The pattern of brain-size change in the early evolution of cetaceans

Abstract

Most authors have identified two rapid increases in relative brain size (encephalization quotient, EQ) in cetacean evolution: first at the origin of the modern suborders (odontocetes and mysticetes) around the Eocene-Oligocene transition, and a second at the origin of the delphinoid odontocetes during the middle Miocene. We explore how methods used to estimate brain and body mass alter this perceived timing and rate of cetacean EQ evolution. We provide new data on modern mammals (mysticetes, odontocetes, and terrestrial artiodactyls) and show that brain mass and endocranial volume scale allometrically, and that endocranial volume is not a direct proxy for brain mass. We demonstrate that inconsistencies in the methods used to estimate body size across the Eocene-Oligocene boundary have caused a spurious pattern in earlier relative brain size studies. Instead, we employ a single method, using occipital condyle width as a skeletal proxy for body mass using a new dataset of extant cetaceans, to clarify this pattern. We suggest that cetacean relative brain size is most accurately portrayed using EQs based on the scaling coefficients as observed in the closely related terrestrial artiodactyls. Finally, we include additional data for an Eocene whale, raising the sample size of Eocene archaeocetes to seven. Our analysis of fossil cetacean EQ is different from previous works which had shown that a sudden increase in EQ coincided with the origin of odontocetes at the Eocene-Oligocene boundary. Instead, our data show that brain size increased at the origin of basilosaurids, 5 million years before the Eocene-Oligocene transition, and we do not observe a significant increase in relative brain size at the origin of odontocetes.

New Skeletons of the Ancient Dolphin Xenorophus sloanii and Xenorophus simplicidens sp. nov. (Mammalia, Cetacea) from the Oligocene of South Carolina and the Ontogeny, Functional Anatomy, Asymmetry, Pathology, and Evolution of the Earliest Odontoceti

“…The smaller size estimates are influenced by the taxa at the base of Xenorophidae, including Inermorostrum and ChM PV4746 (BZW 185.3 mm), as well as Olympicetus, which is situated near the base of the stem that leads to extant odontocetes (Figure 88). Although the magnitude of the skull size decrease is uncertain, this observation is consistent with previous studies that have inferred a notable body size decrease near the origin of Odontoceti (i.e., [189,191]) but is at odds with the recent work of Waugh and Thewissen [190]. Waugh and Thewissen [190] calculated much greater body sizes for Oligocene odontocetes, including the xenorophids Albertocetus and Xenorophus, with the result that the differences encephalization quotients between basilosaurids and early odontocetes were statistically indistinguishable.…”

“…Although the magnitude of the skull size decrease is uncertain, this observation is consistent with previous studies that have inferred a notable body size decrease near the origin of Odontoceti (i.e., [189,191]) but is at odds with the recent work of Waugh and Thewissen [190]. Waugh and Thewissen [190] calculated much greater body sizes for Oligocene odontocetes, including the xenorophids Albertocetus and Xenorophus, with the result that the differences encephalization quotients between basilosaurids and early odontocetes were statistically indistinguishable. On the one hand, we find the methodologies employed by Waugh and Thewissen [190] to be a convincing improvement, but we also have high confidence in our finding of a decrease in BZW width along the odontocete stem.…”

“…Previous studies of cetacean body size through time have used skull dimensions as a proxy for body size [165,183] or used equations that include skull measures [98,184,185] or skull and postcranial measures [186,187] to reconstruct evolutionary changes in body length through time [98,185]. Most studies have focused on body size trends within mysticetes [73,165,183,185,186], a few on the fitting of evolutionary models to the entire cetacean clade [184,187], and the few that have focused on body size evolution within Odontoceti have largely done so within the context of reconstructing brain size [1,98,165,[188][189][190].…”

The better to eat you with: morphological disparity and enamel ultrastructure in odontocetes

“…We conducted an exploratory analysis of the relationship between enamel ultrastructure, dental morphological characters, and different paleoecological variables to test whether they could explain the observed morphological variation. The categorical variables defined were as follows: (i) habitat (semi-aquatic, freshwater, coastal marine, oceanic marine, and polar); (ii) diet (freshwater prey, piscivorous, piscivorous and others, teutophagous, and top predator); (iii) body mass (maximum value in kg regardless of whether it was a male or female; for extinct species, we estimated their body mass following 54 or 55 ); and (iv) feeding method (semi-aquatic feeding, raptorial, combination suction, and capture suction; based on 10 , 11 , 13 ) (see further details in Supplementary Table S2 online).…”

Of mice and men: Dendritic architecture differentiates human from mice neuronal networks

“…Initially, it was postulated that human cognitive prowess was linked to the sheer magnitude of our brains (Cairò, 2011), stemming from the notion that intelligence is directly correlated with brain size. However, research has failed to identify similar abilities in other large-brained animals, such as elephants (Hart et al, 2001) and cetaceans (Waugh and Thewissen, 2021). Furthermore, subsequent studies (Hofman, 1988) systematically analyzing the ratio of brain to body mass refuted these theories, demonstrating that human brain size is not distinctive compared to other animals.…”