The digestion of rodents by Nepenthes truncata
by Irmgard and Siegfried R.H. Hartmeyer
The tropical pitcher plant Nepenthes truncata is very popular in CP collections for its actually considerable up to more than 40 cm large adult pitchers, able to catch even large prey. In the past we considered reports on caught mice by i.e. Lyons’ "Parc de la Tête d’Or" (France) or Robert Cantley (Sri Lanka) just as coincidental accidents. But after a total of four caught mice within 15 months inside our small tropical greenhouse, accompanied by a closer examination of the phenomenon, we are now convinced that Nepenthes truncata - and possibly even some more large growing species - developed an adaptation to catch, digest and absorb small vertebrates like rodents.
It is noticeable that all mice have been caught only by our N. truncata although in the same room are even more pitchers of other large species as N. rafflesiana, N. x mixta, N. eymae x clipeata or N. khasiana, certainly able to catch also mice. From all these species only N. truncata develops very broad leaves, which are tough enough to carry an adult mouse without deformation or folding. Is this a first sign of adaptation to guide small vertebrates (which outweigh most arthropodes) to the pitcher? There are reports on caught rats inside the pitchers of N. rajah at its natural growing site, unfortunately our plant (from tissue culture) is still too small to compete and "swallow" such prey. But without any doubt, this species shows also broad and tough leaves.
The only way to achieve more information was to examine how the pitchers cope with such massiv prey. When the third mouse was caught in April 2008 (pitcher size 38 cm), we decided to stay the course of an definitely very unpleasant experiment, which we aborted two times in 2007 because our sleeping room lies next to our greenhouse and a terrible olfactory annoyance (the "big stink") waft through the door. After monting a tight closing new door in December 2007, we were ready to stand the experiment now without any "nose-violation". Fortunately (ignoring the point of view of the victim) whilst this experiment was still running, in July 2008 a fourth mouse has been caught (pitcher size 41 cm), all by different pitchers. So we took the chance to do a kind of repeat determination, while the lifetime of many pitchers without large prey during two decades act as blank value. In our greenhouse approximately 10 to 12 weeks after opening the pitchers begin to dry out from the lid downwards during about two more months, so we define five months as a normal lifetime from opening until withering.
Actually we must clear one more question: why did our about 30 years old N. truncata whose pitchers are certainly since two decades large enough for such prey, never catch mice before 2007? The reason was obviously shrew, which lived until 2006 beneath the stone floor of our greenhouse. They may look similar on a first glance, but shrew are no rodents (Muridae) but distantly marten related carnivorous predators (Soricidae), which don’t hesitate to feed even on young mice. We tolerated the cute and by law protected guests until they used the space behind the baseboards of our sleepingroom as an uptake for their subterranian nests. Actually no laughing matter anymore! So we filled all spaces with silica sand, which is nontoxic but avoids effectively the digging of rugged tunnels and nests. After a short time the shrew really migrated but now normal house mice (Mus musculus) found the greenhouse interesting enough for visits. That was easy to detect as the droppings look very different. While the shrew were not attracted by the plants nectar and therefore did not scale the plants, their feaces could only be found along the floor edges, but now we found house mouse feaces also in pots and on the plants, so this species did clamber up our Nepenthes. Only a few months later the first mouse was caught in April 2007.
The photo of mouse 1 indicates nicely what happened. The droppings on the leaf show, that the mouse lingered there and was then obviously attracted by the sweet nectar drops which are produced below the pitcher lid. It climbed up the pitcher which is rough-textured at the outside, reaching the slippery peristome (the red collar). From here it must straighten up and at the same time lean forward a little to lick the nectar. Now, standing with only two feet on the slippery surface of the peristom one incautious move is enough to plummet directly into the pitcher, without any chance to escape. Rodents have powerfull teeth, so one may think that they easily bite through the plant tissue, but with all four mice the inner surface remained without any damage. Apparently the combination of pitcher shape, surface texture, and liquid level is optimal adapted to resist even desperate escape attempts of teeth armed rodents.
