dense vegetation (Gannon and Jennings, 2004). They use their low-intensity FM calls to avoid overlap between their pulses and echoes of close targets and longer CF components which detect the fluttering of insect wings, which come back as acoustical glints in the returning echoes. (Schnitzler and Kalko, 1998). These bats distinguish these glints by their frequency change, and not by their timing change, which is a common method used in most types of bats (timing change), so that these detections in frequency change ultimately lead to the overlapping of pulses and echoes (Suga, 1990) which finally lead to the detection of prey.

Structures in the ears of P.parnellis and accompanying concentrations of nerve cells tune their auditory systems to very narrow and specific frequencies according to their primary frequency of use. Although they can hear many other frequencies, this specialization gives them great powers of resolution at the frequencies with which they hunt, which are normally CF calls. In a study done by O'Neill and Brimijoin (2002), the experiment evaluating their directional selectivity in FM and CF frequencies, they measured the bats responses to best excitatory frequency (BEF) tones and linear 12-kHz upward and downward FM sweeps centered on the BEF (Oneill and Brimijoin, 2002). The experiment consisted of sweeps that were presented at durations of 30, 12, and 4 ms, yielding modulation rates of 400, 1,000, and 3,000 kHz/s. Spike count versus level functions were obtained at each modulation rate and compared with BEF controls. BEFs clustered at 58 kHz, corresponding to the dominant CF component of the sonar signal (Oneill and Brimijoin, 2002), which naturally is P. parnellis’ primary echolocation sweep to find prey. This gives a lot of importance to the way they use the Doppler Effect, which provides a significant potential source for error. The auditory specializations of the Pteronotus family as a whole allow them to actually exploit the Doppler-shifted echoes from their fluttering targets, giving them an excellent way to find flying insects.

The sonar signals in P. parnellis, which are emitted from the mouth, are long, constant-frequency types with a type of "missing" fundamental in their first pulse, so that the second and third harmonics contain most of the pulse energy (Hartley and Suthers, 1990), which reveals the mustache bat’s reliability on its use of specifically its second pulse. On experiments done to further discover its second and third harmonics, the study revealed that the pulse structure used a part of the bat’s tract, called the supraglottal vocal tract, which acts as an acoustic notch filter for the fundamental (Hartley and Suthers, 1990). Hartley and Suthers used light and heavy gasses to control the types of pulses that Parnell’s bat emitted and concluded that at the second and third harmonics, sound radiation is highly directional and consistent with the idea that mouth acts as a conical horn. The mustached bat overcomes interference by suppressing the first harmonic in its own sonar pulse to about 20dB of the total signal energy. It is then so weak that other bats may not even hear it. The bat itself hears its own harmonic directly in its own vocal membranes and cochlea (Altringham, 1996). This first harmonic is used to open a neural gate which enables the bat’s auditory system to receive and process the echo from that call.

Spectrogram:

(B) The echolocation call of P. parnelli portoricensis, which demonstrates that most energy is found in its second harmonic (Jennings, 2004).

As said earlier, a large fraction of the inner ear and most the neurons in auditory neural centers are devoted to analysis of a narrow range of frequencies (58khz) around the CF and this neural configuration has been defined as the “acoustic fovea” (Schuller and Pollak, 1979, Grinell, 1995). P. parnellis controls the frequency of the emitted signal to compensate for Doppler shifts of returning echoes so that the echo CF stays within the acoustic fovea and thereby is always effective (Grinell, 1995, Fenton, 1995), so that the auditory neural centers are specifically directed toward a preferred type of frequency, something which further enhances their hearing, sort of like our own primary and periphery vision (with its visual fovea).