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Publications

Peer Reviewed Publications

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.




Pages

Conference Presentations

Gearty, W., McClain, C.R., and Payne, J.L. 2016. The evolution of aquatic mammals toward a nearly universal large size? Evidence from phylogenetics and fossils. Geological Society of America Abstracts with Programs. Vol. 48, No. 7.
[GSA Poster Presentation]


Crown mammals have invaded the water at least six times. Of these six invasions, four clades are extant: Sirenia, Cetacea, Pinnipedia, and Lutrinae. Three of these aquatic clades are substantially larger-bodied than their closest land-dwelling relatives, but the extent to which this pattern reflects common constraints of an aquatic lifestyle that favor larger body size versus a simple coincidence remains to be tested quantitatively. Here we use previously published and publicly available data on the body masses of 3832 living and 3005 fossil mammal species in addition to the most recent mammal supertree and up-to-date fossil ranges to examine the evolutionary trajectories of body size in aquatic mammals through both comparative phylogenetic analysis and direct interrogation of the fossil record. Both of these methods confirm that the evolution of an aquatic lifestyle has driven three of the four extant aquatic mammal clades toward a size attractor at approximately 500 kilograms. Our results indicate that these aquatic mammals have independently evolved toward larger body sizes due to the existence of this body size attractor and relatively rapid selection toward, and limited deviation from, this attractor. We believe that this optimum represents the culmination of selective forces resulting from thermoregulation, skeletal support, habitat area, locomotion energetics, and protein abundance differences between terrestrial and aquatic ecosystems, all of which are theorized to promote larger body sizes in aquatic systems. However, selection against extreme body size persists even in the aquatic realm, causing optimal body size convergence within these three groups. Furthermore, this balance of selective forces has caused increased rates of evolution towards this optimum and prevented deviation from it without the evolution of key innovations.



Benjamin, M., Gearty, W., Payne, J.L. 2015. Evolution of Larger Body Length during Transitions from Terrestrial to Aquatic Habitats in Snakes (Suborder Serpentes). Stanford Bio-X Interdisciplinary Initiatives Symposium.
[Poster Presentation]


Transitioning from terrestrial to aquatic habitats presents species with a host of new evolutionary challenges. However, few studies have examined how these pressures affect body size, a fundamental aspect of species' biology. Snakes have traversed this habitat change multiple times, so we investigated if and how snake body size varies across these habitat transitions. We collected body length and habitat use data from published literature. After finding aquatic snakes to be significantly larger than terrestrial, we used phylogenetic comparative methods to account for the non-independent nature of species. We fit seven models of body size evolution and evaluated each for their likelihood. An Ornstein-Uhlenbeck model with separate values for selective pressure, size optima, and variation intensity for each habitat best represented the data. This model suggests aquatic snakes are undergoing relatively strong selection towards a larger optimum.


Pages

Other Published Writing

Gearty, W., 2014, "The Good, the Bad, and the Ugly: The Potential Drawbacks of Manually Controlling Our Climate", Yale Scientific Magazine
Gearty, W., 2014, "Fossils Reveal that Many Marine Reptiles Separately Joined the Dark Side", Yale Scientific Magazine
Gearty, W., 2013, “Fly Guts Reveal Rainforest Biodiversity”, Yale Scientific Magazine
Gearty, W., 2013, “125-Million-Year-Old Biplanes: New Evidence Suggests the Earliest Bird Species had Feathers on their Hind Limbs”, Yale Scientific Magazine
Gearty, W., 2013, “Google Maps meets Biodiversity: A New Interactive Map of Life Aims to Integrate Biodiversity Distribution Knowledge”, Yale Scientific Magazine
Gearty, W., 2013, “Volcanoes Implicated in Murder of Dinosaurs: New Evidence Points to Volcanism as the Main Cause of Dinosaur Extinctions”, Yale Scientific Magazine
Gearty, W., 2012, “Fate of Australian megafauna Discovered through Prehistoric Dung”, Yale Scientific Magazine