Evolution of the blowhole in whales and dolphins | Polarjournal
Fin whales are the second largest whales after blue whales. They are common in all oceans and also roam in polar waters during the summer months. Photo: Julia Hager

Dolphins and whales spend their entire lives in the ocean and must come to the surface for each breath. The location of their “nose”, the blowhole, on the top of their head allows them to exchange breathing air efficiently during the sometimes brief surfacing. But how does the blowhole of whales and dolphins actually get on top of the head? Two scientists from the University of Washington and Duke University also thought about this question. In their study, which recently appeared in the Journal of Anatomy, they found an answer.

Whales and dolphins evolved from an ancestor of a four-legged land mammal into marine mammals more than 50 million years ago, and they have adapted well to life in the water during their evolution. One of the adaptations is the peculiar position of their nostril on top of their head. It rises at an angle to the roof of the mouth and ends at the top of the head as a blowhole. During the embryonic development of whales and dolphins, the nasal passage initially lies parallel to the palate and opens at the tip of the snout, similar to other mammals. So far, experts have not been able to decipher how the nasal passage changes to an angled orientation during embryonic and fetal development and then ends in a blowhole.

Images of a representative sample of pantropical spotted dolphin embryos and fetuses used in this study. The white arrow indicates the position of the developing blowhole. Image: Roston and Roth, J. Anatomy, 2021

According to Rachel Roston, a postdoctoral researcher at the University of Washington School of Dentistry and lead author of the study, such a developmental process does not occur in any other mammal. “It’s an interesting question to see what parts remain connected, what parts shift orientation and how might they work together through a developmental process to bring about this change.”

Roston and biology professor Louise Roth, co-author of the study, used photographs and CT scans to accurately measure anatomical details of bottlenose dolphin (Tursiops truncatus) and pantropical spotted dolphin (Stenella attenuata) embryos and fetuses and compared them to data from fin whale (Balaenoptera physalus) fetuses. They obtained the specimens and data, respectively, from the Smithsonian Institution’s National Museum for Natural History and the Natural History Museum of Los Angeles County.

This allowed the two scientists to identify the anatomical changes during development that shift the orientation of the nasal passage upward. “We discovered that there are three phases of growth, primarily in the head, that can explain how the nasal passage shifts in orientation and position”, Roston said.

Representative sample of side-view CT scans of pantropical spotted dolphin (A, B and C) and fin whale (D, E and F) fetuses. Image: Roston and Roth, J. Anatomy, 2021

The three stages of growth are:

  1. Initially parallel, the roof of the mouth and the nasal passage become separated as the area between them grows into a triangular shape. This phase begins during embryonic development after the face starts forming, which, for the pantropical spotted dolphin, is in the first two months after fertilization.
  2. The snout grows longer at an angle to the nasal passage, further separating the nostrils from the tip of the snout. This phase begins later in fetal development and may continue even after birth.
  3. The skull folds backward, and the head and body become more aligned. This rotates the nasal passage up so that it becomes nearly vertical relative to the body axis. This phase begins in late embryonic development and continues through fetal development.

Roston explains that these three stages of growth are distinct developmental transformations that do not occur one step at a time, but overlap in time and collectively shift the nasal passage to the top of the head.

In addition, they found that there are significant differences between the pantropical spotted dolphin, which belongs to the toothed whales, and the fetuses of the fin whale, a baleen whale. According to the study, in fin whales, the skull folds in the posterior region, near where the skull connects to the spine while in spotted dolphins, the fold is in the middle of the skull. Toothed and baleen whales separated over 30 million years ago.

Pantropical spotted dolphins are found worldwide in all temperate and tropical oceans. Considered threatened before the introduction of “dolphin-friendly” tuna fishing, they are now one of the most common dolphins. Photo: Marie Hill/NOAA/NMFS/PIFSC

“I’m struck by two interesting discoveries that emerged from this work,” Roth said. “Although they both develop blowholes, there are key differences between a baleen and a toothed whale in how they reorient their nasal passages during development. Moreover, surprisingly, accompanying the processes of developing upwardly oriented nostrils there are profound changes within the braincase.” Roston further describes that in the future, studying more species from both lineages could reveal whether all baleen and toothed whales differ in this way.

The results of the study provide an integrative model for this developmental process in dolphins and cetaceans that could shed more light on their evolution. “This model gives us a hypothesis for the developmental steps that had to occur to make that anatomical transformation happen, and will serve as a point of comparison for additional studies of growth and development in whales, dolphins and porpoises”, Roston says.

Julia Hager, PolarJournal / Original text: James Urton, University of Washington

Link to the study: Rachel A. Roston, V. Louise Roth. Different transformations underlie blowhole and nasal passage development in a toothed whale (Odontoceti: Stenella attenuata ) and a baleen whale (Mysticeti: Balaenoptera physalus ). Journal of Anatomy, 2021; DOI: 10.1111/joa.13492

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