The Evolutionary History of Cetacean Brain and Body Size

Abstract

Cetaceans rival primates in brain size relative to body size and include species with the largest brains and biggest bodies to have ever evolved. Cetaceans are remarkably diverse, varying in both phenotypes by several orders of magnitude, with notable differences between the two extant suborders, Mysticeti and Odontoceti. We analyzed the evolutionary history of brain and body mass, and relative brain size measured by the encephalization quotient (EQ), using a data set of extinct and extant taxa to capture temporal variation in the mode and direction of evolution. Our results suggest that cetacean brain and body mass evolved under strong directional trends to increase through time, but decreases in EQ were widespread. Mysticetes have significantly lower EQs than odontocetes due to a shift in brain:body allometry following the divergence of the suborders, caused by rapid increases in body mass in Mysticeti and a period of body mass reduction in Odontoceti. The pattern in Cetacea contrasts with that in primates, which experienced strong trends to increase brain mass and relative brain size, but not body mass. We discuss what these analyses reveal about the convergent evolution of large brains, and highlight that until recently the most encephalized mammals were odontocetes, not primates.

Ultrasonic hearing and echolocation in the earliest toothed whales

“…Recent analyses of early odontocetes show that even the most archaic stem taxa possessed cranial morphology functionally linked to generation of high-frequency signals and therefore the potential to echolocate [5,7,8]. The xenorophid inner ear described here corroborates the latter, demonstrating that early diverging stem odontocetes had cochleae sensitive to high-frequency sounds pivotal in echolocation.…”

“…The advent of echolocation is thought to be a key innovation that supported exploitation of a vast pelagic biomass-vertically migrating organisms, especially cephalopods-and explosive diversification of odontocetes [3,4]. Bony correlates of nasofacial tissues linked to ultrasonic signal production have been identified in archaic fossil odontocetes [5,6], but until now we have lacked anatomical evidence from their inner ear to test for high-frequency hearing, and verify functional echolocation [6][7][8]. Here, we describe the cochlear anatomy in a member of the Oligocene Xenorophidae, one of the earliest diverging odontocete clades [6,8], showing that the most archaic odontocetes could detect high-frequency sound, although probably not in the upper range of some living odontocetes and retained greater sensitivity to lower frequencies.…”

Is Cetacean Intelligence Special? New Perspectives on the Debate

“…The available information suggests that social communication and structure are often complex in mysticetes [92], and include long-term social bonds, long-range communication, cooperative hunting, cultural traditions, and fission-fusion like social behavior [91]. These behaviors are observed in some large-brained odontocetes like the Sperm whale (Physeter macrocephalus) and primates, and are considered cognitively demanding [92].…”

“…The available information suggests that social communication and structure are often complex in mysticetes [92], and include long-term social bonds, long-range communication, cooperative hunting, cultural traditions, and fission-fusion like social behavior [91]. These behaviors are observed in some large-brained odontocetes like the Sperm whale (Physeter macrocephalus) and primates, and are considered cognitively demanding [92]. For instance, the discovery of complex learned songs, i.e., structured sequences of vocalizations that cycle with a period of 5-25 min and clever fish corralling strategy techniques, including the use of bubble clouds to encircle prey, the production of loud feeding calls and the waving of their large pectoral flippers as a feeding tactic [93] certainly suggest that humpback whales (Megaptera novaeangliae) are quite intelligent [13,91].…”

ASPM and mammalian brain evolution: a case study in the difficulty in making macroevolutionary inferences about gene–phenotype associations

“…Instead, they rely on previous assumptions about cetacean evolution to highlight key branches. Recent comparative analyses unfortunately suggest that these assumptions are not valid [7]. Furthermore, their results demonstrate that positive selection was again pervasive and not limited to a subset of branches.…”