Gearty, W., McClain, C.R., and Payne, J.L. 2018. Energetic tradeoffs control the size distribution of aquatic mammals. Proceedings of the National Academy of Sciences. doi: 10.1073/pnas.1712629115.
Four extant lineages of mammals have invaded and diversified in the water: Sirenia, Cetacea, Pinnipedia, and Lutrinae. Most of these aquatic clades are larger bodied, on average, than their closest land-dwelling relatives, but the extent to which potential ecological, biomechanical, and physiological controls contributed to this pattern remains untested quantitatively. Here, we use previously published data on the body masses of 3,859 living and 2,999 fossil mammal species to examine the evolutionary trajectories of body size in aquatic mammals through both comparative phylogenetic analysis and examination of the fossil record. Both methods indicate that the evolution of an aquatic lifestyle is driving three of the four extant aquatic mammal clades toward a size attractor at ∼500 kg. The existence of this body size attractor and the relatively rapid selection toward, and limited deviation from, this attractor rule out most hypothesized drivers of size increase. These three independent body size increases and a shared aquatic optimum size are consistent with control by differences in the scaling of energetic intake and cost functions with body size between the terrestrial and aquatic realms. Under this energetic model, thermoregulatory costs constrain minimum size, whereas limitations on feeding efficiency constrain maximum size. The optimum size occurs at an intermediate value where thermoregulatory costs are low but feeding efficiency remains high. Rather than being released from size pressures, water-dwelling mammals are driven and confined to larger body sizes by the strict energetic demands of the aquatic medium.
Racicot, R.A., Gearty, W., Kohno, N. and Flynn, J.J. 2016. Comparative anatomy of the bony labyrinth of extant and extinct porpoises (Cetacea: Phocoenidae). Biological Journal of the Linnean Society, 119(4), pp.831-846. doi: 10.1111/bij.12857.
The inner ear anatomy of cetaceans, now more readily accessible by means of nondestructive high‐resolution X‐ray computed tomographic (CT) scanning, provides a window into their acoustic abilities and ecological preferences. Inner ear labyrinths also may be a source for additional morphological characters for phylogenetic analyses. In this study, we explore digital endocasts of the inner ear labyrinths of representative species of extinct and extant porpoises (Mammalia: Cetacea: Phocoenidae), a clade of some of the smallest odontocete cetaceans, which produce some of the highest‐frequency clicks for biosonar and communication. Metrics used to infer hearing ranges based on cochlear morphology indicate that all taxa considered could hear high‐frequency sounds, thus the group had already acquired high‐frequency hearing capabilities by the Miocene (9–11 Mya) at the latest. Vestibular morphology indicates that extant species with pelagic preferences have similarly low semicircular canal deviations from 90°, values indicating more sensitivity to head rotations. Species with near‐shore preferences have higher canal deviation values, indicating less sensitivity to head rotations. Extending these analyses to the extinct species, we demonstrate a good match between those predicted to have coastal (such as Semirostrum cerutti) preferences and high canal deviation values. We establish new body length relationships based on correlations with inner ear labyrinth volume, which can be further explored among other aquatic mammals to infer body size of specimens consisting of fragmentary material.
Gearty, W. and Payne, J.L. 2017. Convergent body size evolution of crocodyliformes upon entering the aquatic realm. Evolution 2017.
[Evolution Poster Presentation]
[Honorable mention for Ruth Patrick Student Poster Award]