Zoopharmacognosy–Do Animals Self-Medicate?

Before becoming a veterinarian, I did a master’s degree in animal behavior, working primarily with chimpanzees. These are fascinating creatures, with many of the cognitive abilities of a young human child (and with a similar emotional temperament as well, which can be a problem in an animal far stronger than any human and equipped with large canine teeth!). One of the most interesting observations about chimpanzees in the wild is that they are sometimes seen eating plants, dirt, or other substances not part of their normal diet, substances which are thought to have possible therapeutic values in preventing or treating parasitic infestations, infections, and other medical problems. Other mammals, birds, and even insects have been seen engaging in similar behaviors, and the suggestion has been made that these creatures are, in some sense, deliberately medicating themselves, a phenomenon labeled zoopharmacognosy.

This is, not surprisingly, a popular idea among proponents of herbal medicine and other medical approaches that claim natural remedies and intuitive knowledge are central to health. However, as a skeptic I am well aware of how easily anecdotes, and elaborate theories built on them, can turn out to be descriptions of what we hope or wish to be true rather than of the world as it actually is. Humans are masters of seeing what we want to see and ignoring things that contradict our beliefs. So do animals self-medicate? 

How Can We Find Out?
The first question to ask in order to explore this subject is how would we demonstrate self-medication and distinguish it from other behaviors? A variety of criteria have been suggested for identifying self-medication in wild animals. And as always in science a consistent pattern that holds up even when attempts are made to explain it with alternative hypotheses is necessary for a reliable conclusion.

Ideally, animals should be seen to seek out and ingest plants or other substances that are not part of their normal diet. They should do so only when they have some objectively identifiable marker of a specific illness. There should be some plausible explanation for why the substance ingested could potentially affect the illness the animal has (e.g. laboratory evidence that a chemical consistently found in a plant can reduce the levels of a GI parasite; NOT simply an unproven folk belief in the medicinal value of the plant). The marker of illness should consistently resolve after the animal has ingested the substance thought to be therapeutic.

Of course, all of these steps could be present and still not definitively prove self-medication, especially given that most behavior of wild animals isn’t observed, and there are many ways in which observers can skew results be selectively attending to or noting some behaviors more than others. Controlled experimentation would also be needed to rule out other explanations. This is, of course, rarely possible with great apes or other wild mammals, though some work has been done with laboratory animals and insects.

Another question to consider is what mechanism might explain self-medicating behavior if it were observed. Do animals have theories of disease, as humans do, that lead them to select specific therapies based on predictions from these theories? Do they remember or communicate to others the particular remedies for particular illnesses? Such deliberative thinking seems pretty unlikely, and there is no reliable evidence of it, despite extensive research, even in the most cognitively advanced species such as the great apes.

More likely, self-medicating behavior would be, like all other behaviors, a product of natural selection operating on underlying variation between individuals. The tendency to seek out certain tastes or smells, for example, when experiencing certain symptoms could easily be fixed by selection if doing so improved the reproductive success of individuals with genes prompting this behavior over that of individuals without such genes. Of course, such adaptive explanations can easily be manufactured for every behavior, and they need to be demonstrated experimentally when possible to be truly believable.

What’s the Evidence?

Wild Animals
In any case, what is the evidence for self medication? In wild animals, it consists largely of uncontrolled observations with a significant risk of bias or misinterpretation. Anecdotes abound, but rigorous, repeatable patterns with solid links between symptoms, behavior, and outcome have not been demonstrated. It is common to note the consumption of an unusual substance and begin looking for evidence of a medical problem to explain it, but instances of that same problem or consumption of that same substance that are not related are overlooked. This is a form of recall or confirmation bias that makes all anecdotes unreliable as evidence for a particular theory.

The medicinal value of plant ingested in such cases is often assumed based on local folk medicine practices or the presence of certain chemicals in the plant which have in vitro effects that might be relevant. The same sources of evidence are often used to validate all kinds of claims about the medicinal value of herbal remedies, and the reality often turns out to be that there is no such value. These kinds of evidence can only suggest possible relationships, not prove them.

