Some exceptions in biology: Poisonous birds

Credit: snakebuddies.net
The drawing above is a rather accurate representation of a saying that I read a few months ago:
“If you bite it and you die, it’s poisonous. If it bites you and you die, it’s venomous.”
-Credit: @SciencePorn Feb 4, 2014
This saying essentially illustrates the practical difference between toxins and venoms.  Definition-wise, I wrote the following in a previous post:
“… what is the difference between toxins and venoms? Well, toxins are usually referred to as single compounds with a defined molecular target (generally a receptor or any other type of protein). In many instances these compounds are passively secreted (very much like perspiration) and require the “cooperation” of the target. For example, a tobacco plant do not chases after the insect that it wants to avoid. Rather, the toxin is actively ingested by the insect upon trying to feed on the plant.
Venoms, on the other hand, are more or less complex cocktails containing several different types of toxins, each of them with potentially different targets. Additionally, venoms are produced in specialized structures called venom sacs (duh!) and are actively injected to the target animal via specialized structures like fangs, stings, the works.”
When one thinks about poisonous organisms we usually start by thinking about plants, from poison ivy to curare.  We also think about certain mushrooms and more recently, about vertebrates like poison arrow frogs and pufferfish.  We usually (and logically) associate toxins with “passive” defense mechanisms because the organisms involved are weak, slow or not motile at all.  Fair enough. Based on that logic, you would not expect fast or very strong animals to display toxicity right?

“                                                               Done at: memegenerator.net
Yes, there are, and we have known about them for 20-odd years.  In 1992 a group lead by Dr. J.P. Dumbacher, currently at the California Academy of Sciences, studied a group of birds from New Guinea, belonging to the Pitohui genus.  It even made the cover of Science magazine! These are rather pretty, colorful birds.

                                                                    Credit: science magazine
Dumbacher and his collaborators made some curious observations, namely that handling of the specimens caused “numbness, burning, and sneezing on contact with human buccal and nasal tissues”. Dumbacher noted this when upon trapping pitohui, a bird scratched and bit him.  No big deal.  Sometimes when a person gets a small finger cut, he or she licks it. That’s what Dumbacher did and that’s when he noticed the numbness on his lips. In fact, that these types of birds seemed to have toxic properties was known since the 1940s, but no one did much about it until Dumbacher and friends.
You never know what will take you to a scientific discovery.
Eventually, they determined that the chemical responsible for that was a steroid alkaloid called homobatrachotoxin.  This toxin is mainly present in the skin and feather of the birds, with lesser amounts found in the internal organs and muscles.  This molecule interferes with several types of ion channels on the surface of neurons and it also has cardiotoxic activity.

                                                         Credit: www.science-frontiers.com
That’s quite an exciting discovery because to date, the Pitohui genus is the only bird genus that includes poisonous birds.  However the discovery of the exact nature of the toxin deepened the mystery, as homobatrachotoxin was previously found in only another type of organisms, some poison arrow frogs, particularly Phyllobates terribilis.  This species of frogs is found in South America, at the other en of the world, some 11,000 miles away!

                                                             Credit: www.amphibiancare.com/
The most likely scenario that explains how identical toxins found their way to these two animal species separated by thousand of miles and most of an ocean is that the toxin is of environmental origin.  This idea is supported by the fact that poison arrow frogs kept as pets tend to lose their toxicity and frogs born in captivity lack the toxin.  I am not aware of any similar studies with pitouhis but it is a reasonable assumption.
In a 2004 paper, Dr. Dumbacher and collaborators provided evidence of a possible culprit, a beetle species (Choresine genus, shown below).  Homobatrachotoxin, related molecules and even some other weird alkakloids are found in these critters in relatively large quantities. These Choresine beetles most certainly composed part of the diet of pitouhis and similar species are consumed by poison arrow frogs in South America.  Thus, they are strong candidates for the source of toxicity for both the pitouhis and the poison arrow frogs.

Credit: J.P. Dumbacher
 
 
 
 
Nature is resourceful; furthermore, it does not care about how do we humans wish to classify, describe, interpret or think about a particular aspect of it.  We can make all the rules we want, and nature will happily break them.  Perhaps the “rule that rules over all” would be:
“For each biological rule, there will be exceptions”.
Fascinating huh?
If you want to know more:
http://beautyofbirds.com/pitohui.html
http://research.calacademy.org/om/staff/jdumbacher
Dumbacher JP, Spande TF, Daly JW. Batrachotoxin alkaloids from passerine birds: a second toxic bird genus (Ifrita kowaldi) from New Guinea. Proc Natl Acad Sci U S A. 2000 Nov 21;97(24):12970-5.
Dumbacher JP, Beehler BM, Spande TF, Garraffo HM, Daly JW. Homobatrachotoxin in the genus Pitohui: chemical defense in birds? Science. 1992 Oct 30;258(5083):799-801.
Dumbacher JP, Wako A, Derrickson SR, Samuelson A, Spande TF, Daly JW. Melyrid beetles (Choresine): a putative source for the batrachotoxin alkaloids found in poison-dart frogs and toxic passerine birds. Proc Natl Acad Sci U S A. 2004 Nov 9;101(45):15857-60.
Dumbacher JP, Deiner K, Thompson L, Fleischer RC. Phylogeny of the avian genus Pitohui and the evolution of toxicity in birds. Mol Phylogenet Evol. 2008 Dec;49(3):774-81.
Weldon PJ. Avian chemical defense: toxic birds not of a feather. Proc Natl Acad Sci U S A. 2000 Nov 21;97(24):12948-9.
Daly J, Witkop B. Batrachotoxin, an extremely active cardio- and neurotoxin from the Colombian arrow poison frog Phyllobates aurotaenia. Clin Toxicol 1971 4(3) 331-42.
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