Ch. 15: Nervous system and behaviour II

Neurobiology of Behavior - Bats Hunting Noctuid Moths

Historical Context

  • Lazaro Spallanzani (1794):

    • Conducted early experiments with bats.

    • Blinded bats showed no collisions while hunting, demonstrating unimpeded ability to hunt without sight.

    • Used tubes in bats' ears filled with substances that disrupted navigation; confirmed that bats navigate without sight, relying predominantly on hearing.

    • 18th-century authors marveled at bats' ability to hunt in the dark, positing extraordinary vision as their main navigation method.

Key Contributions in Echolocation

  • Donald Griffin (1938):

    • Utilized ultrasound microphones to record bats, producing the first conclusive evidence supporting the use of echolocation in bats:

    • Definition: Echolocation is the emission of pulses of high-frequency sound (ultrasound) that reflect off obstacles and prey, allowing bats to receive and interpret weak echoes.

Detailed Observations on Echolocation

  • Myotis lucifugus (Little Brown Bat):

    • Griffin's research from the 1940s to 1950s indicated:

    • During cruising, bats emit 4-5 pulses per second.

    • Frequency of emitted pulses can reach up to 100 kHz.

    • Sound Pressure Level (SPL) can reach up to 120 dB.

    • Call rate increases when echoes are detected and when bats are closing in on prey.

Griffin's Perception Experiment

  • Conducted experiments in rooms with obstacles:

    • Bats successfully oriented and hunted even with numerous obstacles in their environment.

    • Broadcast tests:

    • When audible sounds between 1-15 kHz were played, bats navigated successfully in the presence of obstacles.

    • When ultrasound (20-100 kHz) was played, bats crashed into obstacles and rested on the ground.

Advances in Bat Echolocation Research

  • Subsequent studies indicated that bats can:

    • Both locate and identify objects using echolocation with frequencies ranging from 10 to 200 kHz and sound intensities over 120 dB.

    • Examples: SPL levels comparable to nearby jet engines; U.S. Department of Labor recommends exposure of less than 15 minutes at 115 dB, which is 2000 times louder than echoes from prey.

Mechanisms Maintaining Auditory Sensitivity

  • Mechanisms that prevent bats from deafening themselves confirmed through cellular recordings:

    1. Middle Ear Muscles: Contract before the emission of sound pulses to dampen vibrations from the ossicles.

    • Attenuation achieved is to 1% of the original sound; relaxes 2-8 milliseconds after pulse emission.

    1. Inhibitory Interneurons: Block auditory transmission to the brain during signal emission, further attenuating sound to 0.01% of the original.

    2. Echo-detector Cells: In the bat’s brain, these cells respond maximally to the second of two sound pulses.

Intraspecific Competition and Jamming

  • Aaron J. Corcoran and William E. Conner:

    • Discussed the phenomenon of sonar interference (“jamming”) during food competition among bats.

    • Active sensing animals can alter the frequency of their emissions to avoid jamming.

    • Example research revealed:

      • Echolocating bats (Tadarida brasiliensis) engage in jamming behaviors during food competition, employing specialized calls to interfere with each other’s echolocation, impacting prey detection. More dominant bats jam the signals of subordinate bats, so they can’t catch any bugs.

  • Field Observations:

    • Interactions recorded showed bats emitting sinusoidal frequency-modulated calls, specifically timed to disrupt the echolocation of conspecifics pursuing prey.

Neurophysiology of Predator Evasion in Noctuid Moths

  • Behavioral Responses of Noctuid Moths:

    • Preferred prey of little brown bats.

    • Exhibited specific evasive tactics based on bat proximity:

    • When the bat is distant, flies directly away; when close, flies erratically, reducing predation chances by 40%.

    • Moths possess neural mechanisms for evasion that are relatively simple but effective, relying on minimal receptor cells (A1 and A2).

Detection Mechanisms in Moths

  • A1 and A2 Receptors:

    • Each moth ear has only 2 receptor cells responsible for detecting sound from bats:

    • A1 receptors begin firing when a bat is 30 meters away, while the detection threshold for the bat is merely 3 meters away.

    • A1 firing rate correlates with intensity and distance, providing crucial locational information to the moth.

Directional Detection

  • Horizontal Plane Detection:

    • Differential firings in left and right A1 receptors enable the moth to discern the bat's position.

  • Vertical Plane Detection:

    • A1 cells exhibit cyclic changes in firing when a bat approaches from above, while no change occurs when a bat approaches from below.

Anti-detection Strategy of Moths

  • Behavioral Adjustments:

    • Orients body to balance firing rates in both ears to detect the direction of the bat effectively.

    • Engaging in erratic flight increases chances of hiding from the bat's detection radius, where detection by the bat's echolocation dims significantly beyond 3 meters distance.

Evasive Mechanisms Under Threat

  • Behavioral Response When Within 3 Meters:

    • Engages in erratic flight patterns (loops, dives), making capture difficult, masking their echo signature, but simultaneously risking injury.

Physiological Basis of Evasion

  • Receptor Response Mechanism:

    • When ultrasonic signals are detected within 3 meters, A2 receptors fire, leading to significant physiological responses:

    • A2 fibers connection with interneurons generates inhibitory signals that disrupt coordination of wing muscular movements, facilitating evasive flights.

Sensory Receptors and Evolution

  • Neural Specialization:

    • Identified neurons in the moth evolved specifically for executing avoidance flight when ultrasonic chirps from predators (bats) are detected.

Stimulus Filtering in Moths

  • Biological Relevance of Stimuli:

    • Animals filter stimuli to attend only to those biologically relevant; moths’ nervous systems are selectively attuned to essential stimuli in their environment, particularly the ultrasound pulses from predatory bats.

Example of Adaptive Filtering in Moths

  • Neural Activity During Sound Exposure:

    • Moths respond distinctly to low-intensity, moderate-intensity, and high-intensity stimuli with appropriate neural activity, emphasizing their selective responsiveness to frequency ranges associated with threats.

Additional Studies on Acoustic Response

  • Ormia Ochracea and Cricket Songs:

    • Female flies locate male crickets by tuning their auditory receptors to frequencies of cricket songs so they can lay their eggs in the crickets; male flies are not similarly adapted.

Case Study of Adaptive Sensitivity in Fish

  • Lythgoe et al (1994):

    • Examined 12 species of snapper across 4 distinct habitats; noted adaptive visual pigment sensitivity based on local ambient light conditions, showing similar adaption with species at varying water depths.

  • The study illustrates the intricate sensory responses that permit prey (moths) to effectively evade predators (bats) and highlights complexities within predator-prey dynamics and acoustic communication in the animal kingdom.