April 28 (UPI) -- All whales have blowholes, but not all of them evolved them the same way -- according to a new study, the two major forms of cetaceans turned their noses into blowholes in different ways.
Cetaceans, the group of marine mammals that includes whales and dolphins, evolved from land mammals. The earliest cetaceans had noses a lot like their land-based relatives, but at some point, the forward-pointing nose became an upward-facing blowhole.
Often, scientists trying to understand the evolutionary origins of a distinct anatomical feature focus on fossils. For the latest study, presented this week in the Experimental Biology meeting, scientists observed the development of spotted dolphin and fin whale embryos and fetuses.
During embryonic and fetal development, researchers watched as the nasal passage initially formed like a nose before migrating back to its position atop the body, where blowholes are found.
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"The main difference is in which other parts of the skull change orientation in relation to the nasal passage," study lead author Rachel Roston told UPI in an email.
Scientists were surprised to find the development of the blowhole during embryonic and fetal development followed two distinct patterns.
"In dolphins, which are toothed whales, odontocetes, those changes occur in the middle of the skull. But, in fin whales, which are baleen whales, mysticetes, we did not see the same changes the middle of the skull as the dolphins," said Roston, a postdoctoral fellow at the University of Washington.
Instead, the anatomical transformation in fin whales involves the rear of the skull, at the nexus of the neck and vertebral column.
"Other closely-related species seem to follow each pattern," Roston said. "So, it seems there are at least two ways to reorient the nasal passage into a blowhole during development, one in toothed whales and another in baleen whales."
Because most previous studies have focused on the shapes and structures of whale blowholes, Roston and her colleagues wanted to look at how the nasal passage relates to the other parts of the head and body.
The differences in the way the nasal passage reorients itself during prenatal development may help explain the functional difference in the blowholes of toothed and baleen whales.
"Baleen whales use their nasal passages for breathing, and toothed whales use their nasal passages for both breathing and echolocation," Roston said.
"So, the differences in development that we've identified are accompanied by many other interesting functional and anatomical differences in the two groups," Roston said.
As so often happens, researchers said their findings have raised more new questions about blowhole evolution than they have answered.
"It will be interesting to see how these developmental differences relate to other differences in blowhole and head anatomy among fossil and living cetaceans," Roston said.
"Likewise, it will be interesting to see how these discoveries in cetacean development reshape how we think about skull and head development and evolution in other mammals," she said.
