There's ample circumstantial evidence for all these creatures: eyewitness accounts, blurry photographs, mysterious footprints. For many cryptozoologists—the people who search for legendary animals—that evidence is enough to confirm a monster's existence.

But it will take more than shadowy sightings to convince Daniel Loxton and Donald R. Prothero that Bigfoot or any of the other monsters are real. What Loxton and Prothero want is scientific evidence. In their new book, Abominable Science! Origins of the Yeti, Nessie, and Other Famous Cryptids, they analyze the history of mythic beasts and the clues to their existence.

Loxton and Prothero come at cryptozoology from different directions. Loxton, a staff writer for Skeptic magazine, was an ardent believer in monsters as a kid, having spotted a Bigfoot print in the woods and a pterodactyl winging over his backyard. (Now, he suspects the print was a prank and the pterodactyl was a great blue heron.) Prothero is a paleontologist, who is also trained in biology and geology. He has written over 250 scientific papers and 28 books, including five textbooks on geology.

National Geographic's Rachel Hartigan Shea spoke with the two authors about bringing skepticism and science to the study of cryptids.

First of all, what is a cryptid?

DP: A cryptid is any animal that has never been described by science, usually something very unusual along the lines of a Loch Ness monster or Bigfoot, something that stretches the limits of what is scientifically plausible.

DL: It's based on the word cryptozoology, which means hidden life or animals. It implies a creature that's been recorded through folklore, something that we have reason to suspect exists.

What can science tell us about cryptids?

DP: The first thing, of course, is that a cryptid can't be a single animal. If there's one of them, there's got to be many of them. You can talk about their population density, the size of range they should have based on their estimated body size. All of that tends to weigh against them being real because they should have had huge ranges, and they should have been spotted a long time ago if they really did exist. And then there's other aspects, like geology, something you never hear the cryptozoologists mention. All the lake monsters, not just Loch Ness but the ones here in North America, in Lake Champlain and Lake George, were all under a mile of ice 20,000 years ago. The cryptozoologists never asked the question, "Well, how did the monster get in the lake if the lake was completely under ice, the lakes are all landlocked, and there's no way for a marine creature to get there at all?" Those are all things that are not news to geologists, they're not news to biologists, but they're apparently news to cryptozoologists.

All the cryptids that you discuss in the book – Bigfoot, the Yeti, the Loch Ness Monster, Mokele Mbembe – are very similar to things that exist or existed in the past: bears, primates, plesiosaurs, sauropods. Why the similarity?

DL: In some cases I think it's because they are the same. Bears are often associated with ogres or wildmen in folklore because they're pretty humanlike. Once that folklore is underway, you have the opportunity for people to make these misidentification errors where they see a bear and think it might be a bigfoot. (Read a National Geographic magazine story about Europe's wildmen.)

DP: These animals look like something familiar to us because the myths grow around whatever we've already just seen. Daniel pointed out in the book that the Mokele Mbembe myth emerged right about the time that large sauropod skeletons were first mounted in New York City and illustrated by people like Charles R. Knight. Then lo and behold, someone starts reporting one in the Congo, where it doesn't have any history prior to that.

So Mokele Mbembe definitely does not exist?

DP: We have an excellent fossil record of Africa. We have very great confidence that there have been no dinosaurs around in the last 65 million years because we have bones of large animals from Africa of all kinds but they're all mammals. Same goes for plesiosaurs. Worldwide, there are no bones of plesiosaurs in any marine deposit after about 70 million years ago. There are plenty of places where they should show up if they actually lived, but they don't. That to me is not just absence of evidence, that's very strong evidence that they don't exist.

That sentence -- the absence of evidence is not the evidence of absence – occurs a lot in the book.

DL: It's a really good thing for people to keep in mind, but it's not always true. If the claim that you are advancing implies some kind of evidence, then failing to find that kind of evidence is evidence that that thing does not exist. Take, for example, the idea that there might be plesiosaurs in Loch Ness. Well, plesiosaurs had bones. That implies that there should be bones littering the loch. Well, they've dredged the loch to see if there are any monster bones down there, any plesiosaur bones, and there aren't. That goes to the truth of the claim.

Do you ever encounter people who say, "No, I saw it!"

DL: Oh yeah. I have a lot of sympathy for that. If you have the experience of seeing something with your own eyes, it's natural that that should trump my "talking head" skepticism and Don's arguments about why that's probably not so. But there's only so much I can do with your personal experience that I did not share. I accept that it's compelling to you, but it cannot be as compelling to me.

DP: By and large, all of the evidence for these really strange cryptids is from eyewitness testimony. People are fooled by their senses, especially sight, because we are notoriously bad witnesses. One of the sightings of the Yeti, or the abominable snowman, turns out to be a rock outcrop. The guy saw it move the first time and then he had to leave. He came back finally a year later--after his sighting had been all over the media--and it turns out that it was just a rock he was shooting pictures of.

What do you think the connection is between people believing in cryptids and the level of scientific literacy among the general public?

DP: Lately cryptozoology has been connected to creationism in a lot of ways. People who actively search for Loch Ness monsters or Mokele Mbembe do it entirely as creationist ministers. They think that if they found a dinosaur in the Congo it would overturn all of evolution. It wouldn't. It would just be a late-occurring dinosaur, but that's their mistaken notion of evolution.

Is there any one cryptid that you wish was real?

DL: All of them.

DP: I'm a paleontologist. I'd love to have Mokele Mbembe and a plesiosaur!

This interview has been edited and condensed.