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

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, P e (in kg), for a given body length, L i (in cm), is given by log 10 P e =2.784 • log 10 L i − 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 (ER TC ), is the difference between observed log 2 brain weight (E i in g) and expected log 2 brain weight (E e in g), where the latter is estimated from body weight (P i in g), as log 2 E e =0.740 • log 2 P i −4.004. ER TC 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 ER TC scale intuitive, in uniform units of halving or doubling. Encephalization quotients (EQ) are unsuitable for comparison because they are proportions on a nonuniform scale. Middle Eocene archaeocetes have ER TC values close to −2 (two halvings compared to expectation), while late Eocene archaeocetes have ER TC values close to −1 (one halving compared to expectation). ER TC is not known for fossil mysticetes, but living mysticetes have ER TC values averaging about −2. Oligocene-Recent odontocetes appear to have ER TC 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.

Comparison of Dolphins' Body and Brain Measurements with Four Other Groups of Cetaceans Reveals Great Diversity

“…In evolutionary history, the animals with the largest brains relative to body mass, the Delphinidae, have experienced a large decrease in body mass [Montgomery et al, 2013]. Studies such as Marino et al [2004] and Montgomery et al [2013] relied heavily on EQ derived from endocranial volume [also see Gingerich, 2015]. Given the vast differences in nonbrain endocranial tissue, we are skeptical of these results.…”

“…The smallest adult La Plata dolphin, Pontoporia blainvillei , may weigh as little as 15-20 kg [Kamiya and Yamasaki, 1974;Pirlot and Kamiya, 1975] while the largest blue whale, Balaenoptera musculus , may approach 200,000 kg [Lockyer, 1976]. It is obvious from many studies that brain size does not vary strictly with body size [Pilleri and Gihr, 1970;Pirlot and Kamiya, 1975;Miyazaki et al, 1981;Ridgway and Brownson, 1984;Worthy and Hickie, 1986;Marino et al, 2004;Turner et al, 2006;Connor, 2007;Montgomery et al, 2013;Gingerich, 2015;Mallette et al, 2016]. Data on brains of cetaceans of known maturity have been sparse.…”

The Origin of Filter Feeding in Whales

“…A recent study inferred that the most recent common ancestor of all extant cetaceans was 167 kg [35], whereas another suggested that this taxon was about 2.5 m in length [34]. Using equations that relate body mass to length [37], a cetacean this long should be about 175 kg. Coronodon is much larger: an equation that estimates body length from width across the zygomatic process results in a length of 4.9 m [34].…”

“…Coronodon is much larger: an equation that estimates body length from width across the zygomatic process results in a length of 4.9 m [34]. This length corresponds to a mass of 1,150 kg, very similar to the mass estimated for the archaeocete Dorudon atrox [37]. Determining whether Coronodon simply retained the body size of archaeocetes or represents a dramatic increase over a small ancestral neocete will be difficult.…”

“…Body Mass Body mass in kg (BM) was estimated from body length in cm with this equation [37]. logðBMÞ = 2:799 Ã logðTLÞ 4:464…”