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Body Weight and Relative Brain Size (Encephalization) in Eocene Archaeoceti (Cetacea)

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Abstract

Calibration of the Brownian diffusion model of Felsenstein indicates that phylogeny may have an influence on body length and other phenotypic measures in Cetacea for as many as 10,000 generations or about 180,000 years, which is negligible in the 35 million year history of extant Cetacea. Observations of phenotypic traits in cetacean species living today are independent of phylogeny and independent statistically. Four methods for estimating body weight in fossil cetaceans are compared: (1) median serial regression involving a set of multiple regressions of log body weight on log centrum length, width, and height for core vertebrae; (2) regression of log whole body weight on log body length for individuals; (3) regression of log whole body weight on log body length for species means; and (4) regression of log lean body weight on log body length for individuals. These yield body weight estimates for the Eocene archaeocete Dorudon atrox of 1126, 1118, 1132, and 847 kg, respectively, with consistency and applicability to partial skeletons favoring the first approach. The whole-body weight expected, Pe (in kg), for a given body length, Li (in cm), is given by log10 Pe = 2.784 • log10 Li − 4.429. Negative allometry of body weight and body length (slope 2.784 < 3.000) means that small cetaceans are shorter and more maneuverable than expected for their weight, while large cetaceans are longer and more efficient energetically than expected for their weight. Encephalization is necessarily quantified relative to a reference sample, most mammals are terrestrial, and terrestrial mammals provide a logical baseline. The encephalization residual for living terrestrial mammals as a class (ERTC), is the difference between observed log2 brain weight (Ei in g) and expected log2 brain weight (Ee in g), where the latter is estimated from body weight (Pi in g), as log2 Ee = 0.740 • log2 Pi − 4.004. ERTC is positive for brains that are larger than expected for a given body size, and negative for brains that are smaller than expected. Base-2 logarithms make the ERTC scale intuitive, in uniform units of halving or doubling. Encephalization quotients (EQ) are unsuitable for comparison because they are proportions on a non-uniform scale. Middle Eocene archaeocetes have ERTC values close to −2 (two halvings compared to expectation), while late Eocene archaeocetes have ERTC values close to −1 (one halving compared to expectation). ERTC is not known for fossil mysticetes, but living mysticetes have ERTC values averaging about −2. Oligocene-Recent odontocetes appear to have ERTC values averaging about +1 (one doubling compared to expectation) through their temporal range. Definitive interpretation of the evolution of encephalization in Cetacea will require better documentation for Oligocene–Recent mysticetes and odontocetes.

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Acknowledgments

Publication was prompted by Marino et al. (2003), who noted that the computer program of Gingerich (1998) for estimating body weight in fossil cetaceans was still unpublished. This was rewritten in R to carry out the analyses presented here (Online Resource 2). I thank Bruce D. Patterson, Field Museum of Natural History, and Philip Myers, University of Michigan Museum of Zoology, for access to mammal specimens in their care. John L. Gittleman checked several questionable published measurements, and Graham A. J. Worthy supplied important marine mammal references. Michael Foote, Gregg Gunnell, Serge Legendre, and Philip Myers read early versions of the manuscript. Two anonymous reviewers made suggestions and raised questions that improved the text substantially.

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  1. Museum of Paleontology, University of Michigan, Ann Arbor, MI, 48109-1079, USA

    Philip D. Gingerich

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Correspondence to Philip D. Gingerich.

Electronic supplementary material

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Online Resource 1

Measurements of vertebral centra for eleven extant marine mammals used to predict body weight from centrum length, centrum width, and centrum height. (XLSX 19 kb)

Online Resource 2

R script used to predict body weight in Eocene Archaeoceti from vertebral centrum length, centrum width, and centrum height. (PDF 18 kb)

Online Resource 3

Measurements of vertebral centrum length, centrum width, and centrum height used to predict body weight for the Eocene basilosaurid archaeocete Dorudon atrox. (XLSX 11 kb)

Online Resource 4

Measurements of body length (meters) and body weight (metric tonnes) for 1711 individuals representing all 12 or 13 families, 32 genera, and 59 species of extant cetaceans. When available , weights of blubber, meat, bone, and viscera (metric tonnes) are included. (XLSX 116 kb)

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Gingerich, P.D. Body Weight and Relative Brain Size (Encephalization) in Eocene Archaeoceti (Cetacea). J Mammal Evol 23, 17–31 (2016). https://doi.org/10.1007/s10914-015-9304-y

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