Bauplan of a Blowhole: Testing a Developmental Model for Nasal Passage Reorientation in Echolocating Whales (Odontoceti) and a Baleen Whale (Mysticeti)

Abstract

Adaptation to secondarily aquatic lifestyles has transformed cetacean (whale, dolphin, porpoise) heads by shifting the nares posterodorsally to form a blowhole. Reorientation of the nasal passage (NP) away from the anteroposterior axis has evolved rarely among mammals, yet all extant cetaceans possess blowholes and dorsally-rotated NPs. Nevertheless, anatomical and functional differences between the two cetacean sub-orders, echolocating whales (odontocetes) and baleen whales (mysticetes), suggest that NP reorientation may be the result of convergent or parallel evolution rather than homology. To assess whether nasal passage reorientation in members of the two sub-orders results from similar developmental transformations during ontogeny, we first used morphometrics on photos and CT scans to characterize fetal series of an odontocete (Stenella attenuata) and a mysticete (Balaenoptera physalus). Although the NP and plane of the foramen magnum became more parallel in both species, we found that basicranial retroflexion played an important role in NP reorientation in S. attenuata, but found no evidence of basicranial retroflexion in B. physalus. We then used the results from S. attenuata to develop a model of how developmental transformations reorient the NP during ontogeny in odontocetes. Testing the generality of this model against published observations of other fetal odontocetes suggests that the model is widely applicable. Our findings add evidence suggesting that NP reorientation evolved twice within Cetacea and lay a foundation for future syntheses of data from fossils and other species.