Baleen wear reveals intraoral water flow patterns of mysticete filter feeding

Abstract

A survey of macroscopic and microscopic wear patterns in the baleen of eight whale species (Cetacea: Mysticeti) discloses structural, functional, and life history properties of this neomorphic keratinous tissue, including evidence of intraoral water flow patterns involved in filter feeding. All baleen demonstrates wear, particularly on its medial and ventral edges, as flat outer layers of cortical keratin erode to reveal horn tubes, also of keratin, which emerge as hair-like fringes. This study quantified five additional categories of specific wear: pitting of plates, scratching of plates, scuffing of fringes, shortening of fringes, and reorientation of fringes (including fringes directed between plates to the exterior of the mouth). Blue whale baleen showed the most pitting and sei whale baleen the most scratching; gray whale baleen had the most fringe wear. The location of worn baleen within the mouth suggests that direct contact with the tongue is not responsible for most wear, and that flowing water as well as abrasive prey or sediment carried by the flowing water likely causes pitting and scratching of plates as well as fringe fraying, scuffing, shortening, and reorientation. Baleen also has elevated vertical and horizontal ridges that are unrelated to wear; these are probably related to growth and may allow for age determination.

Sling, Scoop, and Squirter: Anatomical Features Facilitating Prey Transport, Processing, and Swallowing in Rorqual Whales (Mammalia: Balaenopteridae)

“…D–F). This unusual phenomenon has been documented (Werth et al, ), but the widespread extent of its nature was not known until numerous specimens of nearly all balaenopterid species were examined for this study. Laterally projecting baleen fringes at this location are visible not only in post mortem exams but also in full or partial baleen racks in museum collections.…”

“…It is widely known that every rorqual species possesses 180–400 triangular plates or laminae of baleen in each of two paired racks (Young et al, ). A recent study of wear patterns (Werth et al, ) has shown that baleen fringes (also called bristles, hairs, filaments, or fibers) frequently (40%–60% of the time) are redirected by consistent intraoral water flow patterns, and that the orientation of the fringes, and microscopic scuff marks on the baleen fringes and plates, reveals directions of water flow. In further examining baleen's fringe orientations and related structural arrangement for this project, we found, additionally, that there are markedly distinct patterns of fringe orientation evident in the baleen racks of rorquals (Fig.…”

Hydration affects the physical and mechanical properties of baleen tissue

“…Initial microscopic analysis [14] indicates a scuffed, scale-like surface in dried baleen, as tiny, cuticle-like keratin plates form a rough, ridged exterior. Pfeiffer [34] noted this in baleen fringes, but it has not yet been thoroughly described in a survey of baleen plates.…”

“…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]. …”

Oral cavity hydrodynamics and drag production in Balaenid whale suspension feeding

“…Curvature may be important not only for understanding the drag production by those hydrofoils, but also the lift that they may also produce (more energy dissipation) and the possible baleen-tip bending and movement that could follow. The latter would help further understand the wear patterns of baleen [39]. Finally, more work will be needed to clarify the role of the tongue’s shape and bulk within the oral apparatus.…”