One of the most bizarre biological phenomena that I have ever observed occurred when I was 18 years old and a freshman at Bowdoin College. It was early fall, probably September, and I was walking with my boyfriend at the Bates-Morse Mountain Conservation Area on the Phippsburg Peninsula in Maine. It was early afternoon and we were headed home. As we approached the trailhead, we saw a mouse sitting in the middle of the trail. I am no expert on rodent identification, but based on my memory of its appearance, the habitat (wooded, mixed coniferous-deciduous) and the species of mice native to Maine, I am going to make an educated guess that it was a woodland jumping mouse (Napaeozapus insignis).
A mouse sitting in the open in the middle of the day is bizarre enough, but its subsequent behavior was even stranger. As we approached, the mouse did not move, but remained in the middle of the trail happily munching something green. I walked right up to the mouse, and squatted down to get a closer look at what it was eating. The “green thing” was, on closer inspection, a somewhat flattened caterpillar. It was of the green, hairless variety that as a non-lepidopterist (studier of moths and butterflies), I can not identify, but which in its semi-flattened state looked like a green Swedish Fish and the mouse was gobbling it with a passion that suggested that it was indeed a piece of candy.
The mouse was so intent on its meal that when I gently poked it in the side with my finger, it did nothing but continue to eat. Fascinated, and feeling mischievious, I gently pulled the caterpillar from its paws and dragged it along the ground. The mouse followed. Bizarre! Thoroughly enjoying this strange scene and fearing for the mouse’s safety I dragged the caterpillar, mouse following, to the side of the trail and into the weeds.
When I later described the mouse’s behavior to my ecology professor, Dr. Nat Wheelwright, he hypothesized that perhaps the caterpillar was toxic. Many species of lepidopterans eat poisonous plants when they are larva (caterpillars). They are able to tolerate the toxins and even sequester them in their own tissues to deter predators. The most well-known example is the Monarch Butterfly which eats milkweed as a caterpillar and sequesters the cardiac glycoside from the plant. When a predator tries a Monarch for the first time and gets sick, it will learn to steer clear of them in the future. The famous photo that appears in most biology textbooks is a blue jay eating a monarch and then promptly horking it back up.
A toxic caterpillar made a lot of sense but why was the mouse eating it so enthusiastically? Most of the substances that humans ingest to get a buzz or high are actually toxic, hence the word “intoxicated”. Alcohol is fun, but it will kill you if you drink too much. Caffeine and nicotine are widely used toxins that are made by plants. Animals are not immune to these effects and become intoxicated when they consume certain toxins. Cedar waxwings are known to eat older, fermented fruits and berries in the fall and get drunk, flying into windows and otherwise behaving stupidly. For a long time, many people believed that elephants sought out and got drunk off of fallen, fermented marula fruit. Recently, a study published in Physiology and Biochemical Zoology reported that this is not truehttp://www.sciencedaily.com/releases/2005/12/051205235555.htm. Researchers showed that the elephants in fact preferred the non-fermented marula fruit still on the tree, so they weren’t getting drunk on alcohol. They hypothesized that the animals instead became intoxicated from eating the bark of the tree that contained toxic beetle pupae.
A fairly thorough search of the primary science literature and internet have not turned up other reports of rodents getting “high” from eating caterpillars. Toxic caterpillars may not be a routine choice for mice. One possibility is that this was a young animal trying out an unknown food. Perhaps, like many a college freshman, the mouse would wake up in the morning with a wicked headache and regret his meal choice. I am guessing that unlike the freshman, the mouse would probably not make the same mistake twice.