Vampire mania aside, bats don't go for a diet that would make for an exciting Hollywood blockbuster. Most are insectivores, chomping on insects. Some large species prefer fruit. And yes, a few like blood.
But over the 52-million-year history of these flying mammals, a few have evolved a taste for their fellow vertebrates. Now biologists at the University of Washington and the Burke Museum of History and Culture are shedding light on how these so-called "carnivorous bats" adapted to the daunting task of chowing down their backboned prey.
"Vertebrate prey are a unique challenge for carnivorous bats," said lead author Sharlene Santana, a UW assistant professor of biology and curator of mammals at the Burke Museum. "They eat flesh, bones and everything else within their prey, and we wanted to understand the evolutionary changes that help them accomplish this."
Santana and co-author Elena Cheung, a UW undergraduate, wanted to understand how these adaptations influenced changes in skull shape and size. When talking about diet, this is no small question.
"The skull and mandible provide attachment points for the jaw muscles, and variation in these attachment sites results in differences in bite force, and how wide of a gape the jaws are capable of," said Santana.
Their findings, published May 11 in the Proceedings of the Royal Society B, reveal surprising patterns of change that helped carnivorous bats catch and eat vertebrates. Though there are currently more than 1,300 species of bats, only a few dozen eat vertebrates, from fish to land animals -- including a few species that eat other bats. This evolutionary transition -- from insects to vertebrates -- has occurred at least six times over bat history.
Santana and Cheung took high-resolution images of skulls from 140 bats across 35 species, representing all six lineages of carnivorous bats as well as bats that eat insects, or a combination of vertebrates and insects. The skulls were from the Burke Museum and the Los Angeles County Museum.
They used these images in a complex computer-based comparison of landmarks on the skulls, which takes into account the position, scale and orientation of those features to determine differences in shape among species.
"The unique features found in the skulls of carnivorous bats may reflect the adaptations that would have enabled them to adopt a diet of vertebrate prey instead of insects," said Santana.
Through this process, Santana and Cheung also discovered that larger animal-munching bats -- whether they ate insects or vertebrates -- tended to have longer snouts, which may allow them to consume relatively larger prey. Carnivorous bats tended to be larger and had skulls that emphasized a strong bite force when the jaws are opened wide.
Surprisingly, the main exception to this trend were carnivorous bats that ate a particular subset of vertebrates -- fish. The skulls of fish-eating bats were optimized for a strong bite force at a relatively narrow jaw gape.
"Many fish have flatter bodies compared to land vertebrates, which may explain the distinctive jaws and bite force of fish-eating bats," said Santana. "In addition, fish-eating bats must spend a lot of time chewing the carcass thoroughly, breaking up those sharp and tiny bones into chunks that are easier to swallow and digest."
Santana and Cheung also collected data from the skulls of other carnivorous vertebrates, including a polar bear, puma, lion and several species of hyenas and wolves. This wider comparison helped them understand if the skulls of carnivorous bats showed adaptations shared by other mammalian carnivores.
"This is important to understand because, unlike other mammalian carnivores, carnivorous bats don't have the strong, blade-like teeth that can tear flesh -- these bats chew and consume the whole body of their prey, bones and all," said Santana.
She and Cheung found that the skulls of carnivorous bats emphasized a strong bite force at the expense of gape width when compared to other mammalian carnivores, perhaps indicating that some of the key trade-offs in feeding and chewing strategies enabled these bats to subsist on a vertebrate-rich diet.
Santana hopes their conclusions will inform ecological studies of carnivorous bats, which by and large reside in tropical and sub-tropical environments around the globe. In addition, Santana says this study demonstrates how ecological factors like diet can so heavily depend on the adaptive changes that evolution provides.
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