When cetaceans have a bone to pick…

Cetaceans are a diverse group of air-breathing, diving mammals that utilize many different niches in their aquatic environments. When swimming, their broad, lunate-shaped fluke moves dorsoventrally, generating large forces for swimming (caudal oscillation). Cetaceans often have a rigid neck, chest, torso, and fluke, but the degrees of rigidity vary due to regional variations in the vertebral morphology. For example, vertebrae in rigid regions have disc-like centra that are taller and wider than they are long, while vertebrae that are longer and spool-shaped can be found in areas of greater flexibility. Those vertebrae with more surface area have greater axial muscle attachment leading to increased stiffness. At the tissue level, the vertebral trabecular bone is highly responsive to changes in load direction and intensity. Most importantly, the trabecular bone structure is tightly correlated with an animal’s behavior and movement in its environment.

To understand the functional and morphological variation between 10 species of delphinids and kogiids, Ingle and Porter (2022) established three groups based on vertebral morphology and diving behavior. Group 1 contained the shallow-diving delphinids whose vertebral morphology indicates that they are fast swimmers with limited bending capabilities due to their rigid torsos. The delphinids in Group 2 have similar vertebral morphology and torso rigidity, but routinely dive to depths of about 100m and glide while foraging. Finally, the kogiids in Group 3 have a more flexible body and often forage at depths greater than 100m where they glide rather than actively swim. Ingle and Porter quantified the trabecular bone mechanical properties (yield strength, apparent stiffness, and resilience) and compared the properties across regions of the vertebral column (thoracic, lumbar, and caudal).

The authors found that the shallow-diving delphinids in Group 1 had the greatest vertebral trabecular bone strength, stiffness, and resilience, while the deep-diving kogiids in Group 3 had the least strong, stiff, and resilient bone. Group 2, deep-diving delphinids, fell within intermediate ranges of bone strength, stiffness, and resilience. In other words, the species that swim more and swim faster experience greater stress on their vertebral columns than those that glide. Furthermore, they found that the region really did matter: greatest bone strength and resilience was found in the central/posterior caudal region for all three groups, and the lowest values were from the thoracic region.

Figure 1, adapted from Ingle and Porter (2022): Yield strength (σy; MPa) (A), Apparent stiffness (Eapp; MPa) (B), and Resilience (Ur; J/mm3) (C) varied significantly among the functional groups (Groups 1, 2, and 3). Group 1 displayed the greatest bone strength (A), stiffness (B), and resilience (C).

Another interesting piece of the puzzle that they authors tackled was how the cetacean vertebral bones compare to those of terrestrial animals. What does the lack of weight-bearing and gravitational loading do to bones? Ingle and Porter found that cetacean, and manatee, bone was less stiff and bent more easily than cow and human bones.

This study by Ingle and Porter (2022) is a prime example of the new insights that can be reached when we focus on the intersection of morphology, ecology, and behavior. Through the lens of both behavioral ecology and vertebral morphology, the authors present a very satisfying answer as to what determines bone mechanical behavior. Their work helps us better understand how and why cetaceans interact with their environment as they do.

Dr. Shirel Kahane-Rapport is a post-doctoral scholar at California State University, Fullerton, in Dr. Misty Paig-Tran’s lab. She is interested in physiology, ecology, and large marine animals. You can reach her on Twitter @shirelkr or at skahane-rapport@fullerton.edu.

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