On the Trails: What happens when animals eat poisons?

Amanita mushrooms, like the one pictured, are known to be toxic to humans. However, rodents have been observed nibbling on them and the affects of such behavior are unknown.

I have reports, from two good observers, of marmots on Gold Ridge eating monkshood, a plant that is known to be highly toxic. In fact, extracts of this plant (genus Aconitum) have been used, historically, by hunters that coated their arrow points, spears, or harpoons with such material, not to mention its use by human murderers. The toxins are very quick-acting, and symptoms begin to appear almost immediately. Although the root is most commonly cited as the principal source, all parts of the plant are said to contain the poisons, and the poisons reportedly can even be absorbed through the skin (although individual sensitivity may vary). That being the case, why in the world would those marmots be eating this stuff? There are a number of possible explanations, none with documentation for this particular case:


Both marmots were observed to be foraging selectively on monkshood, bypassing other plants, and also selecting chiefly the flowers (and incidentally a few of the uppermost leaves). Perhaps the toxins are less concentrated there. Or, perhaps marmots have some physiological means of detoxifying the poisons. Or maybe the marmots then eat something that is an antidote (for example, tropical parrots eat toxic seeds and then gather at clay banks to eat the clay that counters the toxins). Possibly the concentrations of the toxin very during the day or at different stages of growth (as is known for some other species). Perhaps there is some medicinal value in small doses of the toxins (as is true for digitalis, for example, used to treat heart problems in humans, but high doses kill). The effects of small doses are variable: small doses might prepare the body’s physiology to deal with subsequent larger doses (“hormesis”) but small doses of certain compounds may accumulate to lethal levels. Asian medicine features a number of medicinal uses of monkshood, and in some human cultures, the root is even used as food (after boiling in several waters).

Virtually every wild plant, and domestic ones as well, produce (or accumulate substances from the soil) chemical compounds that are poisonous to at least one kind of animal, and often to many species. Many, probably most, of these compounds have evolved as deterrents to would-be consumers. Consumers, in turn, generally evolve counter-measures that render the plant defenses less harmful. In effect, there is a continual co-evolutionary battle between consumer and consumee. Some insects have gone one step farther and sequester the plant’s defensive compounds for their own defense against predators; in certain cases, these insects have become totally dependent of the kinds of plants that produce particular re-usable compounds. Perhaps the best-known example is the monarch butterfly, whose larvae eat milkweeds and sequester the plants’ cardiac glycosides, which cause digestive upsets in birds that try to eat the larvae or the adults and thus deters predators. (More on this topic of recycled weapons in a future essay.)

Of course, our local flora is full of plants that contain toxic substances, in varying amounts. All members of the buttercup family (such as monkshood) are suspect, wild irises “disagree” with some herbivores, pine needles have been reported to make livestock sick. Consumption of false hellebore (a.k.a corn lily) has nasty results for many kinds of animals. The list goes on!

Mushrooms are known for many hallucinogenic or pathological effects upon consumers. Among the most famous are the Amanitas. Our species is fairly common and quite attractive: the cap is red or yellow, with warty growths that might suggest dots of streusel topping on coffee cake. It is reportedly less toxic than some other species of the genus, but it can produce very nasty effects in humans. Nevertheless, rodents are known to nibble on the mushroom cap: we often see signs of their teeth there. I have not found any reports of the effects of amanita on rodents.

Another local species of some toxicological interest is baneberry (Actaea rubra). It is reported to be highly poisonous — perhaps especially the attractive red (or sometimes white) berries. At least one moth species can eat the seeds, and I recall that, in the Midwest, grouse eat the berries. I once fed berries to several small mammals without obvious deleterious consequences. Nevertheless, for humans, discretion is the better part of valor.

Although there are many studies of the effects of various plant toxins on livestock, it is hard to find reports of plant poisoning in wild, free-ranging animals. The reproductive success of California quail was reduced in years when their preferred forage plants were scarce and the birds ate more plants that contained protective chemicals. Voles in Japan also had lower reproductive success in habitats that contained more plants with certain protective chemicals.

Most cases of poison deaths in free-ranging wildlife seem to be associated with situations where foraging opportunities are limited and the animals don’t have full access to their preferred foods (to which they are adapted). In short, wild animals are usually able to avoid poisoning themselves, unless they have little or nothing else to eat.

That generalization leaves open the question of fruit-eating birds, such as waxwings, that eat fermented fruit and get so drunk that they cannot fly or even perch successfully. They then become very vulnerable to predators and disturbances that might send them crashing into something. Do they eat fermented fruits only when other ripe fruits are hard to find?

• Mary F. Willson is a retired professor of ecology.


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