After ten thousand years of domestication, can livestock still self-medicate?

To determine if livestock can self-medicate, we first showed that sheep fed high-grain diets ingest sodium bicarbonate or bentonite, substances that attenuate acidosis and restore acid-base balance. We then showed that sheep and goats regulate intake of polyethylene glycol (PEG), which alleviates the aversive effects of consuming high-tannin diets, in accord with the amount of tannin in their diet. Finally, we showed that sheep ingest dicalcium phosphate to counteract foods high in oxalic acid. To learn whether sheep make multiple illness-medicine associations, we conditioned sheep to use three medicines – bentonite, PEG, and dicalcium phosphate – and then offered them grain or food with tannins or food with oxalic acid and gave them access to the three medicines. Sheep chose the medicine that rectified the malady.

Sheep and goats infected with internal parasites eat more tannin-rich forage than non-infected animals. As parasite loads increase, they increase their intake of plants with tannins, which decreases parasite loads. Livestock are less inclined to self-medicate when they are provided with anti-parasitic drugs. Parasitized sheep reduce intake of high-tannin food when their parasite infection is terminated with ivermectin. Likewise, goats treated with anthelmintic drugs eat less tannin-containing heather than do goats infected with internal parasites.

Collectively, these findings show livestock self-medicate, even after 10,000 years of domestication. Biochemically mediated flavor-feedback associations, where cells and organ systems alter liking for foods as a function of needs, enable livestock to self-medicate. To do so, however, livestock must have access to phytochemically rich foods and learn to use them.