Department of Geological Sciences, Stanford University, Stanford, CA 94305;
Louisiana Universities Marine Consortium, Chauvin, LA 70344.
Proc Natl Acad Sci U S A. 2018 Apr 17;115(16):4194-4199. doi: 10.1073/pnas.1712629115. Epub 2018 Mar 26.
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.
海牛目、鲸目、鳍足目和鼬科。这些水生类群的平均体型比它们最接近的陆生亲属要大，但潜在的生态、生物力学和生理控制因素在多大程度上促成了这种模式仍然没有得到定量检验。在这里，我们利用先前发表的关于 3859 种现存和 2999 种化石哺乳动物物种的体重数据，通过比较系统发育分析和对化石记录的研究，来研究水生哺乳动物的体型进化轨迹。这两种方法都表明，水生生活方式的进化正在推动现生的四种水生哺乳动物类群中的三种向约 500 公斤的体型吸引子进化。这种体型吸引子的存在，以及对该吸引子规则的快速选择和有限偏离，排除了大多数关于体型增加的假设驱动因素。这三个独立的体型增加和一个共同的水生最优体型与在陆地和水生领域，能量摄入和成本函数的缩放与体型之间的差异控制相一致。在这种能量模型中，体温调节成本限制了最小体型，而摄食效率的限制则限制了最大体型。最佳体型出现在体温调节成本较低但摄食效率仍然较高的中间值。水生哺乳动物并不是因为摆脱了体型压力而被驱动和限制在更大的体型，而是因为受到水生介质严格的能量需求的驱动和限制。
