Poison Rat: Did It Evolve?

The African crested rat has a unique way of deterring predators.  It chews the bark from a poisonous tree (the same one native hunters use to poison their darts), and licks it onto its fur.  Any predator that tries to eat the rat becomes very sick, and quickly learns to keep its distance.  This kind of defense has been seen in other animals, but is the first known case of a mammal using a substance from another organism to make itself toxic to predators. Is it a classic case of evolution?

Science Daily reported the find.  Jonathan Kingdon and colleagues studied how this rat, known to be poisonous, acquired its defense.  Several things need to be explained.  One is how the rat became immune to the toxin.  Another is the behavior: i.e., how the rat learned to find the poison and place it on its back.  A third is how the hairs on its back achieved a unique structure, full of pores that absorb and store the poison.  According to New Scientist, each of these aspects are hardwired into the rat’s DNA.

The “don’t eat me” defense is known elsewhere in the animal kingdom.  Monarch butterflies are able to eat milkweed, toxic to other animals, and store the poison in their wings to deter birds.  Some nudibranchs (a kind of marine mollusc) are able to digest tiny poison-dart structures called nematocysts and transfer them, unexploded, onto their backs for protection.  Poison-dart frogs ingest toxins from arthropods they eat.  Now this defense mechanism has been found in a mammal.

Kingdon remarked, “This is an extraordinary thing to have evolved.”  In the New Scientist article, he added, “Evolution has mimicked something that hunters do.  It [the crested rat] is borrowing from the plant just as the hunters are borrowing from the very same plant.”  According to a co-author of the study quoted by Science Daily, “The African crested rat is a fascinating example of how a species can evolve a unique set of defenses in response to pressure from predators.” 

As for how this rat can ingest the poison without dropping dead, Kingdon said, “We don’t have the slightest idea how that could be done.”  He and his team is hoping that insights from this toxin might lead to health benefits, much like digitoxin (a related chemical) has been used for decades in the treatment of heart failure.

Variability is not the same thing as evolution as Darwin meant it (molecules to man).  There is not really any innovation here; if anything, there is loss of function that happened to have some benefits in a particular environment.  The rat already had fur; the tree already had the chemical.  In a forest with predation, it is not unexpected that rats would try ingesting all kinds of plant substances in their habitat.  Loss of sensitivity to toxin could be due to loss of function – malfunction of a receptor that normally would react to the substance.  (By analogy, a man with loss of hands becomes immune to handcuffs.)  Rats unaffected by the toxin would proliferate over those affected.  Or maybe the rats were immune already.  Different animals respond differently to chemicals; deer can browse on poison ivy, for instance, while humans suffer dermatitis from it.

Once you have rats immune to the toxin, put them under the pressure of predation.  Those that lick their fur after eating the bark would deter predators more effectively than those that did not, and would proliferate.  So would those with fur that absorbs and stores the chemical more effectively.  It is not clear from the articles whether the scientists established that the behavior is hardwired rather than learned and passed on from parent to offspring, like bird song.  Either way, these changes fit into the kind of horizontal variation accepted by creationists as designed adaptive capacities for survival in changing environments.  It does not represent upward innovation of new information-rich structures. 

Contrary to what some evolutionists think, creationists do not believe in fixity of species.  They believe the Creator has designed a lot of variability into plants and animals.  Think of the variation in dog varieties – yet they are all one species, Canis familiaris.  Let these rats evolve a new organ de novo, and we might be impressed.  If this is all the African crested rat can come up with after tens of millions of years in the evolutionary rat race, it does not qualify for a Darwin award.