Penguins have a poor sense of taste, and it is just getting worse over time, a new University of Michigan study reveals.
In over 30 million years of evolution, the animals have lost the ability to recognize bitter and sweet, along with recognition of umami, or a savory, meaty flavor. Genetic analysis was utilized to determine how the sense of taste has evolved in the birds over time.
Taste receptor genes can be perceived in animals through examination of their genetic structure, and used to infer the ability of a given species to perceive various tastes. Fish make up the vast majority of the diet of penguins, so a reduction in the perception of umami flavor was particularly surprising to researchers.
"Penguins eat fish, so you would guess that they need the umami receptor genes, but for some reason they don't have them. These findings are surprising and puzzling, and we do not have a good explanation for them. But we have a few ideas," Jianzhi "George" Zhang from the University of Michigan said.
Birds are believed to have a poor sense of taste. The animals do not possess teeth for chewing food, which means little surface area is exposed to taste buds, sensors which are also less common in birds than other animals. Receptor genes for sweetness have never been identified in any species of bird.
When researchers from a genetics institute in China noted they could not find other receptor genes in Emperor and Adelie penguins, they asked Zhang to examine their analysis. His team was unable to find such genes in any of the five penguin species evaluated.
Genes for recording tastes of umami and bitterness have evolved into pseudogenes, which are unable to direct proteins to take action in bodies. Multiple mutations can sometimes result in the formation of pseudogenes.
These tastes are present in some bird species, which lack the ability to recognize sweetness. This suggests the loss of gene receptors for sweet foods was lost in birds before penguins lost the ability to sense the other tastes.
Penguins evolved from tubenose seabirds around 60 million years ago, developing into several separate species about 37 million years later. During that period, Antarctica experienced a period of extreme cooling, which could be responsible for the loss of these tastes, researchers believe.
A protein known as Trpm5, essential for the recognition of sweet and bitter tastes, along with umami, functions poorly in cold temperatures, possibly leading to the changes seen in penguins. Future research will examine how well Trpm5 functions at temperatures similar to those experienced by penguins in the wild.
Analysis of the sense of taste in penguins was detailed in the journal Current Biology.