Module 7

Whisker & Sensory Systems

Seal whiskers (vibrissae) are among the most sensitive mechanoreceptors in the vertebrate world. Harbor seals track fish wakes for more than 30 seconds after passage — demonstrated in the landmark Dehnhardt 1998 experiments — using specialised undulated-shape whiskers whose geometry suppresses flow-induced self-noise.

1. Vibrissal Anatomy

Each whisker is innervated by hundreds of mechanoreceptors — Merkel cells, Ruffini corpuscles, Pacinian corpuscles — in the follicle sinus. Harbor seal vibrissae carry 1 500–2 000 axons each (>10× a cat whisker). The follicle is a complex blood-filled sinus that amplifies and directionally filters mechanical stimuli.

2. Undulated Cross-Section

Most phocid whiskers are not round but carry a streamlined undulated profile (Hanke 2010) with ~6 cycles around the circumference. The undulations suppress von Kármán vortex shedding, reducing the seal’s own swim- induced noise and preserving faint hydrodynamic signatures from prey wakes. PIV (particle image velocimetry) visualisations confirm the wake-quieting effect.

3. Wake Tracking (Dehnhardt 1998)

Dehnhardt trained harbor seals to track a submarine that had passed 10–30 s before. With blindfolds but intact whiskers, seals achieved >70% success up to 20 s delay. Whisker-masked seals performed at chance. The experiment demonstrates that long-lived turbulent wakes leave a persistent signature the seals can follow — essentially bloodhound-by-hydrodynamics.

Simulation: Whisker Wake Tracking

Python

script.py53 lines

import numpy as np, matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt

# Harbor seal whisker geometry: undulated cross-section (Hanke 2010)
theta = np.linspace(0, 2*np.pi, 400)
r_round = 0.3 * np.ones_like(theta)
r_undul = 0.3 + 0.05*np.sin(6*theta)

# Wake tracking: fish swim ahead of seal -> vortex street persists
# Dehnhardt 1998: harbor seals track fish 30+ s after passage
t = np.linspace(0, 40, 400)
wake_strength_round = np.exp(-t/10)        # baseline cylinder
wake_strength_undul = np.exp(-t/25) * 1.3  # undulated preserves wake

# Follow-track success rate vs fish-passage delay
delays = np.array([5, 10, 15, 20, 30, 45])
success_blind = np.array([95, 85, 70, 55, 35, 15])
success_masked = np.array([20, 15, 10, 8, 5, 3])

fig, axes = plt.subplots(1, 3, figsize=(17, 5), facecolor='#0a0a1a')
for ax in axes:
    ax.set_facecolor('#111827'); ax.tick_params(colors='#cbd5e1')
    for s in ax.spines.values(): s.set_color('#334155')
    ax.grid(True, color='#334155', alpha=0.3)

ax = axes[0]
ax.plot(r_round*np.cos(theta), r_round*np.sin(theta), color='#94a3b8', lw=2)
ax.plot(r_undul*np.cos(theta), r_undul*np.sin(theta), color='#fbbf24', lw=2.5)
ax.set_aspect('equal')
ax.set_title('Round vs. undulated whisker', color='#e0f2fe', fontweight='bold')

ax = axes[1]
ax.plot(t, wake_strength_round, color='#94a3b8', lw=2.5, label='Round')
ax.plot(t, wake_strength_undul, color='#fbbf24', lw=2.5, label='Undulated')
ax.set_xlabel('Time since fish passage (s)', color='#cbd5e1')
ax.set_ylabel('Wake detectable (relative)', color='#cbd5e1')
ax.set_title('Wake persistence', color='#e0f2fe', fontweight='bold')
ax.legend(facecolor='#1e293b', edgecolor='#334155', labelcolor='#cbd5e1')

ax = axes[2]
ax.plot(delays, success_blind, color='#34d399', lw=2.5, marker='o', label='Blindfolded seal')
ax.plot(delays, success_masked, color='#f87171', lw=2.5, marker='s', label='Whisker masked')
ax.set_xlabel('Delay (s)', color='#cbd5e1')
ax.set_ylabel('Track success %', color='#cbd5e1')
ax.set_title('Dehnhardt 1998 wake tracking',
             color='#e0f2fe', fontweight='bold')
ax.legend(facecolor='#1e293b', edgecolor='#334155', labelcolor='#cbd5e1')

plt.tight_layout()
plt.savefig('output.png', dpi=120, bbox_inches='tight', facecolor='#0a0a1a')
print('Dehnhardt 1998: harbor seals follow fish wakes >30 s after passage')
print('Undulated whisker cross-section suppresses vortex-induced vibration')

Click Run to execute the Python code

Code will be executed with Python 3 on the server

4. Vision & Amphibious Hearing

Seal eyes are adapted for underwater vision with large, spherical lenses (minimising refractive-index mismatch at the cornea). Pupils range from slit (daylight) to fully dilated round (deep dark). Retinal sensitivity to blue wavelengths is enhanced, matching the peak transmittance of polar water. Hearing is sensitive in both air and water — external auditory meatus closure during diving prevents water ingress; underwater hearing uses bone conduction.

Key References

• Dehnhardt, G., Mauck, B., Hanke, W. & Bleckmann, H. (2001). “Hydrodynamic trail-following in harbor seals.” Science, 293, 102–104.

• Hanke, W. et al. (2010). “Harbor seal vibrissa morphology suppresses vortex-induced vibrations.” J. Exp. Biol., 213, 2665–2672.

• Marshall, C. D. et al. (2014). “Morphology, innervation, and function of pinniped vibrissae.” Anat. Rec., 297, 2077–2094.

• Mauck, B. et al. (2000). “How a seal turns a pelican into a dove: behavioural evidence for discrimination of hydrodynamic stimuli.” Mar. Mamm. Sci., 16, 526–539.

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