Why Can’t Rats Vomit?

This article pursues the evolutionary reasons why rats don’t vomit. Is it beneficial for rats to be unable to vomit?

As of yet, no empirical research has been done on whether the inability to vomit benefits the rat in some way. Davis et al. (1986) provides some interesting speculation on this topic, however. Remember that Davis et al. suggested that there are hierarchical lines of defense against toxins (first food avoidance, then detection of toxins in gut, and lastly detection of toxins in circulation, followed by vomiting). Davis et al. notes that rats have extremely sensitive senses of smell and taste (Roper 1984). The rat uses its senses of smell and taste to avoid foods that made it feel ill in the past (Garcia et al. 1966, Rozin and Kalat 1971). In fact, rats avoid foods in response to cues that cause vomiting in other species (Coil and Norgren 1981). So the rat who avoids foods that made it feel ill should not ingest lethal amounts of that food in the future.

Davis et al. speculates that because rats have such an extraordinarily well-developed first line of defense against toxins (conditioned food avoidance), the rats’ later lines of defense (vomiting in response to gastric or circulatory cues) have become redundant and were therefore lost over time. Rats can, in fact, detect toxins in the stomach (Clarke and Davison 1978), and in the circulation (Coil and Norgren 1981) but they don’t respond by vomiting, instead they avoid that food in the future. So, the theory goes, rats have lost the ability to vomit because they no longer need it: rats never eat lethal amounts of toxic foods in the first place.

However, an alternative theory is that rats developed their hyper-sensitive food avoidance to compensate for the inability to vomit. It makes sense for a rat to scrupulously avoid ingesting toxic food if it can’t get rid of it later. So, it might indeed benefit the rat to be able to vomit, but as vomiting isn’t an anatomical option, the rat has developed other methods of protecting itself, including food avoidance.

Also, rats do still need a strategy to cope with ingested toxins. Rat food avoidance isn’t foolproof. Rats do experience nausea and have evolved an alternative to vomiting: pica, the consumption of non-nutritive substances. When rats feel nauseous they eat things like clay, kaolin (a type of clay), dirt and even hardwood bedding (eating clay and dirt is a type of pica called geophagia). Their consumption isn’t random, though: rats offered a mixture of pebbles, soil and clay after being given poison prefer to eat the clay (Mitchell 1976).

Update: Yesterday, I didn’t put the long passage in quotations, giving the impression that the analysis was all mine. I can assure you that I have absolutely no background in zoology and
am not even qualified to even make quack pronouncements about such a thing. BTW, for more cases of literary types dabbling in zoology, see Materlinck’s fascinating Life of the Bees.

From Mr. Sun, who also points me to plushy gigantic microbe dolls.

Why Visual Studio rots the mind. I’ve only started this piece, but he argues that the tool determines design decisions. Duh!