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  Behavioral Education for Human, Animal,
Vegetation & Ecosystem Management

Stories of Applied Animal Behavior
Created by members of a graduate Foraging Ecology Class
     at the University of Idaho and Washington State University
     under the direction of Drs. Karen Launchbaugh and Lisa Shipley

How do Wild Herbivores Cope with Plant Toxins?

By Elvia Lopez-Perez

What are Plant Toxins?

Plants contain a variety of toxic compounds commonly called "secondary compounds" that affect the behavior and productivity of wild and domestic animals. There are many classes of these toxic compounds, however soluble phenolics, alkaloids, and terpenoids are the most common. Soluble phenolics include flavonoids, isoflavonoids, and hydrolysable and condensed tannins. The excessive consumption of flavonoids can produce abortions, sterility, or liver damage. The major problems found in wildlife have occurred in captive animals fed on soybean based feed or clover. Tannins generally have a bitter or astringent taste. In ungulates, tannins affect the digestibility of plant cell walls by binding with microbial enzymes in the rumen and cell tissues of rumen microbes, killing the microbes and reducing fiber fermentation, digestive efficiency and protein availability. As a result, the protein requirements of herbivores and their nutritional status could be negatively affected especially in those animals that need high protein levels such as lactating females and those feeding on low nutritional quality and/or tannin-containing plants, mainly during wintertime. In the case of alkaloids, intoxication or poisoning in wildlife is rarely reported. However, elk are exposed to a variety of alkaloid-containing plants such as locoweed and hemlock. With respect to terpenoids, the bactericidal effects of these components reduce the digestive efficiency of ungulates. For example, juniper contains volatile oils that inhibit the action of deer rumen microorganisms.

Why Do Herbivores Ingest Plants With Toxic Compounds?

Although toxic plants can cause negative effects, herbivores may still consume them because of the plant's nutritional quality, palatability, availability, or addictive chemical properties.

Nutritional quality and palatability. Some toxic plants taste good and have high nutritional quality. For example, locoweed contains 20% protein during early spring, which encourages animals to ingest this plant. Larkspur, another toxic plant also has high protein content and tastes good, which induces its consumption.
Addiction A few alkaloid-containing plants such as locoweed, buttercup, nightshade, laurel and oak have been regarded as addictive. It has been hypothesized that animals become addicted to the flavor of these plants. Locoweed and swainsona, which contain swainsonine, have been described as some of the most addictive plants to livestock and some wildlife species like mule deer and pronghorn antelope. Some field observations indicate that animals refused to take any other kind of food and greedily seek those addictive plants once they consume them. However, presently there are no clear answers about the addictive qualities of locoweed and other toxic plants. Perhaps positive reinforcement encourages animals to feed on these plants.

Animal nutritional state Nutritional stress can contribute to animal consumption of toxic plants. An animal's perception of toxic plants may change when it is starved or deprived, as undernourished or hungry deer may select less palatable or toxic plants that they would reject when forage is plentiful.

Developed Mechanisms Against Plant Toxins

Grazing animals have developed behavioral and physiological mechanism to reduce or avoid poisoning by plant toxins.

Behavioral strategies Behavioral mechanisms involve the reduction or elimination of plant toxins through innate knowledge, and/or post-ingestive consequences.
1. Cautious sampling
. Herbivores use odor and taste to detect and avoid toxic plants. Thus, they are attracted to sweet flavors and repelled by bitter flavors. For example, black-tailed deer first sniff a new plant; if the odor is acceptable, it tastes the plant and if it is still acceptable, eats it. Once familiar with the plant, deer eat it directly. Also, some studies indicate that feeding habits are genetically controlled by food identification mechanisms, however it is not well understood.
2. Mixing diets.
Herbivores eat more than one plant in each meal. By eating a mixture of plants containing different toxins the negative effects of ingesting one toxin may be diminished. Thus, mule deer are able to eat two times more of a toxic plant when they ingest a mixed diet than when they eat one poisonous plant alone.
3. Cyclic consumption
. Herbivores can avoid toxicities by limiting or varying the consumption of a specific toxic plant each day or until toxins cause negative feedback. For example grazing studies with tall larkspur showed that consumption above 25 to 30% of this plant for 1 or 2 days reduced the intake on subsequent days.
Physiological strategies Physiological strategies deal primarily with how animals handle toxins internally when consumed and center on detoxification in the gut by rumen microbes or in the liver.
1. Ejection of the toxin.
The first defense against against digesting a toxin compound is to get rid of it as soon as possible after eathing it.  This usually accomplished through vomiting or diarrhea.
2. Detoxification mechanisms
. Browsing herbivores such as moose and mule deer, counteract the negative effects of tannins by secreting tannin-binding salivary proteins which make tannins inactive and providing these animals with the ability to maintain greater digestion of fiber and protein when ingesting tannin-rich forages. Detoxification in the mouth may allow animals to ingest some toxic plants. For example, during chewing some terpenoids are lost through volatilization as in the case of pygmy rabbits when they eat sagebrush. In other cases, some plant toxins are bound with other eaten material as soil. Mineral licks are often high in several clays and some clay naturally binds to various toxins. Other detoxification mechanism includes toxin dilution in the rumen and/or degradation by rumen microbes. Once plant toxins are absorbed from the gut into the blood, they are transported to the liver, metabolized and excreted in urine.
3. Tolerate the toxin.
Although little is known about the tolerance of wildlife species to plant toxins, tolerance to toxic compounds might be the best way for animals to diminish the risk of poisoning. Different animal species and individuals within a species are more or less tolerant to toxic plants than others. For example, mule deer are more tolerant of locoweed than pronghorn antelope, and elk are more tolerant of ponderosa pine than bison. Microbial adaptations in the gut can be induced by consumption of small quantities of plant toxins and thus provide an opportunity for the animal's system to adapt to the toxin.


Information provided in this review may be useful to wildlife managers in developing management practices to reduce the consumption of toxic plants. For example, planting high quality forage could be a solution to reduce problems caused by toxic plant compounds, which may enhance the survival of wildlife species.

Wild herbivores are able to cope with plant toxins using both behavioral and physiological adaptations. However, toxic plants could play an important role in the survival of wild herbivores mainly when forage is insufficient and animal density is high because it may force wild animals to eat less preferred plants, which often contain toxic compounds.

Additional Readings

Cheeke, P. R. 1998. Natural toxicants in feeds, forages, and poisonous plants (2nd Ed). Interstate Publ., Inc., Danville, IL.

Fowler, M. E. 1983. Plant poisoning in free-living wild animals: a review. J. Wild. Diseases 19: 34-43

Owen, J. B. 1992. Genetic aspects of appetite and food choice in animals. J. Agric. Sci, Cam. 119: 151-155.

Pfister, J. 1999. Behavioral strategies for coping with poisonous plants. Pp 45-59. In: K.L. Launchbaugh, K. D. Sanders, and J. C. Mosley (eds.). Grazing behavior of livestock and wildlife. Idaho Forest, Wildlife and Range Exp. Sta. Bull 70, University of Idaho. Available online http://www.uidaho.edu/range/publications.html

Provenza, F. D. 1996. Acquired aversions as the basis for varied diets or ruminants foraging on rangelands. J. Anim. Sc. 74: 2010-2020.

Robbins, C.  1993. Wildlife feeding and nutrition. Academic Press, San Diego, CA.

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Learn more about the Foraging Ecology Class by visiting http://www.cnr.uidaho.edu/range556/