Structural orientation and density in cetacean humeri

Abstract

The cetacean humerus is a short, robust bone without an open medullary cavity. It lies in the base of the resilient, streamlined pectoral limb (flipper) between the only free articulation (the glenohumeral) and approximately the body contour. The humerus is acted upon by muscles of the shoulder complex and receives loadings from the flattened distal portion of the limb as this hydroplane acts against body inertia and water resistance in control of body attitude.This study is an analysis of development and structure of the humerus in terms of the unique functional role of the flipper of finback, beluga and pilot whales. Gross external and internal architecture are depictued by photographs and by whole bone and frontal section radiographs. Structural density (bone/unit volume of humerus, with mineral content known to be constant) is analyzed indirectly by photodensitometry of standardized radiographs of sawed sections. Results are shown in graphic reconstructions of sections and of the whole bone. By comparison of radiodensity with the actual weight/volume of excised samples, sections are also reconstructed in terms of absolute density distribution.It is found that the spongy cetacean humerus, from its origin, is without central resorption and that its definitive structure is produced primarily by differential concentration of bone along endoectad gradients of porosity. Thus, the greatest concentration of bone is on medial and lateral sides while bone only half as dense fills the anterior and posterior sides and underlies the most dense regions. The core region is extremely porous. Within the biological context, this is a most reasonable approximation of the engineered box-beam as employed in some aircraft wings. Whole bone and frontal section radiographs show that, within this overall density pattern, the distribution of bone trabeculae resembles the classic illustrations of trajectories in the loaded beam.The ontogenetic and phylogenetic factors with possible bearing on this type of bone development and structure are discussed.

Histology and growth of the cetacean petro‐tympanic bone complex

“…The presence of pachyosteosclerosis in the petro-tympanic complex and ear ossicles of recent cetaceans has been interpreted as the vestige of an evolutionary stage in which this condition was (supposedly) generalized in the skeleton (Lancaster, 1990). However, it is noteworthy that: (1) the pachy-osteosclerosis exhibited by the post-cranial skeleton of the archaeocetes corresponds to the model commonly encountered in mammals (persistence of calcified cartilage: Buffrénil et al, 1990), and not to that observed in the periotic; (2) in the course of cetacean evolution, there is a complete reversal of the generalized pachy-osteosclerotic condition of the archaeocetes: the skeleton of recent toothed and baleen whales (except their otic region) presenting an extremely light, osteoporoticlike structure (Felts & Spurrell, 1965;Felts, 1966;Buffrénil & Schoevaert, 1988;Robineau & Buffrénil, 1993). It may, therefore, be incorrect to associate the structural specialization of the otic bones with the condition of the archaeocete skeleton.…”

Monophyly or Diphyly in the Origin of Whales

“…20) To my knowledge no archaeocete has been previously investigated with respect to bone structure, and my own observations are incomplete, so the pattern of divergence of the recent suborders is unknown. The supposed difference in structure between the recent suborders has not been published; a recent study by Felts and Spurrell (1965) on the bone of humeri did not report any important difference between Balaenoptera and two odontocetes. In fact, Felts and Spurrell found apparently more difference between the bone of the odontocetes Globicephala and Delphinapterus than between the latter and Balaenoptera.…”

Bones as biofuel: a review of whale bone composition with implications for deep-sea biology and palaeoanthropology

“…Bones that are primarily spongy in structure will have a greater capacity to contain oil-rich marrow than bones with higher tissue densities. The density of bone tissue decreases from the outer cortical layer of each bone to the inner cancellous bone [27,28]. The effect that this can have on the measurement of bone composition is illustrated by Heyerdahl [18] who measured oil content in samples taken from the outer part and inner part of a vertebra and found that they differed by 17 per cent.…”