ABSTRACT
The scale dependence of locomotor factors has long been studied in comparative biomechanics, but remains poorly understood for animals at the upper extremes of body size. Rorqual baleen whales include the largest animals, but we lack basic kinematic data about their movements and behavior below the ocean surface. Here, we combined morphometrics from aerial drone photogrammetry, whale-borne inertial sensing tag data and hydrodynamic modeling to study the locomotion of five rorqual species. We quantified changes in tail oscillatory frequency and cruising speed for individual whales spanning a threefold variation in body length, corresponding to an order of magnitude variation in estimated body mass. Our results showed that oscillatory frequency decreases with body length (∝length−0.53) while cruising speed remains roughly invariant (∝length0.08) at 2 m s−1. We compared these measured results for oscillatory frequency against simplified models of an oscillating cantilever beam (∝length−1) and an optimized oscillating Strouhal vortex generator (∝length−1). The difference between our length-scaling exponent and the simplified models suggests that animals are often swimming non-optimally in order to feed or perform other routine behaviors. Cruising speed aligned more closely with an estimate of the optimal speed required to minimize the energetic cost of swimming (∝length0.07). Our results are among the first to elucidate the relationships between both oscillatory frequency and cruising speed and body size for free-swimming animals at the largest scale.
FOOTNOTES
Competing interests
The authors declare no competing or financial interests.
Author contributions
Conceptualization: W.T.G., P.S.S., D.E.C., J.P., F.E.F., A.S.F., J.H.L., M.S.S., J.A.G.; Methodology: W.T.G., P.S.S., K.C.B., D.E.C., J.P., F.E.F., A.S.F., J.K., J.H.L., M.S.S., J.A.G.; Software: W.T.G., D.E.C., J.K.; Formal analysis: W.T.G., K.C.B., J.P., J.K.; Investigation: W.T.G., P.S.S., K.C.B., D.E.C., J.P., F.E.F., J.D., J.d.C., A.S.F., D.W.J., S.R.K.-R., M.O., G.P., M.S.S., M.S., S.K.V., F.V., D.N.W., J.A.G.; Resources: K.C.B., D.E.C., J.P., J.D., J.d.C., A.S.F., D.W.J., J.K., M.O., G.P., M.S., S.K.A.V., F.V., D.N.W., J.A.G.; Data curation: W.T.G., P.S.S., K.C.B., D.E.C., J.D., A.S.F., D.W.J., S.R.K.-R., J.K., M.O., G.P., M.S.S., M.S., S.K.A.V., F.V., D.N.W.; Writing - original draft: W.T.G.; Writing - review & editing: W.T.G., P.S.S., K.C.B., D.E.C., J.P., F.E.F., J.D., J.d.C., A.S.F., D.W.J., S.R.K.-R., J.K., J.H.L., M.O., G.P., M.S.S., M.S., S.K.A.V., F.V., D.N.W., J.A.G.; Visualization: W.T.G., M.S.S., J.A.G.; Supervision: J.P., F.E.F., A.S.F., J.H.L., M.S.S., J.A.G.; Project administration: A.S.F., J.A.G.; Funding acquisition: J.P., F.E.F., A.S.F., J.A.G.
Funding
This research was funded in part by grants from the National Science Foundation (IOS-1656691, IOS-1656676, IOS-1656656; OPP-1644209), the Office of Naval Research (N000141612477), a Terman Fellowship from Stanford University, and a National Science Foundation INSPIRE Special Projects grant (1344227). Additional Monterey Bay CATS tag deployments were funded by grants from the American Cetacean Society Monterey and San Francisco Bay chapters, and by the Meyers Trust. Bryde's whale tagging operations in South Africa were funded by the Percy Sladen Memorial Fund, PADI Foundation, Torben and Alice Frimodts Fund, and Society for Marine Mammology.
Supplementary information
Supplementary information available online at http://jeb.biologists.org/lookup/doi/10.1242/jeb.204172.supplemental
- Received March 29, 2019.
- Accepted September 24, 2019.
- © 2019. Published by The Company of Biologists Ltd
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