Prenatal Development of the Humpback Whale: Growth Rate, Tooth Loss and Skull Shape Changes in an Evolutionary Framework

Abstract

Extant baleen whales (Mysticeti) share a distinct suite of extreme and unique adaptations to perform bulk filter feeding, such as a long, arched skull, and mandible and the complete loss of adult dentition in favor of baleen plates. However, mysticetes still develop tooth germs during ontogeny. In the fossil record, multiple groups document the transition from ancestral raptorial feeding to filter feeding. Fetal specimens give us an extraordinary opportunity to observe when and how this macroevolutionary transition occurs during gestation. We used iodine-enhanced and traditional CT scanning to visualize the internal anatomy of five fetuses of humpback whale representing the first two-thirds of gestation, and we combine these data with previously published reports to provide the first comprehensive qualitative description of the sequence of developmental changes that characterize the skull and dentition. We also use quantitative methods based on 3D landmarks to investigate the shape changes in the fetuses in relation to a juvenile cranial morphology. We found similarities in the ossification patterns of the humpback and other cetaceans (dolphins), but there appear to be major differences when comparing them to terrestrial artiodactyls. As for the tooth germs, this developmental sequence confirms that the tooth-tobaleen transition occurs in the last one-third of gestation. Analysis of cranial shape development revealed a progressive elongation of the rostrum and a resulting posterior movement of the nasals relative to the braincase. Future work will involve acquisition of data from other species to complete our documentation of the teeth-to-baleen transition.

Prenatal developmental sequence of the skull of minke whales and its implications for the evolution of mysticetes and the teeth‐to‐baleen transition

“…Detailed external measurements following Lanzetti et al. () were taken on all described fetuses up to 1.21 m in total body length due to the excessive weight and size of larger specimens (Supporting Information Table ). Since the Antarctic minke whale was not recognized as a separate species until recently (Rice, ) , specimens at both institutions were labeled as either ‘minke whale’ or ‘ B. acutorostrata ’, as they were collected from the late 1800s to the 1960s, although most of the specimens had a known collection location, with fetuses from Japan being collected in the Southern Pacific Ocean and those from Denmark being collected in Greenland.…”

“…A Fetal Stage (FS) of development was assigned to each of the specimens based on previous work on the development of the humpback whale (Lanzetti et al. ) and the pantropical spotted dolphin ( Stenella attenuata ; Sterba et al. ; Thewissen & Heyning, ).…”

“…) and traditional CT scanning, based on the high‐quality results obtained using these non‐invasive methods in another mysticete species, the humpback whale (Lanzetti et al. ). Three‐dimensional (3D) geometric morphometric (GM) landmark analysis was used to quantify skull shape changes in a developmental series of minke whales, from the earliest fetal stages to adult.…”

Extreme Anatomy: Gear for the Pioneer

“…Baleen whales underwent additional extreme changes, losing the dentition prenatally, and evolving a feather‐like array of keratin (baleen) that serves to filter prey from ocean water. In this issue, Lanzetti, Berta, and Ekdale (this issue) investigate the prenatal development of the humpback whale. They confirm that humpbacks, like other mysticetes, retain teeth beyond the midgestation age, and they conclude that transition from teeth to baleen occurs in the last one‐third of prenatal development.…”

“…The authors discuss developmental evidence that the same growth factor promotes both the presence of tooth germs and the initial formation of baleen. Postnatally, Lanzetti et al (this issue) assert that growth of the buccal cavity drives growth of the rostral skull and this likely account for the overall trend in which mandibular growth is rapid and parallels the growth of overall body size. This developmental account of a mysticete whale reveals how changes in growth rates produce a profoundly large oral cavity, and that teeth alone can sometimes not suffice for survival in the ocean.…”