Vampire bats' strict blood diet has made them lose much of their ability to taste bitter flavours, a study has found.
Bitter taste acts as a natural defence against eating poisonous foods and was thought to be indispensable in animals.
Researchers say the bats' special diet and use of smell, echolocation and heat could have made taste less important.
Their work shows poor bitter taste is more widespread in animals than previously thought.
The findings are reported in the journal Proceedings of the Royal Society B.
Toxins typically taste bitter to animals but bottlenose dolphins and some whales have been shown to have reduced bitter taste, probably because they swallow their food whole, making taste unnecessary.
Vampire bats are the only mammals to feed solely on blood meaning they are unlikely to encounter toxic foods in the wild. The research team wanted to find out if that had left them with a lack of bitter taste.
Lead author, Professor Huabin Zhao, from Wuhan University, previously discovered that vampire bats had lost sweet and umami (savoury) tastes.
The researchers analysed the genomes of four bat species - representing two major subgroups of bats - and identified bitter taste receptor genes, which allow animals to taste bitter flavours.
As a result they predicted all bats should have bitter taste.
They compared the sequences of nine taste receptor genes in all three species of vampire bats and 11 species of non-vampire bats.
Vampire bats lost bitter taste very recently'
End Quote Professor Huabin Zhao Wuhan University
They found vampire bats had a higher percentage of non-functioning bitter taste receptor genes (called pseudogenes) (47%) compared to non-vampire bats (4.3%). This demonstrated a greatly reduced bitter taste in vampire bats.
Vampire bats were found to still have some functioning bitter taste receptors but the authors say these are unlikely to play a major role in selection of food and may suggest vampire bats' ancestors did not originally feed on blood.
'From our study we understand that vampire bats do not need bitter taste, but some bitter taste receptor genes are still needed in vampire bats for other functions,' Prof Zhao said.
'Unlike other mammals, vampire bats lost bitter taste very recently, suggesting they may switch their diet from insects to blood in a recent time range.
'More importantly bats represent a unique case in mammals for studying the driving force in taste receptor evolution.'
The researchers are now planning to look more closely at the role of functioning bitter taste receptors in vampire bats.
Join BBC Nature on Facebook and Twitter @BBCNature.