Assuming that N. truncata really extended its prey pattern from arthropods to small vertebrates, the pitchers must be able to digest such meaty prey without being untimely destroyed by aggressiv putrefactive bacteria and fungi. Every Nepenthes grower knows this problem: too much prey at once leads very soon to rotting pitchers. For that reason the presence of mosquito larvae and numerous other small animals living within the liquor is beneficial for the plant. Commensal animals help the plant to break down prey protein what counteracts putridity, and their feaces dispensed directly into the liquor return parts of the food even optimised for the nutrient uptake of the plant. In nature such pitchers show complex food chains ranging from protozoans over mosquito larvae up to ants and spiders, especially adapted to this mini-environment. Unfortunately we are not able to say wether there are special commensal organisms at the growing site of N. truncata (Philippines) or not, however, inside our tropical greenhouse we observed the following course for Mouse 3 and 4.
In early June the lid began to turn brownish, this quite normal withering proceeded downwards the pitcher for three weeks and stagnated about 1 cm above the liquor level. Below that margin the pitcher remaind inside as well as outside coloured and without any damage. That appears reasonable as the pitcher would certainly not cope with an additional caught rodent at this time without rotting. Not until early September also the lower part started to become brownish. As mentioned above the pitchers of our N. truncata without large prey wither averaged within 4-5 months after opening, so we can state that the digestion of an adult mouse did not change the time span notably. Even when we decided to cut the pitcher open on September 16th the liquor level was still nearly constant.
Equiped with a respirable dust mask, gloves and a scalpel, Siggi exhausted the liquor and opened the pitcher, which appeared very thick fibrous and even with the scalpel not easy to cut at its lower part. The precautions are suitable, as meanwhile a high percentage of rodents and shrew in Germany carry the dangerous, and sometimes even deathly Hantavirus. More on that increasing desease at the end of this report.
Precautions for Hanta virusMice are actually not rare in greenhouses, so in case that somebody likes to repeat our examinations we would like to mention an increasing thread for health at this place which was reported by Germanys’ first news magazine Der Spiegel. Assumedly after 1945 the North Amarican Hanta virus, which is comparable versatile like the flu virus, has been introduced to Europe. Since that time it spreads increasingly over the continent carried by rodents and shrew. While the American variant (spread there by other small vertebrates) attacs the lungs after infection, the European form (Puumalavirus) caused in the past mainly slightly to medium flu symptoms and was therefore often not noticed as Hanta. But during the last years heavy kidney-dysfunction has been diagnosed in Germany caused by Hanta and the estimated number of unreported cases is probably high. Since 2001 an average of 200 infections was recorded, but only 2007 that number jumped up to 1688 cases from which nearly every tenth person needed a kidney dialysis. Even more alarming is the fact, that in four cases the Puumalavirus in Germany appeared in his worst form with a haemorrhagic process (inner bleedings) like Ebola, now with a chance of survival of only 50%.
The results of our experiment indicate certainly that N. truncata shows all necessary features to attract, catch and digest (nearly complete, except hair, big teeth and a greasy matter) rodents. The increasing size of the adjacent developed pitchers evidence clearly that the plant took actually its benefit from the nutrients. After the digestion of mice 3 and 4 in late September 2008 a new 45 cm tall pitcher opened, which is the largest since we bought the plant 1984 as a rosette of only 15 cm diameter.
Despite the experiment became in some parts nauseously, we can finally state that it was worth it to document the digestion process in detail. Obviously the remains of rodents and other vertebrates found inside the pitchers of large Nepenthes in nature, are not the result of coincidence but a specific adaptation to profit from such large prey. To circumstantiate this definitly, even more detailed observations should be taken at the natural growing sites regarding the number and species of caught vertebrates in particular plant populations.
Enjoy our additional movie on YOUTUBE:
Cantley, R. (2004/2006), private communication
Clarke, Ch. (1997) Nepenthes of Borneo, Natural History Publications Kota Kinabalu
Hartmeyer, I. & Hartmeyer, S. (2007), Ratz fatz aus die Maus, DAS TAUBLATT 2/2007: 31-36
Hartmeyer, I. & Hartmeyer, S. (2008), Die Truncata lässt das Mausen nicht, DAS TAUBLATT 1/2008
McPherson, S. (2008), private communication
Stockinger, G. (2008), Vom Winde verweht, DER SPIEGEL 35/2008: 154-155