Named by: Milne-Edwards, 1867
Taxonomy: Synapsida, Sphenacomorpha, Sphenacodontia, Sphenacodontoidea, Therapsida, Theriodontia, Cynodontia, Probainognathia, Mammaliamorpha, Mammaliaformes, Mammalia, Theria, Eutheria, Placentalia, Euarchontoglires, Glires, Rodentia, Myomorpha, Muridae
Included species: L. imhausi
Lophiomys are rarely-seen rodents that live in the forests and woodlands of eastern Africa. They are members of Muridae, the clade uniting Old World rats and mice, but, at up to 920 g (2 lb) in body mass, Lophiomys are quite a bit larger than your average rat.
The large size of Lophiomys compared to most other murids has allowed them to evolve a stomach partitioned into five compartments, not unlike the four-part stomach of cows and other ruminants. This in turn allows them to process their mainly plant-based diet in a similar way: by fermenting it in their foregut. Lophiomys are good climbers and regularly forage in trees, but their movements are slow and sluggish.
A large, slow-moving rodent would make a substantial meal for many predators. However, there are strong indicators that Lophiomys are not easy prey. When threatened, the long, gray-tipped hairs that normally cover their body are flared out, erecting a crest along their back and exposing boldly-patterned black and white tracts of shorter hair along their flanks. Such an eye-catching threat display would appear to be a warning to predators of a potent defense mechanism, but what this defense is has not been clearly understood until recently.
Historically, some have speculated that Lophiomys are harmless in themselves and that their black and white coloration are adaptations for mimicking other mammals with more repugnant defenses, such as zorillas (Ictonyx striatus, small carnivorans capable of spraying a noxious secretion) or porcupines. However, anecdotal accounts have long suggested the possibility of a chemical defense in Lophiomys. Dogs that had attacked these rodents were said to exhibit loss of coordination, froth at the mouth, and sometimes even drop dead. Detailed observations of Lophiomys behavior and anatomy, published in 2012, have given us a more complete understanding of antipredator adaptations in these rodents, and the truth is remarkable.
The observations presented in the 2012 study confirm that Lophiomys do indeed employ a chemical defense against predators, but the rodents do not use chemicals that they produce themselves. Instead, they chew the bark of a highly toxic tree species (Acokanthera schimperi), mix the toxin with their saliva, and then carefully apply this mixture to the shorter hair on their flanks. Examination of these hairs underneath a microscope reveals that they have a distinctive spongy structure, which allows them to absorb and retain the toxin. Considering that Acokanthera toxin is also used by native hunters to hunt and kill elephants, any predator that recklessly tries to make a meal out of Lophiomys is undoubtedly in for a nasty (and potentially fatal) surprise.
Even that is not quite the full extent of defensive adaptations in Lophiomys. Given that Lophiomys lack any means of injecting the toxin into would-be attackers, the only way for the toxin to take effect is if a predator grabs a Lophiomys individual in its mouth. As such, Lophiomys also have features that protect them from harm while being handled by large predators. The skin of Lophiomys is extremely dense and tough, their vertebrae are stout, and their skull is box-like and covered in bony granules. Mammals typically have a large opening on either side of skull, but in Lophiomys these openings are sealed off by bone, forming an inbuilt helmet. (In fact, the skull of Lophiomys has been likened to that of turtles, which also have similarly sealed-up skulls.) Though not the most extreme form of armor among mammals, this suite of adaptations is evidently sufficient to prevent serious injury to Lophiomys while the poison does its job.
Much remains unknown about the biology of Lophiomys, but one particular mystery stands out regarding their unique defense mechanism. Given that their defense requires them to regularly chew on highly toxic material, how do Lophiomys protect themselves from the toxin? Perhaps this is where the true significance of their multi-chambered stomach lies. The diet of Lophiomys consists primarily of soft plant parts that should be easily digestible even without foregut fermentation. Breaking down a deadly toxin, on the other hand, might just call for the involvement of an unusually complex digestive system.
- Happold, D.C.D. (ed.). 2013. Mammals of Africa Volume III: Rodents, Hares and Rabbits. A&C Black Publishers Ltd., London. 784 pp.
- Kingdon J., B. Agwanda, M. Kinnaird, T. O’Brien, C. Holland, T. Gheysens, M. Boulet-Audet, and F. Vollrath. 2012. A poisonous surprise under the coat of the African crested rat. Proceedings of the Royal Society B 279: 675-680. doi:10.1098/rspb.2011.1169