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But what about the frogs?
The beginning of this chapter posed the question, “Why should anyone want to make a bunch of frogs throw up?” The short answer is, “Because rats can’t throw up.” The untold heroes of motion sickness research are the test subjects. Human volunteers often suffer for science in this way, but for a time, so also did the animals that humans most identify with: cats, dogs and monkeys. At least that was the case until the animal rights movement gathered traction in the late 1980s and early 1990s. It soon emerged that, as far as the general public was concerned, it was more acceptable to conduct research with animals that humans typically don’t have close relationships with (rats, mice, amphibians), than to conduct research involving the cute ones: dogs, cats and monkeys. Unfortunately dogs, cats and monkeys are the creatures whose responses to nauseogenic stimuli most closely match those of humans.
“Why not just use rats?” came the chorus of well-meaning voices. “Rats aren’t sympathetic at all.” Unfortunately those old laboratory warhorses, rats, can’t vomit. Lacking key muscles and sophisticated neural pathways that could help to co-ordinate vomiting, rats have adapted in other ways to ensure that they don’t eat poisonous foods. That is why it sometimes difficult to get rats to ingest poison. On the other hand, when they do, it also means they are completely vulnerable to the poison’s effects, having no way to expel it from their bodies. It also rules them out as useful test subjects for motion sickness research.
The researchers who sent the frogs on their parabolic flights were a team of four. Akemi Izumi-Kurotani and Masamichi Yamashita worked at the Institute of Space and Aeronautical Research (ISAS), a department of Japan’s Space Utilization Research Center. Tomio Naitoh was a professor at Shimane University on Honshu, the largest of Japan’s islands. Canadian Richard Wassersug was a visiting professor from Dalhousie University in Halifax, Nova Scotia. Their main goal was to determine if unusual postures observed in frogs on the MIR space station could have been evidence of motion sickness. However, the introduction to their joint paper Motion Sickness in Amphibians also noted, “If it can be shown that the emetic response of amphibians [is]… essentially the same as in mammals … then future work on the development of antiemetic medications could use these lower vertebrates in place of mammals.” In part, the mission’s goal was to spare cute furry creature the misery of nausea.
On each of four days of the experiment, the MU-300 jet zoomed through eight to ten (9 + 1) gut-churning parabolic arcs. Inside the cabin, Izumi-Kurotani, who had been chosen to accompany the frogs, knew that any particles of ejected food would be too small to be observed until the frogs were returned to the lab and their individual canisters checked for specks of amphibian vomitus. But several other interesting behaviors were on display.
“We saw frogs doing very different things on these parabolic flights,” says Wassersug. “Some of them would spin wildly and struggle to right themselves or tumble forward, which many animals do when they’re weightless. But we found out that certain frogs don’t do that. Certain frogs stick their arms out to the side and essentially assume a skydiving posture. These are tree frogs that normally jump from tree to tree so they have specialized adaptations for freefall.”
Back at the lab, the scientists eagerly examined the containers. Immediately following the flight, nothing in either group indicated vomiting – no tell-tale specks of regurgitated frog food. But, patiently keeping an eye on their test subjects, they soon made a surprising discovery – frogs appear to have a delayed response to stimuli that provoke motion sickness. Varying by species, the frogs regurgitated small amounts of food anywhere from 30 minutes to 42 hours after the parabolic flights had ended. On average the frogs didn’t throw-up for 14 to 16 hours following their return to terra firma.
This is a very different reaction than one would typically see in humans. Humans and many other mammals tend to become sick from motion either soon after the provocative stimulus is introduced, (e.g. boarding a ship) or once it is withdrawn (e.g. returning to land after several hours or longer); in each case our systems need time adapt to the change in environment and this is when motion sickness is most likely to occur.
Wassersug et al also discovered that the frog species that vomited least was the same one that had calmly spread its limbs in freefall position instead of tumbling madly in a futile struggle to right itself. Far from suggesting that tree frogs have some mechanism that renders them immune to motion sickness, this observation seems to mirror the experiences of human astronauts; the more you move about the more likely you are to become sick.
Many happy returns
In many ways the experiment had been a great success; Wassersug, Naitoh, Yamashita and Kurotani had gathered compelling evidence to support their hypothesis that frogs on the MIR space station had been suffering from motion sickness. They had discovered the building blocks of a new hypothesis; that, like humans, the less frogs move around in weightlessness, the less likely they are to succumb to motion sickness. The team had also observed a hitherto undocumented phenomenon, namely the strange delay between the end of the parabolic flight and the onset of vomiting. But had they discovered a new animal for testing? One perhaps less reprehensible than cats, dogs and monkeys? The frogs’ susceptibility to motions sickness suggests that their neural pathways are at least somewhat similar to those of humans, but the unexpected delay between the onset of motion sickness and actual vomiting could further suggest that comparing frogs to humans is like counting apples and oranges.
The story of the vomiting frogs is a fitting cap to this chapter on motion sickness. It illustrates the powerful fascination that vomiting and regurgitation hold for scientists, researchers and many people who are simply curious. Motion sickness especially seems to be veritable siren call for researchers, possibly because it is an ancient and debilitating misery that people have shared since humans constructed their first primitive boats. Even now, when there are reasonably effective drugs that one can take to minimize the effects of motion sickness, people are still plowing forward with new and often exotic research: some for the sake of knowledge itself, others to investigate new ways of suppressing motion sickness and others who simply find themselves going down a research path they are eager to follow, regardless of where it leads.