Abstract

The traditional view of mysticete feeding involves static baleen directly sieving particles from seawater using a simple, dead-end flow-through filtration mechanism. Flow tank experiments on bowhead (Balaena mysticetus) baleen indicate the long-standing model of dead-end filtration, at least in balaenid (bowhead and right) whales, is not merely simplistic but wrong. To recreate continuous intraoral flow, sections of baleen were tested in a flume through which water and buoyant particles circulated with variable flow velocity. Kinematic sequences were analyzed to investigate movement and capture of particles by baleen plates and fringes. Results indicate that very few particles flow directly through the baleen rack; instead much water flows anteroposteriorly along the interior (lingual) side of the rack, allowing items to be carried posteriorly and accumulate at the posterior of the mouth where they might readily be swallowed. Since water flows mainly parallel to rather than directly through the filter, the cross-flow mechanism significantly reduces entrapment and tangling of minute items in baleen fringes, obviating the need to clean the filter. The absence of copepods or other prey found trapped in the baleen of necropsied right and bowhead whales supports this hypothesis. Reduced through-baleen flow was observed with and without boundaries modeling the tongue and lips, indicating that baleen itself is the main if not sole agent of crossflow. Preliminary investigation of baleen from balaenopterid whales that use intermittent filter feeding suggests that although the biomechanics and hydrodynamics of oral flow differ, cross-flow filtration may occur to some degree in all mysticetes.

Highlights

  • As early as 350 BC, Aristotle differentiated mysticetes from odontocetes by their oral filter

  • A substantial element of tangential rather than through-put IB flow in AP and DV directions (Figs 6–8) is apparent. These are wholly indicative of cross-flow filtration (CFF) instead of the traditional view of throughput filtration (TPF) within the balaenid mouth

  • Particle capture data are further supported by flow and pressure measurements, all of which indicate that intraoral flow is predominantly AP and DV rather than mediolateral (IB) as the standard TPF model predicts, where all flows advance in the same direction

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Summary

Introduction

As early as 350 BC, Aristotle differentiated mysticetes from odontocetes by their oral filter. We can speculate that Aristotle understood the general function of this material as a filtration device. Entries in the Encyclopedia of Marine Mammals, the leading reference work, offer bland generalities on the topic of mysticete filtration: “Periodically the mouth is closed and plankton are removed from the baleen by the tongue, and ingested” [3]; “Baleen whales. Force the water containing food out through the baleen plates, and transfer trapped food back to the gullet. Any shortcomings in our understanding of mysticete filtration are not the fault of the experts who wrote these entries; rather, they stem from limitations (logistical, legal, financial) constraining marine mammal research

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