Toothed Aquatic Baleen Aquatic BM1 BMS OU1 OUM OUMV OUMA OUMVA Afrotheria 0.00 0.03 0.00 0.00 0.29 0.00 0.44 Artiodactyla 0.00 0.00 0.00 0.09 0.31 0.23 0.13 Caniformia [- Lutrinae] 0.02 0.15 0.01 0.04 0.34 0.02 0.41 Musteloidea 0.15 0.27 0.06 0.04 0.06 0.12 0.21 1. Introduction Most mammal species live on land, but the largest mammals live in the oceans. Aquatic and terrestrial habitats clearly impose differing selective pressures on body size. However, the quantitative study of body size evolution in mammals and other major animal clades typically focuses on either terrestrial or marine clades independently, thus failing to capture the dynamics of size evolution associated with the transition between land and water. Consequently, the extent to which the rate, magnitude, and outcome of size change associated with habitat transitions are shared among clades remains unknown, leaving open the question of whether the apparently common phenomenon of size increase associated with the acquisition of an aquatic lifestyle reflects idiosyncratic responses of individual clades versus a common response to universal constraints. The evolution of aquatic mammals toward a nearly universal large size? Evidence from phylogenetics and fossils 2. Materials and Methods William Gearty1,*, Craig R. McClain2, and Jonathan L. Payne1 1Department of Geological Sciences, Stanford University; 2Louisiana Universities Marine Consortium *wgearty@stanford.edu 3. Results 4. Conclusions • 3 out of 4 mammal groups living in aquatic environments have larger optimal body sizes than their terrestrial counterparts. • Results suggest the existence of a body size attractor (~500 kg) that has been discovered independently by these three aquatic clades, coupled with shared relatively rapid selection toward, and limited deviation from, this attractor (not shown here). • Some groups may still be getting larger, although analyses suggest there may be an upper limit without help from key innovations (e.g. baleen). • The sustained small size of aquatic mustelids could indicate the presence of a second attractor at a smaller size or competitive exclusion from the 500 kg attractor. • Analyses of the fossil record find indistinguishable optima (with large error), but produce different model support. References Beaulieu JM, et al. Evolution. 2012 Aug;66(8):2369–83. Bininda-Emonds ORP, et al. Nature. 2007 Mar 29;446(7135):507–12. Butler M, King A. Am Nat. 2004;164(6):683–95. Fortelius M. New and Old Worlds Database of Fossil Mammals. 2015. Heim NA, et al. Science. 2015 Feb 20;347(6224):867–70. Hunt G. Paleobiology. 2006 Dec;32(4):578–601. Jones KE, et al. PanTHERIA. Ecology. 2009 Sep 17;90(9):2648–2648. Smith FA, et al. MOM. Ecology. 2003 Dec 11;84(12):3403. The Global Biodiversity Information Facility. 2015. (gbif.org) Tomiya S. Am Nat. 2013 Dec 14;182(6):E196–214. Silhouettes from phylopic.org Figure 3.2: Comparison of results of phylogenetic and fossil analyses. Overlay of Ornstein-Uhlenbeck processes using average parameters as estimated by OUwie phylogenetic and paleoTS fossil analyses for individual clades. Points and error bars within paleoTS results represent average raw data and variance, respectively, per Myr time bin. Error bars associated with the OUwie curve origins indicate the extents of branches associated with aquatic transitions. Note the differences between the results of the phylogenetic and fossil analyses of Sirenia and Pinnipedia. General Equation of an OU Model: 𝒅𝑿𝒕= 𝜶𝜽−𝑳(𝒕) 𝒅𝒕+ 𝝈𝒅𝑩(𝒕) 𝐿(𝑡): initial body size 𝛂: strength of selection 𝑑𝑋𝑡: change in body size 𝛔: intensity of random drift 𝑑𝐵(𝑡): random variation 𝛉: body size optimum Session 163—Booth #308 Figure 2.1: Habitat reconstruction for the four clades of interest. Tip and node labels indicate terrestrial, toothed aquatic, and baleen aquatic species. Figure 3.1: Body size optima. Model-averaged median optima (θ) as estimated by OUwie analyses (circles) and mean modern body sizes (triangles) separated