And we have to remember that animals also frequently ingest substances that are actively harmful to them. I pull a surprising variety of potentially deadly objects out of the gastrointestinal tract of dogs and cats on a regular basis. And both domestic and wild animals have been seen to succumb to ingestion of poisons as well as indigestible foreign objects. So any theory about intentional self-medication has to also explain this self-injurious behavior. Are these animals committing suicide? Or, as seems more likely, are they mistaking harmful and inedible substances for food or exploring the world through taste and ingestion without a consistent regard for the likely benefit or harm?

There is, of course, little question that animals seek out specific tastes, smells, and colors in their food that are associated with relevant nutrients. It is reasonable, then, to suppose they might similarly seek out such cues in medicinal substances. And the distinction between food and medicine can be unclear when both are made up of substances found in nature and not processed in any significant way. So, all the anecdotes about self-medication behavior may represent an evolved behavior that reduces the burden of disease and parasitism in some individuals, conferring a selective advantage. As of now, however, this is still simply a hypothesis that has not been rigorously demonstrated.

Laboratory Experiments
More convincing evidence of self-medication is available from experimental studies in insects and laboratory animals. One such study, looking at Monarch butterflies, elegantly shows a facultative change in food source associated with parasitism.

These butterflies normally lay their eggs on milkweed. The larvae ingest the plant and incorporate toxins called pyrrolizidine alkaloids into their tissues. These act as a defense mechanism. Predators eating the distinctively colored butterflies will be nauseated by the toxins and will avoid similarly colored butterflies in the future. This doesn’t, of course, help the individual the predators eat, but it can help related individuals who carry similar genes. This is itself a fascinating and amazing example of natural selection in action.

The research study showed that butterflies affected by a certain parasite preferentially lay their eggs on a variety of milkweed which they do not ordinarily use as a food source. This variety contains a higher level of pyrrolizidine alkaloids than the one they usually lay eggs on. Larvae which hatch out on the more toxic variety and ingest its toxins have lower rates of parasitic infestation and a greater reproductive success than parasitized larvae feeding on regular milkweed.

Interestingly, there is a cost to this behavior. Unparasitized larvae that hatch on the more toxin milkweed actually have a reduced survival, so the behavior is only advantageous in the face of parasitism, otherwise it’s a bad choice for the female butterfly. This is also a nice illustration of the fact that all medical therapies, whether “natural” or not, involve balancing risks and benefits in specific circumstances.

This model shows how behavior that can be called self-medicating can evolve in the same way that food preferences evolve. It also shows that the development of such behavior doesn’t require advanced mental abilities or any mystical intuition about the healing value of plants. The trial-and-error processes of natural selection operating over long periods of time are capable of leading to behaviors that appear purposeful even though they are not.

Bottom Line
So do animals self-medicate? Iwould say the answer is a qualified “yes.”

There is good evidence that some animals have evolved adaptive behaviors which include selecting certain food sources preferentially when the individual has a medical problem that that food source can ameliorate. There is considerably less evidence that animals consistently make accurate choice about ingesting specific substances to treat or prevent specific medical conditions. The numerous anecdotes of behaviors which appear to suggest this could easily be matched by anecdotes of behaviors which are clearly self-injurious or neutral with respect to health, so any theory about self-medication has to account for both kinds of behavior.

The theory that most effectively does so is the same general theory of how natural selection shapes food preferences and other complex behaviors over time, through differential reproductive success among individuals with genes predisposing to different behavior patterns selecting for the most adaptive pattern for the current environment. Many such variations are expressed. Some are harmful or maladaptive and eventually die out, but they can be seen at any given “moment” in evolutionary time and mistaken for something that is adaptive. Some variations may hit upon a truly beneficial behavior and, over time, the genes underlying these behaviors will become more frequent in the population, as wil the behavior.

It is important to point out also that this has nothing to do with the complex theories and mythologies used to justify herbal folk medical practices. People may or may not have gotten the idea of using certain plants for treating disease from watching wild animals. Whether or not this is true doesn’t mean those plants are actually effective as medical therapies, in humans or in animals. That has to be proven in the usual rigorous, controlled, scientific way.

And there is no need to imagine a mystical intuition about the healing power of plants to explain self-medication. Butterflies can be manipulated to show such behavior under controlled conditions, and this can be easily explained by established mechanisms of natural selection, without any need for recourse to mystical explanations. Likewise, the medicinal use of natural substances by animals doesn’t suggest this is a highly effective or desirable form of medicine for modern humans. Evolved self-medication and folk medical practices were part of human behavior for millions of years with little improvement in our overall health and longevity. We are fortunate to have stumbled across methods of identifying cases, treatments, and preventive measures for disease that are far more effective even if not “natural” in any atavistic sense.

