Module 7

Walrus & Sea Lion Comparative

Beyond the phocid true seals of M1–M6, two other pinniped families round out the order: Odobenidae (the walrus) and Otariidae (sea lions + fur seals). This module covers walrus tusks and suction feeding, sea lion terrestrial agility, and the sexual-dimorphism patterns across the pinniped clade.

1. The Walrus — Odobenus

Both sexes carry elongated canines up to 1 m (males), 0.5 m (females) used for hauling out onto ice, social display, and defence. Adults reach 1 200–1 500 kg (males) or 600–800 kg (females). Circumpolar Arctic distribution; two subspecies (Odobenus rosmarus rosmarus Atlantic, O. r. divergensPacific). Population ~225 000 globally.

2. Suction Feeding Mechanics

Walruses feed on benthic bivalves using specialised vacuum-suction feeding. The vaulted hard palate + thickened tongue generate rapid sub-ambient pressure that draws clam flesh out of the shell — shells are not ingested. Vibrissae on the muzzle (~400 in adult males) provide tactile search of muddy substrate. A single walrus consumes 4 000–6 000 clams per day across a 100 km² feeding range.

3. Sea Lions & Fur Seals (Otariidae)

Sea lions are the classic performing pinnipeds — external ear flaps, forelimb-powered swimming, fully rotatable hindlimbs enabling quadrupedal terrestrial gait. Steller sea lions (Eumetopias jubatus) are the largest (~1 000 kg males); California sea lions (Zalophus californianus) dominate west-coast docks and Hollywood films. Fur seals (Arctocephalus, Callorhinus) carry dense underfur (30 000–50 000 hairs per cm²) rather than thick blubber — the historical source of the 18th–19th century fur-seal trade that nearly extinguished several species.

Simulation: Sexual Dimorphism

Python

script.py25 lines

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

# Sexual dimorphism across pinnipeds (M/F body-mass ratio)
species = ['N.elephant seal', 'Steller sea lion', 'Walrus', 'California sea lion',
           'Harbor seal', 'Weddell seal', 'Harp seal', 'Hooded seal']
ratio = [3.9, 3.3, 1.4, 2.8, 1.05, 0.95, 1.0, 1.8]

fig, ax = plt.subplots(figsize=(12, 5), facecolor='#0a0a1a')
ax.set_facecolor('#111827'); ax.tick_params(colors='#cbd5e1')
for s in ax.spines.values(): s.set_color('#334155')
colors = ['#f87171' if r > 1.5 else '#60a5fa' for r in ratio]
ax.bar(species, ratio, color=colors, edgecolor='#e0f2fe')
ax.axhline(1, color='#fbbf24', ls='--', label='Equal mass')
ax.set_ylabel('Male / Female mass ratio', color='#cbd5e1')
ax.set_title('Sexual dimorphism: polygynous vs monogamous breeding',
             color='#bae6fd', fontweight='bold')
plt.setp(ax.get_xticklabels(), rotation=25, ha='right')
ax.grid(True, color='#334155', alpha=0.3, axis='y')
ax.legend(facecolor='#1e293b', edgecolor='#334155', labelcolor='#cbd5e1')
plt.tight_layout()
plt.savefig('output.png', dpi=120, bbox_inches='tight', facecolor='#0a0a1a')
print('Dimorphism tracks mating system: harem-polygynous species 2-4x ratio')
print('Weddell and harbor seals monogamous -> near-equal size')

Click Run to execute the Python code

Code will be executed with Python 3 on the server

4. Mating System Correlates

Extreme sexual dimorphism correlates with harem polygyny: elephant seal males at 3.9× female mass defend beach harems of 30–100 females. Monogamous or serially-paired species (Weddell, harbor) are nearly monomorphic. This generalises the classical Darwin–Trivers sexual-selection prediction across pinnipeds with unusual clarity because body size is so readily measured at haul-outs.

Key References

• Kovacs, K. M. & Lydersen, C. (2019). Walrus of the North Atlantic. University of Alaska Press.

• Riedman, M. (1990). The Pinnipeds: Seals, Sea Lions, and Walruses. University of California Press.

• Levermann, N. et al. (2003). “Feeding behaviour of free-ranging walruses with notes on apparent dextrality of flipper use.” BMC Ecol., 3, 9.

• Lindenfors, P. et al. (2002). “Sexual size dimorphism in pinnipeds.” Behav. Ecol., 13, 188–197.

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