Gearty, W. and Payne, J.L. 2018. Convergent body size evolution of crocodyliformes upon entering the aquatic realm. Society of Integrative and Comparative Biology 2018.
[SICB Poster Presentation]
Twenty-four species of crocodile populate the globe today, but this richness represents a minute fraction of the diversity and disparity of Crocodyliformes since their origin early in the Triassic. Across this clade, three major diversification events into the aquatic realm occurred. Aquatic and terrestrial habitats impose differing selective pressures on body size. However, previous research on this topic in Crocodyliformes remains qualitative in nature. In this study, our goal was to quantify the influence of habitat (terrestrial versus aquatic) on the evolution of body size in Crocodyliformes. By compiling an extensive body size database of fossil and modern crocs and using phylogenetic comparative methods, we find a history of repeated body size increase and convergence coupled with increases in strength of selection and decreases in variance following shifts to an aquatic lifestyle, suggesting common selective pressures on life in water spanning multiple independent aquatic clades. Lung volume, which has long been proposed as the main constraint on diving time, is only a constraint at sizes greater than 10 kg, whereas the rate of cooling constrains diving time at sizes less than 10 kg. Therefore, we propose this may be the primary driver of larger body sizes in aquatic crocodyliformes.
Gearty, W., McClain, C.R., and Payne, J.L. 2017. Energetics both promote and limit aquatic mammal gigantism. Geological Society of America Abstracts with Programs. Vol. 49, No. 6.
[GSA Oral Presentation]
Four extant mammalian lineages 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 quantitatively untested. Here we use previously published data on the body masses of 3859 living and 2999 fossil mammal species to examine the evolutionary trajectories of body size in aquatic mammals through both comparative phylogenetic analysis and direct interrogation 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 approximately 500 kilograms. Based on the existence of this body size attractor and relatively rapid selection toward, and limited deviation from, this attractor, we rule out most potential drivers of size increase. Using observed constraints on food intake and energy demand, we show that these independent body size increases and the shared aquatic optimum size are consistent with control by differences in the energetic intake and cost functions between the terrestrial and aquatic realms. We propose that thermoregulatory costs constrain minimum size whereas limitations on feeding efficiency constrain maximum size. The optimum size occurs at an intermediate size where thermoregulatory costs are low but feeding efficiency remains high. Rather than being released from size pressures, aquatic mammals are driven and confined to larger body sizes by the strict demands of the aquatic medium.