a poisonous surprise

Oxford Zoologist Fritz Vollrath is famous for his work on spiders, but sometimes his team also gets involved in finding out amazing things about larger animals, such as rats storing poison in their fur. They have a paper out today, here's an excerpt from the press info:

For many years mystery has surrounded the sudden collapse, sometimes followed by death, usually at night, of dogs in north-eastern Africa. Typically, if the dogs do not die at once, they lose co-ordination, froth at the mouth and show symptoms of acute distress. In some rural areas elders or hunters might identify a wooly grey rodent as the culprit and warn that the rat is extremely poisonous. New research has confirmed that warning.

The Crested Rat, Lophiomys imhausi, 40-50 cm. long, looks quite innocuous as it clambers about in rocky, wooded valleys in Kenya and the Horn of Africa. If disturbed the long fur on its flanks parts to expose a vivid black and white pattern around a leaf-shaped tract of peculiarly specialized hair. Just how specialized the hair is and how well designed each hair is to hold and dispense poison has been revealed by a group of East African and Oxford scientists.

The group have identified the toxin, which comes from Poison-arrow trees of the genus Acokanthera, long celebrated for an extract from their bark and roots which, when applied to an arrow-head, can fell an elephant.

The scientists have observed the rat gnaw Poison-arrow tree bark directly from the plant, chew it and then deliberately slather the resulting colloid onto the flank hairs. These hairs are designed to rapidly absorb the colloid, acting like a lamp wick. The poison,Ouabain, is well-known as a heart-stopping cardiac glycoside and for centuries doctors have used minute doses of Ouabain to stimulate weak hearts.

The Crested Rat is the only mammal known to sequester plant toxins in this way. When more is known about the chemistry and genetics of the Crested Rat’s immunity to and use of Ouabain there may well be applications for human medical therapies.

In 1974 the senior author, East African Jonathan Kingdon, first advertised the mystery of a poisonous rat and invited further study. It has taken nearly 40 years for the mystery to begin to be solved.

A poisonous surprise under the coat of the African Crested Rat.
Jonathan Kingdon, Bernard Agwanda, Margaret Kinnaird, Timothy O’Brien, Christopher Holland, Thomas Gheysens, Maxime Boulet-Audet and Fritz Vollrath

Proc. R. Soc. B published online before print August 3, 2011, doi:10.1098/rspb.2011.1169

full text (open access)