Research Labs: Caitlin OConnell-Rodwell
Our long-term goal is to investigate the role that the propagation and detection of vibrations play in communication and environmental eavesdropping in large mammals, using the elephant as a non-human animal model for both the hearing, hearing impaired and the profoundly deaf. The mouse ear has been the traditional model for the human ear for a number of reasons, including the ease of high throughput screening, the cost of housing subjects, the proliferation of transgenic models and the ability to manipulation the mouse ear at the molecular level. The elephant ear, however, is a better model of the human ear as exemplified by their audiogram (Figure 1, Heffner & Heffner) and the fact that human hearing loss usually involves the loss of higher frequencies, which cuts out the overlap between the human and mouse hearing range.
Elephants transmit and detect high amplitude, low frequency (~20 Hz) vocalizations called rumbles (Figure 2a). The higher harmonics of these vocalizations attenuate over a very short distance in the air as measured from our acoustic arrays (Figure 2b). We found that rumble vocalizations are generated at such high amplitudes (on the order of 90-100 decibels SPL at five meters) that they couple with the ground and propagate along the surface of the earth as Rayleigh waves with a separate velocity than their airborne counterparts in both Asian and African elephants (OConnell et al, 2000, Gunther et al, 2004). The distance traveled and propagation velocities of these signals depend on signal frequency and energy level as well as the geological conditions of the substrate.
We show that elephant family groups can detect and respond to artificially transmitted seismic alarm calls and can distinguish between familiar and unfamiliar callers through the ground (OConnell-Rodwell et al., 2006, 2007). The sophistication in which elephants can detect vibrational cues indicates that the ground may be a very important resource for elephants to send and receive signals.
The generation of low frequency signals may be facilitated by the elephants large diagram, by a larynx with five rather than the nine bones present in most other mammals and an unusually large nasal cavity. The weight of an elephant generating a high amplitude signal facilitates the coupling of this signal into the ground by providing better coupling at the source.
Elephants have two possible pathways for the detection and interpretation of seismic signals, either through bone conduction and/or through vibration sensitive mechano-receptors in their feet and trunk. Elephants have an extremely large middle ear bone called the malleus, which is indicative of bone conducted hearing or seismic sensing. We found that they also have a dense distribution of vibration sensitive cells called Pacinian corpuscles in their feet (Bouley et al., 2007), and other colleagues have described them in the trunk. In addition, the elephant foot is cushioned by a cartilaginous fat pad that could facilitate impedance matching and vibration detection via bone conduction as is suggested for marine mammals (Figure 3).
Both pathways of vibration detection would facilitate the discrimination of high resolution frequency differences in seismic signals. If bone conduction to the ear is utilized, then the frequency discrimination ability will be reliant on the elephants ability to discriminate acoustic frequencies. And since the elephant's cochlea shows the sharpest resonance among seven species studied, they appear to have a keen ability to discriminate frequency changes within a narrow bandwidth.
The range of frequency modulation within an elephant acoustic alarm call is approximately 15 to 19 Hz. The minimum perceptible frequency change (Δƒ) is related to the critical bandwidth (CBW) in the following way: CBW = Δƒ * 20. Therefore, if this equation holds true for elephants, an estimated Δƒ of 0.75 to 0.95 Hz would allow them to detect very small changes in frequency modulation across these calls.
If the pathway of detection is via vibration sensitive corpuscles, then elephants should still be able to discriminate fine frequency differences. The frequency range of the second harmonic of the seismic alarm calls that elephants are able to discriminate varies from about 10 to 19 Hz, which should be within the range of vibrotactile frequency discrimination ability of elephants. Since this measurement has not been made directly in the African elephant, estimates are based on work in other species, using similar sensory structures. The ability of touch receptors to discriminate very small changes in frequency (2 Hz) has been demonstrated in humans and other primates. It is likely that elephants have at least the same vibrotactile frequency discrimination abilities as primates, if not better. In an ongoing investigation, we have designed several experiments to get at this question, one on establishing a vibrotactile threshold for elephants using a captive trained elephant at the Oakland Zoo (Figure 4 and video link), and a second experiment using wild bull elephants and playbacks of estrus calls (Figure 5 and video link).
The video of the playback experiment depicts an experiment to assess the effectiveness of playing estrus calls in both the air and the ground to bull elephants that are in musth. The video is sped up and shows a musth bull turning from his departure path to investigate the broadcast of the estrus call with his trunk, feet and ears. He then made a straight path to the origin of the sound and kept going until the edge of the clearing when the sound was broadcast again. Each bull was tested in this way three times to see if they would turn from their departure path to follow the signal. [The estrus calls used in this experiment were originally recorded by Joyce Poole in Amboseli]. Please visit our nonprofit website, Utopia Scientific, for videos of other playback experiments http://www.utopiascientific.org/Research/mushara.html.
Physiological studies have shown that auditory thresholds are based on temporal summation, thus longer signals should reduce the noise floor, facilitating signal detection. In addition, repeated signals appeared to facilitate detection. Elephants may benefit from these two physiological traits as family groups vocalize within interactive bouts which create multiple repetitions of the same signal that is three times longer on average than one produced by a single individual. If temporal summation and repetition itself increase the detection probability, then it would be easier to detect and process signals at greater distances.
We are in the process of assimilating lessons learned from our elephant vibrotactile studies and applying them to humans. Our first prototype of the Hearing Hand is currently being developed together with engineers at Hnu Medical (Figure 6).
All studies and research mentioned within this section can be found in the book chapter: Vibration generation, propagation and detection in elephants by OConnell-Rodwell & Wood, in The Use of Vibrations in Communication: properties, mechanisms and function across taxa.(2010) OConnell-Rodwell CE. (ed). Research Signpost.