If they're so smart, then why do they die?
1. Not all toxins cause a food aversion. In order for an animal to form a food aversion, the toxin in the food must cause nausea. The alkaloids in locoweed, for example, don't cause food aversions, so animals can’t regulate intake of locoweed. Plants that cause chronic problems, such as liver disease, or birth defects may not be aversive enough to prevent animals from eating the plant at levels that cause chronic problems. Of the toxins that have been tested, most produce a food aversion. Unfortunately, most toxins have not been tested.
2. No time to learn. If plants are acutely toxic, a few mouthfuls will kill them, they can’t learn about the toxic effect of the plant. In some areas with acutely toxic plants, livestock reared in the area won’t eat the plant but animals unfamiliar with the area and the plant will eat it and die. Some producers in these areas stomach tube new animals with a solution of the toxic plant and water. When the plant material is stomach tubed animals likely learn that the plant causes illness by regurgitating it during rumination, tasting the novel flavor of the plant and feeling sick.
3. Nutrients send mixed signals. Many toxins do produce food aversions. Unfortunately, many toxic plants are also nutritious. Intake of nutritious foods that contain toxins tends to be cyclical. Animals increase intake of a nutritious, toxic food until the animal experiences illness from the toxin then it decreases intake of the food. After it recovers from illness the animal again increases intake due to the feedback from nutrients in the food and the cycle repeats. Sometimes they eat too much and die. On the right is a graph of larkspur intake by one cow over 30 days.
4. Changes in concentration. Animals must to be able to taste the toxin or a flavor paired with the toxin to detect changes in its concentration. If the toxin concentration increases but the flavor of the plant doesn't change then animals cannot detect the increase in the toxin and they may eat too much of the plant.
Frost turns some plants deadly. For example, plants that contain cyanogenic glycosides become much more toxic with frost because the cyanide compounds and the enzymes that break them down remain separate until frost ruptures cell membranes. After a frost, the flavor of the plant doesn't change but its toxicity does.
5. Stress makes toxins more toxic. For example, aside from being aversive, the alkaloids in larkspur causes muscular paralysis and respiratory failure. If an animal is stressed, such as being chased by a predator, muscles cannot function properly due to the toxin in larkspur and animals die of respiratory paralysis.
New environments also cause stress. The same dose of a toxin has a much greater effect in an unfamiliar environment compared to a familiar one. The added stress heightens the toxin's action on the animal, probably by reducing an animals ability to detoxify toxins in the same way chronic stress suppresses the immune system. Thus, cattle that ingest amounts of toxic plants that were not lethal in familiar environments may be lethal in unfamiliar environments.
6. Lack of water or alternative foods. Thirsty animals often have no appetite. If poisonous plants are abundant near watering points, animals may eat too many of these plants soon after they drink while waiting for the rest of the herd to water.
If animals have a choice between eating a toxic food or starving, in most cases they'll eat the toxic food. In addition, hungry or thin animals are less able to detoxify diets high in toxins.