Echolocation

What is echolocation? The ability to locate an object in space by emitting a call and monitoring the soundwaves being reflected back to you The frequency of the sound emitted is related to the frequency of the sound returned so one can recognize one’s own call The returning echo provides information on the location, size and distance of the prey or object Interaural time difference, interaural intensity difference Also, the direction of travel, if the echo is lower pitch, the object is moving away, if it is higher pitch, it is getting closer

Echolocation detection Orange=non-echolocating Blue=echolocating

Convergent evolution in echolocation Two echolocating bat species (red), and dolphins (purple) Gene alignments of cadherin 23 and protocadherin 15 (tip link proteins)

Convergent evolution of echolocation Otoferlin: Ca2+ sensor necessary for NT release in inner hair cells Otoferlin mutations cause deafness

Evolution of high frequency hearing in bats: KCNQ4 gene

Anatomy of the moth ear

Auditory circuit in moth

Moth auditory receptor recordings Vary intensity, vary output of A1 and A2 neurons

Vary the pattern of stimulus—pulse vs steady Exact frequency doesn’t change the response, ie. 30kHz=50kHz in terms of AP generated No response to low frequency sounds

A1 and A2 auditory neurons Differential sensitivity to loudness (A1) A1 receptors fire more rapidly and with less delay with more intense stimulation and increase the firing rate more to pulses of sound than steady tones Neither is specifically frequency selective Use the combined input to determine where the bat is in space and where it’s headed

Anti-detection Responses The moth must avoid detection for only a few seconds to remain relatively safe. Neural impulses from the A1 receptors will trigger motor responses through the thoracic ganglia motor neurons and wing muscles to steer in a direction away from the approaching bat. When the activity of the left and right A1 receptors is synchronized, the moth will be heading directly away from the bat.

Evasive maneuvers When the bat is within ~3 meters of the moth, it’s too late to fly away and avoid detection Power dives Asynchronous wing beating, mediated by A2 receptors shutting down the normal regulated steering mechanisms.

A2 receptors totally shuts off at the peak of bat ultrasonic vocalizations, the point at which evasive measures would be taken. Question the role of this receptor in that escape behavior.

A2 activity is weaker in endemic species with no bat predators

The flying cricket neural circuit for bat evasion Cricket ears are sensory receptors like A1 that are found on the forelegs. The signals of these receptors are sent to an interneuron pair, AN2 or int-1. One is located on each side of the cricket’s body. Inactivate int-1, no response to vocalizations. Activate/stimulate int-1, get abdominal bending in the absence of any ultrasonic noise. Int-1 is necessary and sufficient to evoke an avoidance response. A,B,C abdominal bending in response to different frequency sound D tuning curve of int-1 cells, most sensitive to male calls in the 5-6kHz range, and >40kHz, the bat frequency

Mechanism of flight path alteration Int-1 relay neurons must synapse onto motor neurons in the central nervous system to effect the desired flight path change Away from the ultrasonic vocalizations Observational studies of tethered crickets responding to ultrasonic bursts showed a slowing of the hindwing opposite of the source

Crickets with experimentally removed hindlegs take about 40 ms longer to initiate a turn and dive maneuver.

Stimulus filtering A1 receptors Only respond to sounds Only respond to high frequency sounds (ignore non-threatening sounds) Respond to all high frequency sounds the same (do not distinguish between different frequencies in a range) Thus A1 receptors do only one job-detect echolocating predators

Stimulus filtering occurs in every animal with sensory capability Seasonal changes in frequency sensitivity in midshipman fish to detect courtship sounds Sex difference in parasitic fly hearing: Tuning of the female auditory system is optimized to detect crickets on which to lay eggs. Males have no such need, so their thresholds are very different.