Dolphins without sense of taste can identify own mother's milk

Various functions found in cetaceans, such as whales and dolphins, have evolved or degenerated due to their aquatic adaptations. As they began to eat by directly swallowing fish and shellfish without chewing, they no longer needed to recognize tastes, and their sense of taste degenerated. At least the genes involved in recognizing sweetness, umami, and bitterness are broken. Meanwhile, cetaceans are mammals and grow up on their mother's milk. Infants of land-dwelling mammals perceive sweetness and umami in their mother's milk, but what about infants of cetaceans?

A research group including Third-year Doctoral Student Hinako Katsushima at the Graduate School of Environmental Science, Hokkaido University, Assistant Professor Takashi Hayakawa at the Faculty of Environmental Earth Science, Hokkaido University, and researchers at Kagoshima City Aquarium, Mikurashima Tourist Association, and San-Ei Gen F.F.I. has discovered that genes for fat taste receptors and lipolytic enzymes are expressed on the tongue of infants of the genus of bottlenose dolphins possibly for recognition of fatty acids in breast milk as a taste. These findings were published in Marine Mammal Science.

The research group considered that infant cetaceans might use a "sixth taste" to recognize their mother's milk, and the taste of fat has been proposed as this. The milk of cetacean mothers stands out from that of other mammals in its high-fat content. The infant dolphin's tongue has projections called the marginal papillae at the tip and a depression called the V-shaped row on the throat side. These structures disappear in adulthood. Therefore, they hypothesized that fat taste receptors are expressed in the marginal papillae and V-shaped row to perceive fat taste in fat-rich mother's milk.

To test this hypothesis, they conducted three analyses: expression of fat taste receptor genes on the tongue of infant dolphins, the fat composition of dolphin milk, and whether infant dolphins can identify their mother's milk. Gene expression analysis of the tongue of an Indo-Pacific bottlenose dolphin revealed that various types of fat taste receptors and related genes were expressed in the marginal papillae and V-shaped row. These genes were prominently expressed in the V-shaped row, where FFAR4, a receptor for long-chain fatty acids, the related gene PLCB2, and the lingual lipase (LIPF) were expressed. They performed a component analysis of the milk of common bottlenose dolphins and detected 7 unsaturated long-chain fatty acids, including essential and 16 saturated long-chain fatty acids.

In a pool with a spreader containing mother's milk and a spreader containing a milky white fluid floating on water, an infant common bottlenose dolphin significantly approached the spreader containing a milky white fluid. Although it remains unclear why the dolphin avoided the milk, the dolphin's different responses to the two containers indicate that it could identify the milk.

Journal Information
Publication: Marine Mammal Science
Title: Fat taste receptors and fatty milk in dolphins
DOI: 10.1111/mms.13195