While bats have been closely associated with vampires for centuries, there are actually only three species of bats that drink blood. Most of them eat fruits, insects, nectar and small animals, like frogs and fish, instead. Blood is low in calories, while being rich in iron, protein and little else, making it a terrible, terrible food source. Now, a team of scientists has figured out how and why those vampire bat species are the only mammals that can live solely on a diet of blood.
Upon comparing the genome of common vampire bats to 26 other species, the scientists found 13 genes in the blood-sucking mammals that either no longer work or are missing. Three of those losses had been reported in another study published in 2014, with all of them indicating a reduced sense of taste reception in vampire bats. The remaining 10 gene losses are new discoveries, according to the team.
The loss of a gene called REP15 indicates enhanced iron uptake in the animals' gastrointestinal cells, which they also shed and excrete quickly. This prevents iron overload that can have severe detrimental effects. The absence of two other genes allow glucose to remain longer in the bats' bodies and prevent hypoglycemia, since blood contains minimal carbohydrates. Another absent gene might also be the consequence of the "extensive morphological and physiological modifications" in the stomach of common vampire bats. Instead of being a muscular organ, their stomachs are expandable structures used to store large amounts of liquid and serve as a major site of fluid absorption.
The loss of one gene even contributed "to the evolution of vampire bats’ exceptional social behaviors." Since they can't survive too long without feeding, seeing as blood is very low in calories, vampire bats can regurgitate their meals and share with others. They can also keep track of who shared with them in the past and will extend extend help to them in the future if needed. Hannah Kim Frank, a bat researcher at Tulane University, told AP: "It's totally bizarre and amazing that vampire bats can survive on blood — they are really weird, even among bats."
The study revealing the loss of genes that allow them to live off blood doesn't make vampire bats any less weird, or intriguing. You can read the whole study in the Science Advances journal.