Module 5

Swimming & Aquatic Foraging

“Maritimus” in the scientific name is no accident: polar bears swim distances that no other ursid approaches. Durner 2011 satellite-tracked a female that swam 687 km in 9 days between ice floes, losing 22% of body mass in the process. This module analyses swim kinematics, drag, the thermal regime, and the climate-driven rise of these long-distance crossings.

1. Swim Kinematics

Polar bears swim with a paddle-stroke doggy-paddle gait, using the large forepaws as oars while the hind limbs trail. Speed ~1.5–2 m s^-1sustained, burst to ~3 m s^-1. Drag coefficient ~0.8 is higher than streamlined pinnipeds (~0.15 for a seal) because the bear is not hydrodynamically optimised — they are surface swimmers, not divers. Head held clear of the water adds wave drag.

The thick blubber layer is buoyant enough to keep the bear at the surface without active paddling, but the hollow-shaft pelage becomes waterlogged in sea water and significantly increases mass on emergence.

2. Thermal Regime Underwater

Seawater at −1 °C has an enormous heat capacity. Without blubber insulation, a bear would lose body heat in minutes. With 10–12 cm of subcutaneous blubber and a ~20 cm effective boundary layer of waterlogged pelage, heat loss is ~800–1 200 W at sustained swim speed — still manageable from mass stores but finite. Pagano 2018 measured core-temperature rises during long swims, suggesting heat retention rather than loss is the immediate concern.

Simulation: Long-Swim Power & Endurance

Python

script.py51 lines

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

# Durner 2011 long-distance swim tracking: >500 km continuous
# Drag F = 0.5 rho Cd A v^2
rho = 1025   # sea water
Cd = 0.8     # slightly lower than rigid cylinder
A = 1.2      # frontal area m^2
v = np.linspace(0.5, 2.5, 200)
F_drag = 0.5 * rho * Cd * A * v**2
P_swim = F_drag * v   # power

# 300 kg bear, swim metabolic rate
BMR = 350
# Energetic cost of transport J/m
COT = P_swim / v
COT_BMR = BMR / v

# Max swim duration given 30% body fat reserve
mass = 350
fat_frac = 0.25
fat_kJ = mass * fat_frac * 37000  # total energy in fat kJ
duration = fat_kJ * 1000 / P_swim  # seconds
hours = duration / 3600

fig, (ax1, ax2) = plt.subplots(1, 2, figsize=(14, 5), facecolor='#0a0a1a')
for ax in (ax1, ax2):
    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)

ax1.plot(v, P_swim, color='#38bdf8', lw=2.6, label='Propulsive power')
ax1.axhline(BMR, color='#fbbf24', ls='--', label=f'BMR {BMR} W')
ax1.set_xlabel('Swim speed (m/s)', color='#cbd5e1')
ax1.set_ylabel('Power (W)', color='#cbd5e1')
ax1.set_title('Swim drag power', color='#bae6fd', fontweight='bold')
ax1.legend(facecolor='#1e293b', edgecolor='#334155', labelcolor='#cbd5e1')

ax2.plot(v, hours, color='#f87171', lw=2.6)
ax2.axhline(230, color='#fbbf24', ls='--', label='Durner 2011 record ~230 h')
ax2.set_xlabel('Swim speed (m/s)', color='#cbd5e1')
ax2.set_ylabel('Endurance (h)', color='#cbd5e1')
ax2.set_title('Fat-limited swim duration',
              color='#bae6fd', fontweight='bold')
ax2.legend(facecolor='#1e293b', edgecolor='#334155', labelcolor='#cbd5e1')

plt.tight_layout()
plt.savefig('output.png', dpi=120, bbox_inches='tight', facecolor='#0a0a1a')
print('Durner 2011: a female bear swam 687 km over 9 days without a break')
print('Cub swim capacity much lower -> drowning risk under longer crossings')

Click Run to execute the Python code

Code will be executed with Python 3 on the server

3. Cub Drowning Risk

Cubs have proportionally less insulation and much less fat reserve. Durner 2011 documented cub mortality during long forced swims; Monnett 2006 filed the first widely-publicised observation of polar-bear carcasses at sea off Alaska, attributable to exhaustion. As sea-ice fragmentation increases, the frequency of forced long swims rises, and cub mortality during those swims is an accelerating contributor to population decline.

4. Pursuit Swimming of Prey

Polar bears occasionally pursue ringed seals in open water or under thin ice, though efficiency is low: seals outpace and out-dive them. More commonly, swimming is used for island-to-island transit, accessing seal birthing lairs on offshore ice, and reaching terrestrial haul-outs. True aquatic predation is a minor dietary contribution compared with ice-edge still-hunting (M1).

Key References

• Durner, G. M. et al. (2011). “Consequences of long-distance swimming and travel over deep-water pack ice for a female polar bear.” Polar Biol., 34, 975–984.

• Monnett, C. & Gleason, J. S. (2006). “Observations of mortality associated with extended open-water swimming by polar bears in the Alaskan Beaufort Sea.” Polar Biol., 29, 681–687.

• Pagano, A. M. et al. (2012). “Long-distance swimming by polar bears of the southern Beaufort Sea during years of extensive open water.” Can. J. Zool., 90, 663–676.

• Pagano, A. M. et al. (2018). “High-energy, high-fat lifestyle challenges an Arctic apex predator.” Science, 359, 568–572.

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