Woolly Bear, Heal Thyself

By Carl Zimmer | March 10, 2009 3:05 pm

woolly-bear.jpgAnimals, as I explained in my recent column for Discover, take precautions not to get sick (including the famous anal cannon). We take precautions too–conscious ones, based on what we have learned about how diseases spread, and perhaps also unconscious ones that lower our risk of infection.

But if those precautions fail, we humans sometimes take medicines to kill off the pathogens making us sick. And there’s an intriguing body of evidence suggesting that animals take medicine too.

A lot of that evidence comes from studies on chimpanzees, our closest living relatives. When they get infected, they will sometimes devour leaves and other vegetation they otherwise never touch. In the case of one plant, the chimps have to first peel away an outer covering that is lethally toxic. They  go to these lengths, it appears, because eating these plants can cure them of their ills. Some plants can flush parasites out of the gut and others actually fight the pathogens themselves. In fact, when researchers study the plants chimpanzees eat, they discover new compounds that kill bacteria and other pathogens. They look promising as medicines for people. (pdf).

Given the sophisticated minds of chimpanzees (they can, for example, plot rock-hurling attacks on zoo-goers), it’s possible that chimpanzees teach themselves which medicinal plants to take, and these medical traditions spread culturally. But other animals seem to eat peculiar things when they get sick, too. Even insects do. Obviously, no invertebrate has ever graduated from medical school. (Insert your doctor joke here.) So the question arises, are insects actually self-medicating, or are they getting so sick that their diet goes haywire?

In a new paper published in PLOS One, Michael Singer of Wesleyan University and his colleagues carefully tested these alternatives in an experiment on woolly bears (Grammia incorrupta). A lot of woolly bears get killed by parasitic flies and wasps. The flies and wasps lay their eggs inside the woolly bears, and the larvae hatching from the eggs devour their hosts from within.

But a lot of these parasites die inside the woolly bears, and the hosts recover in good health. The immune system of the woolly bears probably kills off some of the eggs. But Singer has observed that some of the plants woolly bears feed on contain pyrrolizidine alkaloids, which are some truly nasty chemicals. So Singer and his colleagues looked into whether the woolly bears eat the alkaloids to medicate themselves against parasites.

The researchers fed infected and uninfected woolly bears food that included alkaloids or was alkaloid-free. They found that the infected woolly bears that ate alkaloids were 17% more likely to survive long enough to develop into moths. Their success was due to the parasite-killing power of the alkaloids–many fewer parasitic flies emerged from alkaloid-eating woolly bears than from hosts fed an ordinary diet.

But alkaloids are not to be taken lightly–like many medicines, they can have some nasty side effects on their hosts. Uninfected woolly bears that ate alkaloids were 18% less likely to survive to become moths than woolly bears that didn’t eat them.

Singer’s team also found that infected woolly bears eat more alkaloid-laced food than healthy ones. Another test of the medication hypothesis was not so clear, though. The scientists allowed the woolly bears to choose between food with alkaloids and food without them. Putting more fly eggs in a host did not lead it to choose to eat more alkaloids. But among the infected woolly bears that survived to become moths, the ones that were infected with two eggs turned out to have eaten more alkaloids. It’s possible that the ambiguity in the choice experiment was due to the woolly bears killing off the parasite eggs with their own immune systems, making it unnecessary for them to medicate themselves.

Singer and his colleagues propose that woolly bears can medicate themselves with the alkaloids. What’s more, they do so without learning like chimpanzees do. A woolly bear’s immune system recognizes invading parasites, and alters the animal’s nervous system so that it gets a stronger preference for the taste of alkaloids.

Their hypothesis is, to the say the least, intriguing. How many other insects are healing themselves with medicinal plants? Did the common ancestor of insects and humans have this ability too? Did our ancestors once have these hippocratic instincts, which have since been lost? Or do some foods (or even dirt) taste just a little better when our bodies detect an unwelcome visitor?

