Whether swallowing Excedrin® for a headache or sipping a cocktail to take the edge off, you’ve likely self-medicated more than once in your life. Similar to written language and advanced tool use, ingesting an otherwise harmful substance for the purposes of curing what ails you is a behavior usually considered to be uniquely human. But it turns out ants can self-medicate, too. A recent study showed that common black ants changed their diets when they were sick—eating more food supplemented with a medicinal substance—as compared to ants with no infection.
Do animals get high?
You may have heard stories of elephants getting drunk by eating fermented fruit or of dolphins getting high off pufferfish toxin – but as much as people might like these stories to be true, they’re actually driven more by myth than by the reality of evolution.
It shouldn’t be surprising that natural selection would not favor a tendency to get high. Being successful in a predator-prey interaction, for example, requires being substantially more alert than a stoner on their couch eating pizza rolls and watching Netflix.
But what about legitimate medicine? Disease is pervasive in the natural world. If a creature could cure itself of infection by ingesting a substance naturally available to them—and if this behavior was genetically inherited—then we could easily expect the trait to spread through a population over time (in other words, we’d expect the species to evolve the ability to self-medicate).
A study by researchers at University of Helsinki in Finland (published online for an upcoming issue of the journal Evolution) has demonstrated this sort of therapuetic self-medication in the common European black ant, Formica fusca. They found that the ants will intentionally ingest hydrogen peroxide (H2O2) in the presence of a fungal pathogen, and that the otherwise toxic substance helps them recover from the infection.
Specific criteria for self-medication
To show that this behavior is genuinely medicinal, some specific criteria needed to be met. If the ants ate a non-toxic substance that just happened to prevent infection, for example, this could potentially be eaten by all ants as part of their diet—regardless of pathogen exposure—and would not qualify as medicine. In order to be considered self-medication, the ingested substance must: (1) increase the fitness of infected ants (i.e., make it healthier), (2) have a negative effect on uninfected ants, (3) have a negative effect on one or more pathogens, and (4) be deliberately consumed by the ants.
In a cleverly designed study, researchers found that their study subjects fulfilled all four of these criteria. First, they divided ants into two colonies, one of which was fed a standard honey-based food while the other colony was fed the same food, but supplemented with hydrogen peroxide (they use the more general term reactive oxygen species, or ROS, in the paper). Each of these groups was then divided into two, one exposed to the common fungal pathogen, Beauveria bassiana, and the other kept as a fungus-free control group. They tracked daily mortality rates of all four groups for 12 days and found that medicated ants survived at a higher rate than non-medicated when exposed to the fungus, but the opposite was true for the pathogen-free control groups. This satisfied criteria 1 and 2.
To demonstrate that these differential survival rates were the direct result of H2O2 killing the fungal infection, they cultured the fungus in petri dishes with six different concentrations of H2O2, including 0%. This part of the experiment showed a steep decline in the survival of the fungus as the concentration of H2O2 increased, satisfying criterion 3.
Finally, for the most interesting part, the authors tested whether ants would deliberately put themselves into contact with H2O2 after being infected with the fungus. If they did, this would demonstrate that the ants are able to distinguish between medicinal and non-medicinal food, and that they only eat the medicine when they’re sick. You probably know where this is going: when given a choice, infected ants indeed increased their consumption of H2O2-supplemented food, while non-infected ants largely avoided it.
It was a very simple and straightforward experiment, but one that carefully demonstrated all the criterion necessary to say that these ants successfully self-medicate based on the presence of infection. While this work was done in the lab, the authors are careful to point out that ants are also likely to encounter reactive oxygen species such as H2O2 in nature, as they can be present in many ant food sources such as nectar and cadavers.
The authors even speculate that the phenomenon may be part of a mutualistic association between aphids and ants. Aphids secrete a substance called honey dew as they feed on plants, and ants are known to feed on the honey dew. Because plants also produce H2O2 as a defense against aphid predation, infected ants could potentially use the honey dew food source as a way to adjust their intake of H2O2.
Also posted at: http://www.planetexperts.com/self-medicating-ants/
Nick Bos, Liselotte Sundström, Siiri Fuchs, and Dalial Freitak. 2015. Ants medicate to fight disease. Evolution. DOI: 10.1111/evo.12752