Genetic analysis conducted by scientists at Hokkaido University has revealed evidence suggesting that the acoustic fat bodies found in the heads of toothed whales may have originated from their skull muscles and bone marrow. These collections of fatty tissue play a crucial role in enabling toothed whales to communicate, navigate, and hunt using sound, a technique known as echolocation. The researchers analyzed DNA sequences of genes expressed in these acoustic fat bodies, specifically in harbor porpoises and Pacific white-sided dolphins. Their findings, published in the journal Gene, shed light on the genetic origins of these important structures.
Toothed whales have undergone significant evolutionary adaptations to their aquatic lifestyle, including the partial loss of their sense of smell and taste, in exchange for the development of echolocation abilities to aid in underwater navigation. The evolution of acoustic fat bodies, which include the melon in the forehead of whales, extramandibular fat bodies alongside the jawbone, and intramandibular fat bodies within the jawbone, was essential for their use of sound, such as echolocation. Little was previously known about the genetic basis of these fatty tissues, making this research significant in understanding the evolutionary history of toothed whales.
The researchers found that genes associated with muscle function and development were active in the melon and extramandibular fat bodies of toothed whales. This suggests an evolutionary connection between extramandibular fat and the masseter muscle, which plays a key role in chewing in humans. According to Assistant Professor Takashi Hayakawa, who led the study, the evolutionary tradeoff of masticatory muscles for extramandibular fat bodies was crucial in the aquatic adaptation of toothed whales, as they transitioned from chewing to simply swallowing food. This adaptation allowed them to focus on echolocation and communication, rather than chewing.
In their analysis of gene expression in the intramandibular fat, the researchers detected activity related to immune functions, indicating the presence of genes involved in activating elements of the immune response and regulating T cell formation. This finding suggests that the intramandibular fat plays a role not only in acoustics but also in supporting immune functions in toothed whales. The samples used in the study were collected by the Stranding Network Hokkaido (SNH), an organization that gathers specimens of stranded whales in Hokkaido. This collaboration with local communities and researchers has allowed for various studies of whale biology, leading to surprising discoveries such as those revealed in this study.
The research conducted by Hokkaido University sheds light on the genetic origins of acoustic fat bodies in toothed whales, providing new insights into the evolution of these important structures. By studying gene expression in these fatty tissues, the researchers were able to uncover an evolutionary connection between muscle function, immune responses, and the development of echolocation abilities in toothed whales. The collaboration with the Stranding Network Hokkaido highlights the importance of community involvement in scientific research, enabling researchers to study and understand the biology of whales and other marine organisms in greater depth.