One early morning, while on the Cheringoma Plateau, Jen “the Mammal Lady” Guyton came running into the camp. “We caught something really good in our traps!”, she announced breathlessly, sending the media team that was filming our expedition into a state of frenzied excitement. Everybody rushed to see the mystery animal, whose identity Jen refused to reveal. We scrambled through a spiny thicket and arrived at a place where she had set a large trap the night before. My imagination was running wild, and I was really hoping for a pangolin, or at least a baby aardvark. But the animal I saw was neither of these. At first, in the dim light I couldn’t quite make out the large mammal in the cage, and my first thought was of a small kangaroo, which would have been exciting if rather unlikely. “It is a pouched rat!”, Jen said proudly, and I felt all my hopes deflate like a punctured tire.
But my disappointment turned out to be grossly unjustified. To begin, the animal was actually very pretty. It was the size of a small cat, with a sweet face and slow, deliberate movements, quite unlike those of a house rat or any other rodent that I had ever seen. When Jen handed it a snack, it calmly ate it. Its tail was thick, half black and half white, which is a characteristic coloration of this species. Pouched rats (Cricetomys gambianus) are members of the family Nesomyidae, and thus not closely related to the rats that infest human houses, and derive their name from their ability to store food in large pouches in their cheeks. They can reach the weight of about 2 kg, and in some parts of Africa are hunted for their tasty meat.
In Mozambique, however, pouched rats play a very different role. The civil war of 1975-1992 has left many areas of the country infested with land mines, resulting in human casualties, and forcing people to abandon their land and houses. The removal of mines is an arduous process that normally requires the use of metal detectors or highly trained sniffer dogs. But in the late 1990’s Frank Weetjens, a Belgian mine-removal specialist, realized that pouched rats could make great mine detectors – they have an unparalleled sense of smell, and their calm nature and intelligence makes them easy to train. His organization APOPO set up a rat training camp in Tanzania, where young pups of pouched rats are taught to detect the smell of 2,4,6-trinitrotoluene (TNT), the main ingredient of land mines, and signal its presence to the trainer in exchange for a snack. After a 250-day long training the rats are flown into Mozambique, where they are used to systematically de-mine large areas still too dangerous for people to inhabit. Their efficiency in detecting land mines is nearly 100%, and their use in Mozambique has already allowed thousands of families to return to their homes and farmlands. It is not surprising then that in Mozambique they are known as Hero rats, a designation they fully deserve. In an even more amazing development, the pouched rats are now being used in Africa as sensitive and efficient diagnosticians of tuberculosis, a disease that affects many rural populations. Their incredible sense of smell allows them to differentiate between sputum of a healthy and an infected person at a rate and accuracy much higher than those afforded by traditional medical tests.
All this would be enough to make me worship pouched rats, but the animal we caught had a hidden entomological bonus. We had placed it a large container to take a few photos, and then we noticed dozens of large, brown insects running on its fur. One look and I immediately recognized them as something I had only read about before – they were parasitic earwigs! Earwigs are not insects that are usually thought of as something to be found on other animals, but members of the suborder Hemimerina are specialized parasites of rodents. They lack the large abdominal forceps of their free-living relatives and they are blind. Unlike other earwigs, they are also viviparous. On the body of the pouched rat they moved with an amazing speed, and their legs acted like clasps, allowing them to cling to hair and virtually swim through the mammal’s fur.
Adult parasitic earwigs (Hemimerus) have many nymphal characteristics, such as the lack of clasping cerci, wings, and wing musculature, suggesting that they are paedomorphic (reproductive stages that retain larval morphology.)
From the pouched rat’s perspective, having the earwigs is probably not too bothersome. These insects feed only on dead flakes of skin of the mammal, and thus act more as helpful exfoliants, rather than true parasites, and I also imagine that they provide an occasional snack.
All in all, the pouched rat in the trap turned out to be far more interesting than a baby aardvark would have ever been. (But I am still hoping to see one.)