Dolphins and other toothed whales–or Odontocetes–use their heads to make sounds that assist them in communication, navigation, and hunting in their dark marine environment. These sometimes vocal-fry-like sounds reveal important information about their dark marine environment. Some new genetic analysis suggests that the collections of fatty tissues that enable echolocation in toothed whales may have evolved from their skull muscles and bone marrow, altering how these animals eat and perceive the world around them. The findings are described in a study published in the April 2024 issue of the journal Gene.
Toothed whales include numerous dolphin species as well as orcas, sperm whales, belugas, and narwhals. Echolocation produced by a bulbous mass of fat tissue inside of their heads called the melon.
Alongside of the jawbone of dolphins and toothed whales is a group of sound-producing extramandibular fat bodies (EMFB). Another set of acoustic fat deposits called the intramandibular fat bodies (IMFB) are located inside the jawbone. The evolution of the melon, the extramandibular, and intramandibular fat bodies was crucial for echolocation to develop in these marine mammals. However, little is known about how these fatty tissues themselves originated genetically.
“Toothed whales have undergone significant degenerations and adaptations to their aquatic lifestyle,” Hayate Takeuchi, a study co-author and PhD student at Hokkaido University in Japan, said in a statement.
One of these adaptations was the partial loss of their sense of smell and taste, alongside the gain of echolocation. To look closer at this and other adaptations at a genetic level, the team from Hokkaido University studied DNA sequences of genes that are expressed in these acoustic fat bodies. They measured the gene expressions in harbor porpoises (Phocoena phocoena) and Pacific white-sided dolphins (Lagenorhynchus obliquidens).
This dolphin ancestor looked like a cross between Flipper and Moby Dick.]
They found that the genes which are normally associated with muscle function and development were active in the melon and EMFB’s on the outside of the jawbone. There was also evidence of an evolutionary connection between this fat and a muscle called the masseter muscle. In humans, the masseter muscle connects the lower jawbone to the cheekbones and is one of the the key muscles used in chewing.
“This study has revealed that the evolutionary tradeoff of masticatory muscles for the EMFB—between auditory and feeding ecology—was crucial in the aquatic adaptation of toothed whales,” study co-author and genome scientist and evolutionary biologist Takashi Hayakawa said in a statement. “It was part of the evolutionary shift away from chewing to simply swallowing food, which meant the chewing muscles were no longer needed.”
We now understand how baleen whales create sound.]
When the scientists examined the gene activity in the fat inside the jawbone, they discovered active genes related to certain parts of the immune response and the control of a group of white blood cells that combat infection called T cells. These are special cells that fight illness called T cells. The scientists think that this is because the fat is close to the bone marrow, which makes T cells, and more research is needed to confirm this.
The scientists also gave credit to the Stranding Network Hokkaido as another important part of the study, as they provided the samples used in this study. The organization has collected specimens of stranded whales along the seashore and river mouth in Hokkaido. This work with stranded marine mammals has been essential for collecting samples and conducting research to understand more about the potential reasons for strandings and death, as well as their anatomy, physiology, and evolution. ‘Chewing muscles were no longer needed.’
