Screams the tabloid headline…
Is this the plotline for a sequel to The Planet of the Apes in which mistreated lab rats rebel against cruel animal experimenters?
No, it’s actually an accurate (ok, a bit sensationalized) description of the way in which a small African rat has opportunistically found a way to deploy a poison tool (yes, tool, see below) to defend itself from predators.
For years, observers had suspected something was up with the African crested rat (Lophiomys imhausi): it moves sluggishly, acts fearlessly – practically inviting predators to attack it – and twists around to display boldly-patterned black and white bands along its flanks when it’s excited or threated. Some have speculated that these displays could be designed to mimic the appearance of the spiny porcupine or skunk-like zorilla, and over the years there have been reports suggesting that the crested rat may be poisonous, based in part on anecdotes about dogs retreating in fear from the small rodents or showing signs of having been poisoned after crested rat run-ins.
The mystery of the crested rat was cleared up last week, when a team of researchers led by Jonathan Kingdon of the University of Oxford’s Department of Zoology, published their findings about the rat’s unique set of defenses online in the Proceedings of the Royal Society B1.
The researchers found that the crested rat gnaws and chews the roots and bark of local Acokanthera schimperi trees, which contain a substance called ouabain that is used in a traditional African arrow poison known to be capable of killing elephants by amplifying heart contractions. In chewing on the bark and roots, the rat creates a thick gel-like mixture of saliva and plant toxins, which it proceeds to slather onto the distinctively colored fur along its flanks. Here’s a video of the crested rat in which it briefly displays some grooming behavior:
As it turns out, the hairs of the fur in crested rat’s flank-area are highly specialized and extremely well-suited to deliver this self-applied poisonous mixture. These hairs are essentially perforated cylinders containing fiber-like strands that act as wicks, rapidly absorbing the slobbery, poisonous gel and drawing it up by capillary action. When the researchers chemically analyzed the hairs by infrared spectroscopy, they found strong evidence that that they were indeed absorbing and wicking up ouabain from the saliva mixture. Here’s another video of the hairs doing showing off their wicking abilities (that’s red dye in the video, not blood!):
Properly armed with this potent poison and benefited by some additional physical adaptations (an armored skull, enlarged vertebrae, and dense and thick skin), the crested rat enjoys a suite of defenses that allow it to stare down many a predator. The research paper describes the crested rat’s behavior when threatened:
Flaring of the fur is triggered by external interference or attack on the animal, whereupon white and black banding of the longer hairs on either side of the lateral line effects outlines of the tract in a bold white and black ‘target’ design. An aggravated rat pulls its head back into its shoulders and turns its flared tract towards its adversary as if actively soliciting an attack. This display may or may not be accompanied by vocalizations.
No, you don’t want to mess with Lophiomys imhausi.
The researchers characterize the crested rat’s poisonous defense as “toxicity by acquisition” never before reported for a placental mammal, noting that the closest mammalian analogy may be European hedgehogs, who are known to slather their spines with a mixture of toad venom and saliva, presumably to increase the pain and discomfort that their spines can inflict. By contrast, they point out that there’s no evidence that the crested rat needs to create any kind of a wound; rather, the would-be predator is poisoned when it bites – or even just mouths – the crested rat.
So, is the crested rat just a fascinatingly well-adapted defender, or is it a full-fledged tool user?
Tool user! (We at AnimalWise are never shy about making pronouncements … or speaking about ourselves in the “royal we.”)
Although not mentioned in the research report, the crested rat’s deployment of the plant toxins does indeed qualify as “tool use” as defined in Benjamin Beck’s Animal Tool Behavior, the most complete catalog of tool use in animals. The original 1980 version contained what remains one of the most widely-accepted scientific definitions of the term:
[T]he external employment of an unattached environmental object to alter more efficiently the form, position, or condition of another object, another organism, or the user itself, when the user holds or carries the tool during or just prior to use and is responsible for the proper and effective orientation of the tool.2
In 2011, this treatise was substantially revised and updated, and now contains the following definition:
The external employment of an unattached or manipulable attached environmental object to alter more efficiently the form, position, or condition of another object, another organism, or the user itself, when the user holds and directly manipulates the tool during or prior to use and is responsible for the proper and effective orientation of the tool.3
While it’s not all that much fun wading through the definitions (would they read better in verse?), the authors themselves make it clear that they would consider the crested rat’s “self-anointment” behavior to be tool use: the bark/roots are “unattached environmental objects,” the crested rat uses them to provide itself with a more efficient defensive position, it holds (carries) and manipulates the tool, and is responsible for properly and effectively orienting it.
In fact, the authors have come up with what they call modes of tool use, including several – Affix (attaching an object to the body with adhesive), Apply (attaching a fluid or object to the body without adhesive) and Drape (placing an object on the body temporarily) – which are directly applicable here.4
Moreover, considering only rodents (there are other examples elsewhere in the animal kingdom), the authors specifically call out a number of additional examples of “Affix, Apply, Drape” tool use by self-anointers: rice-field rats that apply the anal gland secretions of the weasel, one of their predators, presumably for concealment purposes; and California ground squirrels, rock squirrels, and Siberian chipmunks that anoint themselves with the scent of rattlesnakes by chewing shed snakeskin, applying dirt (substrate) the snake has been contacted with, and/or anointing themselves with snake urine, all most likely for “olfactory camouflage” purposes.5
So, there you have it. The crested rat is bold, it’s brave, it’s poison, and it’s a tool user!
1Kingdon, 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 of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2011.1169.
2Beck, B.B. 1980. Animal tool behavior. New York: Garland (as quoted in Shumaker, Robert W.; Walkup, Kristina R.; Beck, Benjamin B.; Burghardt, Gordon M. (2011-04-28). Animal Tool Behavior: The Use and Manufacture of Tools by Animals (Kindle Locations 299-301). JHUP. Kindle Edition).
3Shumaker, Walkup; Beck & Burghardt 2011 (Kindle Locations 372-375).
4Id. (Kindle Location 601).
5Id. (Kindle Locations 1934-1943).