Mice and rats can't vomit. Ditto squirrels, beavers, and other rodents. Lucky them, perhaps, but there's a poor-us element to that fact, as Katherine Wu explains in the Atlantic. When it comes to rats, the why is fascinating, and multi-pronged: Some of it is just in the makeup of their rodents' bodies: a barrier between the stomach and esophagus essentially prevents it from happening, per DW, and their senses of taste and smell in some ways make it unnecessary. As Wu writes, "their memory for a sickening substance is strong," so much so that, once tried, they'll avoid a problematic food for years or even their whole lives. A 2013 study published in PLoS One determined "an absent brainstem neurological component is the most likely cause."
Humans aren't subjected to piles of rodent vomit in alleys as a result (and rat poison works, because it can't be thrown up, noted Smithsonian), but in some ways it would be good if we did. That's because we test promising drugs on them, and they can't reveal whether vomiting is a side effect. One expert says that's resulted in millions of dollars wasted on treatments that seemed promising until trials involving humans began. Wu flags the example rolipram, a potential antidepressant that emerged in lab tests 30 years ago amid buzz about a possible "game changer" that worked quicker, and at far lower doses, than the antidepressants on the market.
But when the trials moved on to people, the vomiting began. It turned out humans couldn't take it at a dose high enough to be effective without experiencing serious stomach woes. Dogs and cats can puke, but there's a lot of pushback at the idea of using them in lab testing. That has some researchers turning to a rodent-like mammal that also has the ability to vomit: the shrew. (Read her full piece for more.)
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