Baleen Hydrodynamics and Morphology of Cross-Flow Filtration in Balaenid Whale Suspension Feeding

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.

Hydration affects the physical and mechanical properties of baleen tissue

“…The role of anisotropy in resisting crack propagation in mammalian keratin has been explored [47]. In life, such failure would merely promote the erosive wear process that reveals the horn tubes [14], which when exposed form the crucial fringes that perform most of the work of filtration for feeding, especially in balaenid (right and bowhead) whales [18]. …”

“…Baleen in vivo (or otherwise hydrated) is extraordinarily flexible and ductile: more like a soft plastic than a hard plastic. In short, dried baleen is familiar to many biologists, but it is biologically unrealistic.(ii) Consequently, our overall understanding of normal baleen function is also likely to be deficient, especially with regard to the physical, small-scale mechanisms of filtration, such as whether baleen filtration principally involves direct interception or inertial impaction of particles [48], and whether water and water-borne prey items chiefly flow through or along the filter [18], etc. Baleen may function less as a passive sieve and more as a dynamic filter that traps accumulated prey by governing intraoral flow fields [18].…”

A behavioural framework for the evolution of feeding in predatory aquatic mammals

“…These two types of filter may work in different ways, with at least some mysticetes (e.g. right whales) employing longitudinal cross-flow, rather than transverse throughput filtration [22,44].…”

Suction feeding preceded filtering in baleen whale evolution

“…In right whales, lateral lip rotation serves to create a flow channel lateral to the baleen rack during skim feeding (Lambertsen et al, 2005;Werth and Potvin, 2016). This feeding strategy requires a large filtration area, which in right whales is created by the arched rostrum and elongate baleen plates.…”

“…However, with baleen now in place, other methods of filtering no longer reliant on suction also became possible, including the highly specialised skim (Werth and Potvin, 2016) and lunge feeding (Lambertsen et al, 1995) strategies of extant right whales and rorquals, respectively.…”