Why Can’t Rodents Vomit? A Comparative Behavioral, Anatomical, and Physiological Study

TLDR

The vomiting reflex is documented in many mammals but laboratory rats and mice lack it, and it is unclear whether this absence is due to anatomical constraints or missing neural circuits, or whether laboratory rodents represent Rodentia as a whole. The study tested emetic responses across all three major rodent groups (mouse‑related, Ctenohystrica, and squirrel‑related) to determine whether rodents can vomit. Behavioral testing involved administering prototypical emetic agents (apomorphine, veratrine, copper sulfate) and recording retching, vomiting, and associated motor activity, including phrenic nerve and muscular movements. Across all tested rodent species, prototypical emetic agents failed to elicit vomiting, brainstem recordings revealed a lack of coordinated emetic motor patterns, and anatomical constraints such as reduced diaphragm musculature and stomach geometry further limit vomiting, indicating that the inability to vomit is a general rodent trait likely due to absent brainstem neural circuitry.

Abstract

The vomiting (emetic) reflex is documented in numerous mammalian species, including primates and carnivores, yet laboratory rats and mice appear to lack this response. It is unclear whether these rodents do not vomit because of anatomical constraints (e.g., a relatively long abdominal esophagus) or lack of key neural circuits. Moreover, it is unknown whether laboratory rodents are representative of Rodentia with regards to this reflex. Here we conducted behavioral testing of members of all three major groups of Rodentia; mouse-related (rat, mouse, vole, beaver), Ctenohystrica (guinea pig, nutria), and squirrel-related (mountain beaver) species. Prototypical emetic agents, apomorphine (sc), veratrine (sc), and copper sulfate (ig), failed to produce either retching or vomiting in these species (although other behavioral effects, e.g., locomotion, were noted). These rodents also had anatomical constraints, which could limit the efficiency of vomiting should it be attempted, including reduced muscularity of the diaphragm and stomach geometry that is not well structured for moving contents towards the esophagus compared to species that can vomit (cat, ferret, and musk shrew). Lastly, an in situ brainstem preparation was used to make sensitive measures of mouth, esophagus, and shoulder muscular movements, and phrenic nerve activity–key features of emetic episodes. Laboratory mice and rats failed to display any of the common coordinated actions of these indices after typical emetic stimulation (resiniferatoxin and vagal afferent stimulation) compared to musk shrews. Overall the results suggest that the inability to vomit is a general property of Rodentia and that an absent brainstem neurological component is the most likely cause. The implications of these findings for the utility of rodents as models in the area of emesis research are discussed.