There is also ample evidence that animals, including humans, make maladaptive choices about food and medicinal substances. Animals ingest poisons and inedible foreign objects readily. And while humans have a clearly adaptive drive to seek certain nutrients which were scarce in the environment in which we evolved (like sugar, salt, and the ample calories in fat), our inability to curb our desire for these substances now that most of us have an abundance of them is the source of the most serious disease afflicting the developed world. The mechanisms of food selection and self-medication which may be beneficial in one environment, can just as easily be harmful in another. They are not drives towards health and well-being; they are specific behaviors generated by natural selection to better adapt our ancestors to a particular environment.




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11 Responses to Zoopharmacognosy–Do Animals Self-Medicate?

  1. zyrcona says:

    Nice article. Does this have anything to do with why my dogs start trying to scoff everything in sight when they are being or about to be sick? They eat or try to eat everything from grass to the vomit they have already produced to bits off the carpet and their beds.

    “Evolved self-medication and folk medical practices were part of human behavior for millions of years with little improvement in our overall health and longevity.”

    One suggestion. I don’t quite agree that this is an objective statement. There are no records to show when humans started using folk medicine or mortality/morbidity data from such times and prior to them. Folk medicines and superstitions could very likely be as old as the human species, and folk medicines seem to be endemic to primitive societies (I can’t think of any control groups), so it’s not really possible to say anything about how folk medicine affects the health of humans as a whole.

  2. skeptvet says:

    Thanks for the comment.

    Does this have anything to do with why my dogs start trying to scoff everything in sight when they are being or about to be sick?

    That’s the question, isn’t it? 🙂 We all notice our dogs eating grass or other unusual stuff about the time they throw up. Do they do it to throw up? Do they do it to not throw up and it just doesn’t work well? Do they do it for some other health benefit? Or do they do it the rest of the time for no health-related reason at all and we don’t notice or pay attention unless it’s associated with vomiting or illness? I don’t think we have the answers yet, so it’s hard to decide which of the many reasonable guesses we could make is right.

    I don’t quite agree that this is an objective statement. There are no records to show when humans started using folk medicine or mortality/morbidity data from such times and prior to them. Folk medicines and superstitions could very likely be as old as the human species, and folk medicines seem to be endemic to primitive societies (I can’t think of any control groups), so it’s not really possible to say anything about how folk medicine affects the health of humans as a whole.

    I’m not sure if we’re exactly disagreeing here or not. Granted, the kind of epidemiological morbidity and mortality data we have now certainly isn’t available very far back in time. But through cultural anthropology, we have a pretty good idea that, as you say, folk medicine has existed in probably every human culture going back tens of thousands of years at least (and self-medication likely much longer given that it appears to be present in other more ancient taxa of animals).

    And from phsyical and forensic anthropology, we have a pretty good understanding of basic health going back thousands of years. We know that average life expectancy, infant mortality, mortality associated with childbirth, and the general burden of parasitism and disease were much less satisfactory than today and remained relatively unchanged for millenia. And we also know that the advent of science as a way of figuring things out, followed by advances in sanitation, nutrition, and healthcare (expecially interventions such as vaccines and antibiotics) dramatically altered these figures in an unprecedented way.

    So objectively, I think the evidence is overwhelming that folk medicine practices have been with us for a very long time and have not been anywhere near as effective as scientific medicine. The point, of course, of mentioning this is that
    zoopharmacognosy is sometiems cited as evidence that folk medicine practices are rational and effective, or that specific plants should be viewed as potentially therapeutic because some animals appear to use them medicinally. I think this is a leap beyond what can be justified bby the evidence available concenring acual self-medication in animals.

  3. zyrcona says:

    We definitely have data that strongly suggests societies with scientific medicine have better health than those with only folk remedies — I’m not questioning that. What I mean is that I don’t think we have data that shows folk remedies are better or worse than no medicine at all. 🙂 I suppose as a control group for this, you would need to find a statistically significant group of people who don’t believe in any medicines, perhaps believing instead that whatever happens to them is the will of their god and out of their control. Some medicines that started off in the realms of folk medicine have been analysed and developed into more powerful scientific medicines (e.g. aspirin) which suggests it may be reasonable to expect some benefits above baseline from using folk remedies.