Update: Dr. Singer kindly jumped into the comment thread to respond to some questions about his research. Among other things, he notes that woolly bears move around from plant to plant as they feed, so their choice of chow is not limited by the cost of travel.

CATEGORIZED UNDER: The Parasite Files

Comments (10)

  1. James Hathaway

    In many ways, this isn’t news. The partnership of various insects (particularly lepidoptera) with their toxic food plants is a long-studied phenomenon. The most famous examples (known to most school kids) are the monarch butterfly (the adult and the larvae are toxic to predators and parasites because of the toxins in milkweeds) and the pipevine swallowtail, which tastes so nasty to birds that a wide variety of other species of butterflies mimic it (it’s black and irridescent blue) in order to share the avoidance. The fact that some species can selectively switch species of food plant (it’s probably unlikely because caterpillars can’t exactly crawl all over the field looking for the right food plant when they get an egg laid on them) just means that they are generalist feeders and there are benefits to eating the more toxic plants in their repertoire (and costs, such as dealing with the toxins) in terms of predator prevention. If you think about it evolutionarily, heavy pressure by predators (such as parasitic wasps) is probably what favors the evolution of insects that specialize in eating highly toxic plants (like the monarch butterfly).

    Carl: James, thanks for your comments. I was thinking about mentioning the monarch butterflies, etc., in the post but then decided to stick to the woolly bears. Monarchs obviously also use chemical defenses, but they don’t eat extra milkweed when a flock of birds shows up. If you take a look at the paper, Singer and his colleagues explain how they view the woolly bear behavior as a plastic, adaptive response to infection–in other words, as self-medication.

  2. James Hathaway

    I will look at it — thanks! Again, I have my doubts, since it is a huge investment of energy for a catterpillar to move from plant to plant, but it would not have bben the wildest thing for evolution to have build in an increased eating behavior triggered by parasitism.

  3. I think the fascinating point is their (conjectured) proposal that infection itself and immune response may modulate either sense of taste/smell or the drive to eat specific foods.

    It seems like this would be a pretty easy feat for natural selection to accomplish (at least when thinking about it from a simplistic non-expert view, which mine most assuredly is). Some specific immune response could perhaps simply upregulate a few olfactory/gustatory receptors, for example. Or it could involve some more complex regulation of neural pathways controlling eating.

    Carl, I am in awe of your ability to take what might on first glance seem rather uninteresting and instantly bring a deep potential connection to our own behaviors and experiences and give it greater context.

    If the behavior above turns out to in fact link immune response directly with a relatively simple modulation of neural function or response, it wouldn’t be difficult at all to imagine similar phenomena in humans. I’d be interested to know if similar results could be found in any vertebrate. We all know that food tastes different (usually bland) when we’re sick, and we usually attribute this to stuffy noses and such. It would be quite fascinating to find out that certain foods become tastier…

    great post!

  4. James Hathaway

    OK, I read the paper — the results in the “feeding choice” experiment (that involved selecting among artificial foods with or without the toxins) were mixed, with the overall results showing no significant relationship between an increase of parasite infection and an increase in consumption of the toxin, but one significant exception — a significant increase in consumption between those parasitized with only one egg and those parasitized with two. The results are hardly conclusive and Occam’s Razor seems to point to some far more likely explanations than the catterpillars deliberately medicating themselves: the sicker ones might simply have reduced ability to detect toxins (because their systems are under attack) or they may be more desperate for food (parasitic larvae attack stored fat supplies), making them more ravenous and less selective for food plants. Lepidoptera larvae are generally fairly highly adapted to eat specific food plants (as any kid who has tried to raise a caterpillar on grass clippings finds out), but when starved almost all species will sample plant offerings outside their natural selections. This research strikes me as an example of science that is attempting to find animal behavior to fit a news idea (self-medication) rather than a hypothesis that comes from observing the behavior of the organisms, though I am probably being unfair. I still don’t see how an animal in nature would be likely to take advantage of this behavior, as moving from one food plant to hunt for other, perhaps rarer foodplants would be a pretty expensive for the caterpillar in terms of its survival and would require some decision-making that it is hard to see how an animal as neurologically simple as a caterpillar would be capable of.

