African Rat Obtains Toxic Armor from Plants
For many generations, humans have known that toxic plant compounds can be used to poison the tips of arrows, spears and other weapons, vastly increasing the lethality of an attack. The indigenous peoples of South America are perhaps the most famous (or infamous) for this strategy, and the use of curare-tipped arrows (Lee 2005) achieved legendary status after Europeans invaded that continent.
A similar strategy is used by some African communities (Cassels 1985), which take advantage of the natural substances in the bark and roots of Acokanthera schimperi, a small tree that occurs in eastern Africa and parts of Yemen. This plant harbors a type of steroid known as a cardenolide, which is closely related to ouabain, a sodium/potassium ion pump inhibitor. Any mammal unfortunate enough to ingest or be injected with this toxic compound—also known as a secondary metabolite— is likely to succumb to cardiac arrest. This type of poison is so powerful that it is used by humans to hunt massive prey, such as elephants.
A new study reveals that humans are not the only species that are able to exploit A. schimperi trees for weaponization. In a recent issue of Proceedings of the Royal Society B, Kingdon et al. (2011) report that the African crested rat ( Lophiomys imhausi ) has hit upon a strikingly similar strategy. It chews on the bark of A. shimperi, and then licks a specific strip of long hairs along its back. Once this poisonous saliva has been applied, the rat contracts and relaxes muscles along its back, which distributes the toxic slime along the hairs. In addition, the hairs themselves are specially adapted to absorb the poison through their uniquely porous surfaces.
Thus, when a predator descends on the crested rat—which, at over a foot long, is a substantial morsel of prey—the rodent freezes and arches its back, rather than make a futile attempt to amble away. When the attacker grabs its anticipated prey in its mouth, it is immediately exposed to the rat’s toxic armor of plant-derived cardenolides, a fatal error on the part of the hungry hunter.
This is not the only example of an animal using exogenous toxins to fend of predators, but it is one of the most dramatic—hedgehogs are known to deposit saliva containing toxins from toads on the tips of their spikes (Brodie 1977) providing an extra deterrent to would-be predators, but not a fatal dose of poison as in the case of crested rats and their lethal, cardenolide-tainted fur. Also, monarch butterflies obtain cardenolides from the milkweed that they consume as caterpillars, making the insects toxic to predators. This is a more passive process than that of the crested rats, however, as the butterflies simply build up concentrations of the toxins in their tissues, rather than specifically applying it to strategic areas of their bodies.
Why don’t the rats themselves go into cardiac arrest from chewing on the toxic bark? That question remains unanswered, but further research into that issue may yield discoveries with important implications for the treatment of heart disease in humans. In addition, further research may reveal whether this is a mutualistic or exploitative relationship: does the plant derive any benefit from the protection that the rats gain from munching on its roots and bark? This defense mechanisms on the part of the rat is a fascinating phenomenon that, as always, is likely even more complex than it appears at first glance.
Brodie, E. D. Jr. 1977. Hedgehogs use toad venom in their own defence. Nature 268: 627-628.
Cassels, B. K. 1985. Analysis of a Maasai arrow poison. Journal of Ethnopharmacology 14: 273-281.
Kingdon J, Agwanda B, Kinnaird M, O’Brien T, Holland C, Gheysens T, Boulet-Audet M, & Vollrath F (2011). A poisonous surprise under the coat of the African crested rat. Proceedings. Biological sciences / The Royal Society PMID: 21813554
Lee, M. R. 2005. Curare: the South American arrow poison. Journal of the Royal College of Physicians of Edinburgh 35: 83-92.