Penguins have lost three of the five taste senses and cannot distinguish sweet, bitter or umami flavours, new research has shown.
Humans have between 2000 and 8000 taste buds on our tongues; these specialised sensors are activated in response to the different chemicals present in food and send electrical signals to the brains when they engage with sour, salty, sweet, bitter and "umami" flavours.
Penguins, on the other hand, appear to be able to experience only sour and salty sensations, a discovery that University of Michigan scientist Professor Jianzhi Zhang stumbled upon by accident.
"We were sequencing the genomes from two penguin species and we couldnít find some of the [taste] receptor genes, which was puzzling."
Further investigation revealed that the DNA sequences specifying the presence of the taste buds were missing from the genome. Only salty and sour taste genes were present: the sweet, bitter and umami flavour-detecting genes weren't there.
This might be a consequence of the cold environment in which the birds have evolved to live, Zhang speculates, because the missing taste genes make use of a component called TRPM5 - short for Transient receptor potential cation channel subfamily M member 5.
"TRPM5 is specifically required for transducing the sweet, bitter and umami tastes to the brain." But this type of receptor is highly temperature-sensitive, and won't in the cold.
Another possible explanation is the slippery nature of their fishy diet. To ensure that it can cling on to the food they catch, the penguin tongue is covered in bristles, so taste takes more of a backseat and may have been lost for this reason.
On the other hand, the reverse may also be true. "We donít know if the loss of taste receptors led to the change in tongue structure, or if itís the change of tongue structure that caused the loss of taste," says Zhang.