Cetaceans (family members of dolphins and whales) have evolved from terrestrial life to marine environments, and have acquired various novel traits to adapt to aquatic life. For example, in order to hear underwater sounds, the head is filled with ``acoustic fat,'' which allows sound waves to pass through easily, and the sound that reaches the face is transmitted through the skin to the inner ear.

　The research group conducted a comprehensive gene expression analysis of the acoustic fat of two species of dolphins and found that the gene expression of the acoustic fat exhibits a pattern intermediate between that of muscle and fat. This suggests that acoustic fat was originally muscle and evolved as intramuscular fat.

　When we investigated the expression of individual genes, we found that a gene called MYH16 (myosin heavy chain 16) was expressed specifically in the external mandibular fat pad. MYH16 is a gene that is originally expressed specifically in the masticatory muscles of terrestrial animals. In other words, it can be considered that the external mandibular fat pad was originally a masticatory muscle.

　In the process of adapting from terrestrial to aquatic life, dolphins evolved to swallow their prey whole instead of chewing it. This led to the story that the masticatory muscles, which were no longer needed, replaced the acoustic fat that transmits sound. In this way, evolution in which a new trait is acquired by losing something is called trade-off evolution, and in dolphins, trade-off evolution occurred between the trait of chewing and the trait of receiving sound waves underwater. .

　This discovery will lead to understanding the mechanism by which cetaceans have evolved into the ocean, and will also provide new clues to understanding the relationship between muscle and fat, as in humans, muscle can replace fat due to lack of exercise or a high-fat diet. It also becomes.

Paper information:【Gene】A tradeoff evolution between acoustic fat bodies and skull muscles in toothed whales