Masters of chemical warfare
Toxicity has evolved independently across the tree of life many times, which isn’t surprising given how valuable toxins can be for hunting, defence and even parasite deterrence. While many are aware of toxic reptiles or amphibians such as the poison dart frog (Phyllobates terribilis), the existence of toxic birds is little-known. For the first time in over two decades, two more poisonous bird species have been identified, the regent whistler and the rufous-naped bellbird, in some of the Earth’s most pristine rainforests, New Guinea. Along with the poison dart frog, they do not produce toxins themselves, but instead acquire it from their diets. The question has been, how have these species avoided the negative effects of the toxin? Researchers have just published a fascinating new paper revealing how these birds managed to survive while being toxic, which I will explore here.
The toxin that the birds carry is called batrachotoxin (BTX). As I mentioned in the previous paragraph, these birds acquire BTX through their diet. It is thought that they might get the toxin by eating toxic Choresine beetles. Indeed, batrachotoxins are rare chemical compounds in nature, so the fact that the beetles might be the source is likely. Scientists have found evidence of Choresine beetles in a bird’s stomach, which could provide the evidence to support this theory. More research is needed in that field however as the exact source of BTX remains a mystery. BTX works by binding with the NAV channels, channels in our nerves involved in the electrical signalling to our muscles. These channels are kept permanently open by this chemical bonding between them and the BTX, preventing signals to be sent, paralysing the muscles, and leading, ultimately, to death.
BTX is present in bird feathers, which makes the birds toxic to predators as a consequence. Even we can feel the effects of BTX on our own organism. Kasun H. Bodawatta mentioned the effects of the toxin, while working with the birds: “It’s a bit like cutting onions, but with a nerve agent, I guess” How do birds themselves survive the effect of BTX? The researchers identified a mutation in the gene which codes for the NAV channels: SCN4A. This mutation changes the order of the amino acids in the pore channel, making it impossible for BTX to bind to the channel: The birds are immune to the toxin. They can therefore absorb it and deposit it in tissues without being affected.
We now know how toxic birds from New Guinea deal with the effects of BTX. But do poison dart frogs use the same “trick”? In other words, could the mutation be on the same gene as it is on the toxic birds? This is exactly what researchers from Denmark and Germany have been trying to find out. Their research has concluded that this trait is paraphyletic, meaning that the mutation is similar but not identical. Indeed, they found, after a DNA analysis, that despite the fact that the mutation in the poison dart frog and the birds has the same effect on the NAV channels, it does not occur in exactly the same place. This means that they have evolved through a process biologists refer to as ‘convergent evolution’. Both the toxic birds and the poison dart frog have evolved this feature independently; this is called an ‘analogous’ trait. The idea that this toxic superpower can be acquired through so little evolutionary change, using the molecular hardware that many vertebrates have already, is exciting. In fact, toxicity is by no means a unique and isolated feature. It is found in many places in the animal kingdom, perhaps even more are yet to be discovered. More research is needed! We still do not know how they actually deposit the poison in their feathers, for example. There is still so much more to learn about these fascinating birds…
