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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. and Payne, J. Phylogenetic and fossil evidence for a common body size attractor in marine mammals. Geological Society of America Abstracts with Programs. 2015. Vol. 47, No. 7, p.138.
[GSA Poster Presentation]
[Honorable mention for GBGM Division Student Awards]

Evolutionary transitions between terrestrial and aquatic habitats are rare and often have large effects on the evolutionary trajectory of the clade making the transition. Following a single transition from the marine realm to the terrestrial realm, tetrapods have subsequently re-evolved a marine lifestyle at least 30 separate times. At least six of these re-invasions of the water occurred within crown-group mammals and four [Sirenians (Sirenia), whales (Cetacea), pinnipeds (Pinnipedia), and otters (Lutrinae)] clades are extant. Although marine mammals are widely known to be larger than their terrestrial sister groups, the extent to which the body size evolution of these clades reflects common constraints of a marine lifestyle remains little studied. 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 marine mammals both through comparative phylogenetic analysis and direct interrogation of the fossil record. We find that the evolution of an aquatic lifestyle has driven three of the four aquatic mammal clades toward indistinguishable body size attractors of ~1 metric ton. In these clades, phylogenetic analysis favors an Ornstein-Uhlenbeck model of size evolution and both phylogenetic and fossil data predict comparable optimal sizes and evolutionary trajectories. Both fossil and comparative phylogenetic data indicate that the otters (Lutrinae) were the most recent clade to reinvade the water and the only clade to remain at small body size. Our results confirm that mammals living in aquatic environments have higher optimal body sizes and further suggest the existence of a body size attractor that has been discovered independently by three mammalian clades. The sustained small size of aquatic mustelids could either indicate the presence of a second attractor at small size or competitive exclusion from the ~1 ton attractor. The close agreement in statistical inference from comparative phylogenetic and fossil data both strengthens the findings described above and highlights the power of both approaches to capture the macroevolutionary dynamics of body size.


Gearty, W. and Gauthier, J. Resolving the Relationships of the Squamate Tree of Life: An Assessment of New Approaches and Problems. Journal of Vertebrate Paleontology, Program and Abstracts, 2014, 136.
[SVP Oral Presentation]

Since the division of The Deep Scaly Project into separate morphological and molecular teams, a truly integrated project of wide scope has not been attempted. Much more can be done to understand how the members of Squamata are related to one another through an approach that combines the importance of both morphological and molecular evolution. Here we have developed a novel three-step methodological approach to squamate phylogenetics that incorporates the newest phylogeny-creating techniques and data from previous morphological and genetic analyses. First, we analyze a large squamate morphological dataset using Lewis's Mkv model under both a Bayesian and maximum likelihood framework. Second, we incorporate a previously constructed squamate DNA dataset and analyze the combined data within a 'total evidence' framework. Finally, we adopt a methodology that treats genes, rather than nucleotides, as the character of interest.
We find that the separate analyses of the morphological and molecular datasets, even under Bayesian and maximum likelihood frameworks, still result in drastically different relationships between higher-order clades within Squamata. Additionally, we find that the combination of these two datasets results in a phylogeny with limited support for either topology, although it definitively leans in the direction of the molecular results. Finally, by reducing the molecular dataset to gene characters, we find significantly lower support for the higher-order relationships that are strongly supported in previous analyses. By combining these data with our morphological dataset, we discover that we have inversed the effect of the power in numbers problem.
We conclude that combining datasets, although possibly detrimental to results, should be treated as a source of understanding how the datasets may differ and how they may reflect different evolutionary histories.


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