Research on Lolldaiga Hills’ poisonous rodent—the Maned Rat
Sara B. Weinstein, Mpala-Smithsonian Postdoctoral Fellow, Mpala Research Centre
Department of Biology, University of Utah and National Zoological Park, Smithsonian Institution
Many animals defend against predation by co-opting the chemical defenses of their food. Although this poison sequestration behaviour has been most studied in butterflies, similar behaviours occur in other insects, amphibians and even birds (Savitzky et al. 2012, Nishida 2014). Remarkably, although many mammals consume poisonous vegetation (Dearing et al. 2005), the defensive use of sequestered toxins is rare among mammals. The European Hedgehog Erinaceus europaeus coats its quills with noxious chemicals (Brodie 1977) and the Maned (or Crested) Rat Lophiomys imhausi anoints its fur with compounds from the Poison Arrow Tree Acokanthera schimperi (Kingdon et al. 2012).
The Maned Rat defends itself with cardiac glycosides from the Poison Arrow Tree. These large (adult body weight = 600–1200 g) rodents chew the bark and leaves of the Poison Arrow Tree, releasing ouabain, acovenoside, and other cardiac glycosides which they then lick onto specialized hairs on their sides. When disturbed, they expose these areas to reveal warning coloration and the toxin-anointed hairs (Kingdon et al. 2012). The ecology, behaviour and distribution of the Maned Rat remain poorly known (Kingdon 1974, Happold 2013). These rodents are, however, repeatedly observed around Mount Kenya, including numerous sightings on Lolldaiga Hills Ranch (see past blog post on www.lolldaiga.com/maned-rat-lolldaiga). Lolldaiga’s Maned Rat population is being monitored, sampled, and studied in order to better understand the behaviour and ecology of this enigmatic rodent.
Trapping of Maned Rats has focused on the riverine forest along the Timau River on the south end of the Ranch. Trapped rats are examined for poison levels, parasite loads, and behaviour. All tested animals had cardiac glycosides in their specialized hairs, although poison levels varied among individuals.
In most animals, feeding on even a small amount of the Poison Arrow Tree leads to death, as all parts of the plant (except the ripe fruit) are highly toxic. We have filmed Maned Rats chewing and applying masticated Poison Arrow Tree bark and leaves to their fur, thereby confirming previous observations of this behaviour [see link to Kingdon (2012): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3248729/]. Using motion activated infra-red cameras, we have also observed other behaviours, including mating, grooming and social interactions.
Link to chewing video: https://www.youtube.com/watch?v=kbYvPVNmdWw
Link to feeding video: https://youtu.be/D1h_NBejN1Y
Sampled Maned Rats yielded lice and fleas, suggesting that these ectoparasites, like their hosts, have unique adaptations for managing toxin exposure and its effects. Interestingly, ticks have not been observed on Maned Rats.
This research project will continue to study the Maned Rat on Lolldaiga Hills Ranch, while expanding sampling to other areas to further examine the natural history and mechanisms that facilitate toxin sequestration in this unusual rodent.
References: Brodie, E. D. 1977. Hedgehogs use toad venom in their own defence. Nature 268(5621): 627––628. Dearing, M. D., Foley, W. J. & McLean, S. 2005. The influence of plant secondary metabolites on the nutritional ecology of herbivorous terrestrial vertebrates. Annual Review of Ecology, Evolution, and Systematics 36(1): 169–189. Happold, D. C. D. 2013. Lophiomys imhausi Maned Rat (Crested Rat). In: Mammals of Africa. Volume III: Rodents, Hares and Rabbits, Happold, D. C. D., ed., pp. 214–215. Bloomsbury, London. Kingdon, J. 1974. East African Mammals. Volume IIB: Hares and Rodents. University of Chicago Press, Chicago. Kingdon, J., Agwanda, B., Kinnaird, M., O’Brien, T., Holland, C., Gheysens, T., Boulet-Audet, M. & Vollrath, F. 2012. A poisonous surprise under the coat of the African crested rat. Proceedings of the Royal Society of London B: Biological Sciences 279(1729): 675–680. Nishida, R. 2014. Chemical ecology of insect–plant interactions: ecological significance of plant secondary metabolites. Bioscience, Biotechnology, and Biochemistry 78(1): 1–13. Savitzky, A. H., Mori, A., Hutchinson, D. A., Saporito, R. A., Burghardt, G. M., Lillywhite, H. B. & Meinwald, J. 2012. Sequestered defensive toxins in tetrapod vertebrates: principles, patterns, and prospects for future studies. Chemoecology 22(3): 141–158.
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