Animals adapt to changes in their environment and bodies not only through physiology changes that correct for deviation from homeostasis, but also by engaging in certain behaviors. Thus, a behavior like selecting a food containing a needed nutrient is no different than the secretion of insulin from the pancreas in response to rising blood sugar levels: both responses restore homeostasis. Behavioral homeostasis has been shown experimentally. Livestock modify their intake and diet selection to rectify nutritional imbalances. Besides balancing nutrient intake, herbivores are faced with other challenges such as disease. If behavioral homeostasis exists, then sick animals should self-medicate with substances that restore their health, even substances that contain no nutrients or could be potentially toxic at high levels like plant secondary compounds.
Parasitism is one of the greatest disease problems in grazing livestock. Controlling parasites with drugs is challenging, particularly in recent times due to the rise in drug-resistant internal parasites. Evidence suggests that parasitized apes use natural plant secondary compounds (PSC) as anti-parasitic agents. Can parasitized domestic sheep and goats also learn to use PSC? If the answer is yes, they could learn to self-medicate with PSCs and eat PSC-rich vegetation, either on rangeland or pasture, when needed, while having other nutritious and safe forages available to meet their nutritional requirements.
In a controlled experiment, lambs with parasites ate more of a supplement containing tannins than non-parasitized animals, even when the supplement was very low nutrients. In contrast, lambs without parasites ate more of the supplement without tannins than parasitized lambs.
In another study, lambs with and without parasites were given a choice of alfalfa and alfalfa mixed with 10% tannin. Lambs with parasites had a greater preference for alfalfa with tannins than lambs without parasites. These differences in preference did not exist before lambs with parasites experienced the positive effects of tannins or later after parasites were killed with drugs.
Collectively, the information above suggests herbivores are “aware” of the presence of parasites infecting their bodies. If herbivores are able to sense their parasitic burdens and if there are anti-parasitic substances in plants, which can potentially provide relief, then parasitized animals should increase their preference for such plants relative to healthy animals.
Besides being aware of their parasitic burdens, a second step of self-medication is that after eating or using a certain medicinal plant, herbivores should experience relief from the upset or discomfort caused by the parasites. Animals are more likely to learn about the benefits of a medicine when they experience illness or discomfort and then eat a medicine or plant that leads to recovery.
How is self-medication knowledge acquired? Individual foraging behaviors are mainly acquired by learning from social models, first from mother, then from peers, and from individual post-ingestive experiences. The spread of the self-medicative behavior within a group seems to be influenced by social models.
Juan J. Villalba and Serge Y. Landau. 2012. Host behavior, environment and ability to self-medicate. Small Ruminant Research 103:50-59
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