Not Rodents to Mess With

A deviation not directly related to dinosaurs and/or paleontology? Witchcraft! Nothing is to scale.

The use of aposematism (warning signals) as a defense in rodents is an interesting phenomenon. Most rodents avoid predators primarily by evasion; there is a reason "timid as a mouse" is a common phrase. Far be it from most of them to advertise themselves to potential threats. Yet exceptions exist.

Porcupines are the best known and perhaps the most self-evident among these. The quills of both Old World porcupines (Hystricidae, top right) and New World porcupines (Erethizontidae, center right) are boldly patterned in black and white, warning potential attackers of the threat they pose. These quills can easily detach from a porcupine's skin once lodged in the body of an adversary, inflicting painful and possibly deadly wounds even to large predators. Speaking from experience, even a single oral canker sore can greatly impair my will and ability to eat. I can only imagine how much agony a mouthful of porcupine quills must be. Most remarkably, hystricids and erethizontids are not particularly close relatives among hystricognath rodents and almost certainly evolved their formidable defense convergently.

Aposematism is frequently associated with "unusual" weaponry such as the porcupines' quills or chemical defenses, but sometimes aggression can be a sufficient deterrent to predators, especially if you are too small to make a fight worth much of a meal. Some small and medium-sized carnivorans possess distinctive facial masks that have been suggested to warn larger predators of their ferocity. Astonishingly, some rodents appear to have had the same idea. Norway lemmings (Lemmus lemmus, top left) have contrasting black and orange coloration that keeps them cryptic when viewed from a distance, but becomes more eye-catching up close. These lemmings are more aggressive than other small rodents; one experiment showed that most Norway lemmings caught out in the open were able to successfully fend off a long-tailed jaeger (Stercorarius longicaudus) (no mean feat!), whereas gray-sided voles (Myodes rufocanus) that weren't able to reach cover were all killed. Other potentially aposematic rodents that use aggression as a defense are less well-studied, but possible candidates are the European hamster (Cricetus cricetus, center left), which possesses an unusual color scheme that lacks countershading, and Pel's anomalure (Anomalurus pelii, lower left), the largest of the anomalures (which are not flying squirrels!) and colored in contrasting black and white.

My favorite aposematic rodent is the crested rat (Lophiomys imhausi, lower right), a functionally poisonous rodent. The suggestion that this species uses a toxic defense has been suggested for a long time, but was only confirmed in 2011. It chews up the bark of the highly toxic tree Acokanthera schimperi and slathers the toxin onto its flanks. The hair on its flanks, boldly colored in black, white, and brown, have an unusual microstructure that allows them to easily absorb and contain liquids, providing the means to store this nasty surprise for potential predators. (To gain some perspective, said toxin is used by locals to kill elephants.) When disturbed, the crested rat flares its long gray outer layer of hair in an amazing display, as though daring predators to bite into its deadly arsenal. Should an attacker be foolish enough to do so, it would be unlikely to make the same mistake again, provided it even survives the ordeal.

Are there more examples of aposematic rodents? I find myself wondering about the painted tree rat (Callistomys pictus), which has black and white coloration along with spines, and the Cape mole rat (Georychus capensis), which has a boldy-patterned head and an aggressive defense.