Around 70 percent of the world’s 1,240 known bat species feast on mosquitoes, roaches, flies and other insects, while much of the rest prefer nectar, fruit or blood. But there is also a fifth dietary option: In tropical regions around the world, around a dozen mysterious bat species subscribe to a carnivorous menu of lizards, frogs, birds, rodents, fish—or even other bats.
“When we think about carnivorous mammals, our minds tend to go to lions, wolves and things like that,” says Sharlene Santana, an integrative biologist at the University of Washington and curator of mammals at the Burke Museum of Natural History and Culture. “Very few people realize that there are bats with this specialized diet as well.”
The knowledge gap surrounding carnivorous bats also extends to science. Researchers know that meat-eating evolved in several separate events among formerly insectivorous bats, but past studies have failed to reach any consensus about the adaptations that made that dietary switch possible, leaving researchers in the (bat-filled) dark as to whether any commonalities exist between carnivorous bats that eat birds and reptiles in India, for example, and those that hunt amphibians and mammals in South America.
As it turns out, certain telltale physical characteristics are shared across carnivorous species. In the first analysis of its kind, published today in the Proceedings of the Royal Society B, Santana and her colleagues discovered commonalities in body size trends and skull anatomy, indicating that evolution arrived at similar solutions on multiple occasions to allow for bat species to break away from the bug-eating norm.
Santana and her colleagues turned to a method called geometric morphometrics—which uses spatial landmarks to analyze an object’s shape independently of its size—to examine 140 skulls from 35 species of insectivorous and carnivorous bats that were obtained through museum collections. They statistically compared the specimens for similarities and differences in skull size and shape and used data from the scientific literature to examine body sizes across species. The team also integrated information about the evolutionary relationships among species to account for the fact that closely related ones tend to be more similar than distantly related ones, which allowed them to pinpoint similarities in physical attributes that were due to diet rather than relation on the phylogenetic tree.
Carnivorous bats tend to be larger than insect-eaters, the researchers found. Their snouts are also elongated, which may help them take down relatively larger prey and to close their jaws faster, the researchers believe.
Among carnivorous species, however, those that prefer fish have become even more different: Their snouts are slightly shorter and broader than their land animal-eating relatives, and also project upward. Those adaptations, Santana says, would allow fish-eaters to increase their bite force and chew more thoroughly—all the better for dealing with the spiny bones of their prey, which they eat whole.
“The two fish-eating bat species in our study are separated by over 50 million years of evolution and come from completely different families, but their skulls share similar features,” Santana says. “They’re very different from bats that are eating terrestrial vertebrate prey.”
The bat skulls also shared features with other types of carnivores, especially dogs, bears and weasels. The latter are diminutive hunters that nevertheless are capable of taking down prey much larger than themselves. “Weasels are very small but highly predacious, so [the researchers’] results imply that the interaction of small size with predatory behavior selects for specific shapes, which is interesting and could be further explored using CT scans and biomechanical modeling” says Graham Slater, a paleobiologist at the University of Chicago, who was not involved in the research. “Ultimately, this study emphasizes the need to collect more basic data on carnivorous bats.”
Indeed, much remains to be discovered about these mysterious animals, including what their ancestors look like in the fossil record and how they behave in the field. “This novel approach to analyzing cranial shapes sets the table for now showing how these traits function in nature,” says Rodrigo Medellín, an ecologist at the National Autonomous University of Mexico, who was not involved in the work. “I can’t wait to run to the rainforest and play with the carnivores.”