||What are Plant
Plants contain a variety of toxic compounds commonly called
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
Why Do Herbivores Ingest Plants With
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
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
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
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
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.
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.