In genetic investigations genes for taste receptors for the tastes sour and salty were found. The genes for the other taste receptors which perceive the tastes sweet, bitter and umami were not detected. That is why researchers assume that penguins can only taste the qualities sour and salty but this has to be cleared with behavioural experiments.
However, no taste buds were found in the tongues of four investigated penguin species (African, Humboldt, Gentoo and Macaroni Penguins). The researchers noted that there could occur taste buds in the pharynx of the penguins but they did not examined this region. Taste buds contain the taste receptors. It seems that the sense of taste is reduced in penguins during evolution.
Researchers suppose that penguins are not dependent on their sense of taste because they swallow their prey in one piece. The prey persist in their mouth only for a few seconds, so there is only less time for taste perception anyway.
An ion channel* which is called Trpm5 is important for the signal transduction for the tastes sweet, bitter and umami. In the genetic analyses the gene for this ion channel was also not found. This ion channel is temperature dependent. That is why the tastes sweet, bitter and umami cannot be received when its very cold in an organism's environment.
Some researchers are of the opinion that penguins originated in the Antarctic and distributed to other regions from there. It is assumed that these ancestral penguins were not able to perceive sweet, bitter and umami tastes because of the cold temperature in the Antarctic. That is why the ability and genes for the perception of these three tastes might get lost in ancestral penguins during evolution. Because it is not possible to retrieve genes that were lost once, all recent penguin species probably lack of the ability to taste sweet, bitter and umami.
* An ion channel is a protein that you can find in a cell membrane. This protein is able to transport charged particles into the cell or out of the cell.
Kobayashi, K., Kumakura, M., Yoshimura, K., Inatomi, M., & Asami, T. (1998). Fine structure of the tongue and lingual papillae of the penguin. Archives of histology and cytology, 61(1), 37-46.
Zhao, H., Li, J., & Zhang, J. (2015). Molecular evidence for the loss of three basic tastes in penguins. Current Biology, 25(4), R141-R142.