  5. Michael Singer

    Dear James,

    Thanks for your interest in thinking critically about our research on self-medicating caterpillars. A bit of natural history on these caterpillars and their foraging habits will help you understand why we believe our results suggest that these caterpillars really do practice self-medication in the wild. They are unusually mobile for lepidopteran larvae, moving across the ground at a clip of 10 cm per second or faster. They sample small herbaceous plants quite frequently, and graze among them at a rate of 1-2 plant species per hour. This mobility is not likely to be very costly at all because the energetic demands are relatively small compared to flying insects or warm-blooded species which have thermoregulation on top of energetic costs for movement. Moreover, the caterpillar’s diet is rich in carbohydrates from plant tissue, giving them plenty of energy. My colleagues and I have done extensive observations of the caterpillar’s foraging behavior in the wild (Arizona) and in the lab. Based on a series of previous publications (cited in the PLoS paper), we know that the caterpillars feed selectively, and frequently feed on plants containing the alkaloids even when they have not been parasitized. However, the finding in our recent PLoS paper shows evidence that they increase their ingestion of alkaloids when they are fighting off parasitoids (probably against the set of maggots that have escaped encapsulation by the caterpillar’s immune system). The relationship between the ingestion of alkaloids, the immune response, and anti-parasitoid resistance is a current focus of study in my lab.

  6. Arnold Mousetrouser (Australia)

    Ref to the rock-throwing chimpanzee: A letter-writer in the Australian Sydney Morning Herald today says the chimp is even more intelligent than is thought. He is actually piling up the rocks to hide the entrance to the escape tunnel he is digging and the rock-throwing is no more than a way of diverting attention from this while he completes the task. AM (Australia)

  7. CR

    This is a little off topic, but I found this link while searching for info.

    I captured a couple woolybears in Northern California to bring down to Los Angeles for my daughter to raise. I wasn’t sure if they would survive since the climate is obviously much warmer down here. I placed them in a container with dandelion in the garage and one died and the other built a cocoon. I discovered the female one day had emerged and was depositing eggs in clutches on the back of the enclosure.

    After she had layed 30-60 eggs I released her although she had no interest in flight.

    I had read that the moths fly off to breed, so then how is it this moth emerged with eggs without any contact with a male moth (presumably)?

    Not a lot of information is available about the eggs and gestation periods, early food and care etc. The eggs were often found split open like a stepped on ping pong ball but the enclosure screen was too large to keep them from escaping.

    My question is do the months have the ability to breed Asexually?

    Did the caterpillar that died trigger it or could it possibly have mated as a pupae?

    One last thought is, have any other species of moths been known to interbreed?

    Could be a long shot but a Hyles Lineata was spotted flying around my garage that served as my reminder to check on the Isabella Tiger Moth.

    Sounded like you guys have had a successful breeding program and might shed some light or do you just collect them in the wild?

    Thank everyone for their input on solving this mystery.

    CR

  8. JD

    Yeah wow! So, does any body know what tipe of plants they eat? I have one in my kitchen right now because I found it in a bucket on my patio. I thought it was dead but after being in the house for a while it started moving. Now i Have a few types of leaves in the bucket but its not eating them. What types does it eat?

    JD

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The Loom

A blog about life, past and future. Written by DISCOVER contributing editor and columnist Carl Zimmer.

About Carl Zimmer

Carl Zimmer writes about science regularly for The New York Times and magazines such as DISCOVER, which also hosts his blog, The LoomHe is the author of 12 books, the most recent of which is Science Ink: Tattoos of the Science Obsessed.

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