  4. skeptvet says:

    Ah, I see. Yes, it would be challenging to demonstrate that folk medical practices in general (not to mention any particular set of practice,s since they all differ quite a bit) was better or worse than no attempt at all to affect our health.

    As far as the history of science validating folk medical therapies, it is actually pretty abysmal. Again, confirmation bias leads us to focus on the few things that seem to have some value rather than the many that do not. And, of course, if you take any given plant, you can find 100 uses recommended in history for it, so if some chemical purified from that plant turns out to be beneficial for one of those conditions, does that count as a success? Or look at quinine. The story typically goes that people used cinchona bark to treat malarial fever and, lo and behold, quinine in cinchona bark kills the malarial parasite. However, the actual story is that in Peru the indigenous people used Peruvian balsam to treat any fever, generally without any benefit since proven. This was imported to England to treat fevers, and many of these fevers were associated with malaria, for which the Perucian balsam again doesn’t seem to have had any real benefit. Then cinchona bark was substituted since it was easier to get and cheaper and used to treat fever due to “bad air.” By chance, it had some real effect on those subsets of these fevers that were malarial in origin.

    If this is a “success story” for traditional medicine, it doesn’t engender much confidence in the idea that traditional practices will regularly point us towards therapies science will eventually show really work.

  5. Rita says:

    Yes, indeed, nice article, many thanks.

  6. Janet Camp says:

    Great post! I’m glad I kept reading through all the comments. I have a degree in Anthropology and you did my field proud in your responses. I was thinking about the way some in the field have been trying lately to twist the concept of cultural relativity into a justification of “complimentary” medicine. Their arguments go along the lines of what you clarified nicely above.

    My dog, thankfully, does not vomit after eating grass, but I have always wondered why dogs DO eat grass–especially since they (mine anyway) don’t get into much else in the yard. I know there is no known answer to this, but it does arouse the curiosity and it is easy to see how “folk beliefs” arise from such observations–especially when they happen to correlate at times with some outcome/behavior.

    This would be a great post to put up over at SBM, by the way. 🙂

  7. skeptvet says:

    Thanks! IT is an interesting subject, largely because we both know very little and yet have a lot of strong intuitions about what’s going on.

  8. skeptvet says:

    Cool! Thanks for the link.

  9. Neus says:

    Hi there, I am a vet specialized in animal behaviour. I use applied Zoopharmacognosy and works really well. It’s amazing how animals can choose what they need based on smell and taste. It’s based on the secondary metabolites of plants going into their noses and to the limbic system, hippothalamus, respiratory system, to the blood and wherever it is needed. When they self-medicate, there are less secondary effects.

    There are really good studies about animal self-medication. Have you read Michael Huffman? You can also check Caroline Ingraham’s website to check some interesting videos.
    Appart from that, a big part of scientific research is based on observational research.

    When you say “folk remedies” I guess you mean using herbs. That’s what all doctors and vets used before the first pill was created. Also what Hippocrates, so called “father of medicine”, used. He also used colour and sound for healing.

    We have to be open to old things in new ways. Good scientists are those who have curiosity, are open to everything and willing to change stucked believes if they find new evidences.
    Hope it helps 🙂

  10. skeptvet says:

    By “folk remedies” I mean any method with a history of use prior to the avdent of the scientific method and with no subsequent sound scientific research evidence to support it. The fact that such remedies have a long history of use doesn’t mean they work. Bloodlettingw as widely popular with the best and brightest in medicine for thousands of years, and it was far more likely to harm than help patients. Good scientists do not rely on uncontrolled personal observation or experience but of scientific evidence- that’s why they are called scientists. It is not closed-minded to ask for scientific evidence before accpeting claims about medical therapies, it is how we have doubled our life-expectancy and dramatically improved our health int he last 200 years; by finding therapies that actually work and abandoning those that do not.

    So if there are “really good studies” about zoopharmacognosy other than those I referred to in my article, please provide references so that everyone following this topic can read them and evaluate the evidence for themselves, rather than taking my word or your word or anyone else’s word for anything.

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