Fur, Feathers, and Pharmaceuticals
Posted by Richard Conniff on April 12, 2013
House sparrows and finches pad their nests with nicotine-laced cigarette butts to reduce mite infestations. Wood ants do the same sort of thing with an antimicrobial resin from conifer trees, preventing microbial growth in the colony. Monarch butterflies infected with parasites protect their offspring from that irritating fate by laying their eggs on anti-parasitic milkweed. (Well, if they can find any milkweed.) And baboons exploit a well-stocked medicine chest, treating parasitic infections with the fruit of the Balanites tree, halting bouts of diarrhea by eating leaves from the Sodom apple, and relieving menstrual cramps by munching on the leaves of the candelabra tree.
“When we watch animals foraging for food in nature, we now have to ask, are they visiting the grocery store or are they visiting the pharmacy?” says Mark Hunter, co-author of an article on “Self-Medication in Animals,” published online today in the journal Science. “We can learn a lot about how to treat parasites and disease by watching other animals.
“Perhaps the biggest surprise for us was that animals like fruit flies and butterflies can choose food for their offspring that minimizes the impacts of disease in the next generation,” says Hunter, can ecologist at the University of Michigan. “There are strong parallels with the emerging field of epigenetics in humans, where we now understand that dietary choices made by parents influence the long-term health of their children.”
But the authors are actually far more interested in how pharmaceutical choices affect the ecology of the animals. Here’s an excerpt:
We argue that there are at least four major consequences of animal medication.
First, when animal medication reduces parasite fitness, we expect to observe effects on parasite transmission or virulence. Neither consequence has received much attention yet, but two studies indicate that medication can indeed influence the interactions between hosts and their parasites. For example, when gypsy moth caterpillars consume foliage high in phenolics, it reduces transmission of a polyhedrosis virus and facilitates moth outbreaks (12). There is also preliminary evidence that medication affects virulence evolution: increasing parasite virulence is predicted from models of medication behavior by monarch butterflies using toxic milkweed (13).
Second, animal medication should affect the evolution of animal immune systems. Immune responses are costly, suggesting that animals should not use or evolve immunity when they do not need it. Animal medication provides an alternative to cellular and humoral immune responses and may thus result in a reduction or loss of such immune responses. This hypothesis has not yet been tested formally, but there is suggestive evidence. Perhaps most strikingly, honeybees use a series of behavioral immune mechanisms, including the incorporation of antimicrobial resin into their nests (14). Analysis of their genome suggests that honeybees lack many of the cellular and humoral immune genes of other insects, raising the possibility that their use of medicine has been partly responsible—or has compensated—for a loss of other immune mechanisms (14).
Third, host-parasite interactions are often used to explore patterns of local adaptation, yet surprisingly few studies provide evidence for adaptation of parasites to their local hosts or vice versa (15). Most of these studies are based on experiments in which hosts and parasites from multiple populations are exposed to each other in sympatric and allopatric combinations. By not allowing hosts to behave naturally, such studies preclude animals from medicating themselves or their kin. Thus, if animals have locally adapted to their parasites through medication behaviors, studies must be designed such that animals can display their naturally evolved behaviors. It is our expectation that when this is done, more studies will find that hosts have locally adapted their behavior to their parasites.
Finally, the study of animal medication will have direct relevance for human food production and health. Disease problems in agricultural organisms can worsen when humans interfere with the ability of animals to medicate. For example, increases in parasitism and disease in honeybees can be linked to selection by beekeepers for reduced resin deposition by their bees (14). A re-introduction of such behavior in managed bees would likely have great benefits for disease management. In addition, as self-medicating animals, humans still derive many of their medicines from natural products, and plants remain the most promising source of future drugs. Studies of animal medication may lead the way in discovering new drugs to relieve human suffering.
Source: J. C. de Roode, T. Lefevre, M. D. Hunter. Self-Medication in Animals. Science, 2013; 340 (6129): 150 DOI: 10.1126/science.1235824
See also: Shuker, KPN. 2001. The Hidden Powers of Animals: Uncovering the Secrets of Nature. London: Marshall Editions Ltd. 240 p.