Halloween horror aside, vampires are really pretty spineless.
Most have no backbone at all. By one count, some 14,000 kinds of arthropods, including ticks and mosquitoes, are blood feeders. Yet very few vertebrates are clear-cut, all-blood specialists: just some fishes and three bats. Why hasn’t evolution produced more vertebrate vampires?
The question intrigues herpetologist Harry Greene of Cornell University, who “can’t think of a single example among amphibians and reptiles,” he says. (Some birds are opportunists, sneaks or outright meat eaters, but they don’t have the extreme specialization of bats.)
Kurt Schwenk of the University of Connecticut in Storrs, who studies feeding morphology, comes up empty, as well. As he muses over what animals might have precursor biology that could lead to blood feeding, “a leechlike or lamprey-like blood-sucking tadpole should be a real possibility,” he says. The idea gives him “the heebie-jeebies,” but some tadpole species have already evolved mouths that can cling, and plenty of tadpoles are carnivorous.
Looking at the question from a different point of view — asking what would favor, or not, the evolution of blood feeding—he comes up with a less disturbing answer. For carnivorous animals, eating meat is nutritionally better than sipping blood alone, he says. So vampirism might not offer much of an advantage. “If you don’t need to be light and you’re not a parasite,” he says, there’s “no point in limiting yourself to blood.” So maybe vampiric tadpoles aren’t part of some creepy future after all.
Some adult fishes have evolved blood feeding, even mainstream vertebrates with jaws and bones (unlike cartilage-only jawless lampreys). Among the clear-cut bony examples are some Vandellia canidru catfishes, which fasten onto a gill of a much larger fish and let the fish heart pump sustenance into them as they nestle inside the protected gill chamber. (This is different from the supposed, or maybe mythical, tendency of some canidru catfishes to misunderstand fluid streams and swim up the urethras of humans in the water.)
Among vertebrates, vampirism inside or outside of gills might have arisen from ancestors that hitchhiked on big fishes and nibbled off parasites, in the same way modern remoras (also known as suckerfish) do, suggests parasitologist Tommy Leung of the University of New England in Armidale, Australia. Biologists already know about parasite-picker species, such as some cleaner fish, that will cheat and nip mucus or scales if they can get them. Actual blood-sucking cheats could be mere geologic ages away from that evolutionary step. Vertebrates may have relatively few vampires, but a greater number of almost-vampires.
Full-scale vampirism “is a tough way to make a living,” says William Schutt of Long Island University in Brookville, NY, and author of a book on the topic, Dark Banquet. But also, he adds, one big reason why there are fewer vertebrate vampires than arthropod bloodsuckers may be in the numbers. There are just fewer vertebrates: an estimated 60,000 versus a whopping 10 million arthropods.