They’re brightly coloured, beady-eyed and thanks to some unique genetic mutations, carriers of a deadly neurotoxin.
Two bird species – the regent whistler (Pachycephala schlegelii) and rufous-naped bellbird (Aleadryas rufinucha) – were first described around 150 years ago, however now they’ve had a status update: both possess a highly potent neurotoxin.
The findings by a team from the University of Copenhagen exploring rainforests in Papua New Guinea mark the first discovery of toxic birds in two decades.
Both the regent whistler and rufous-naped bellbird are common songbirds in New Guinea, but before the Copenhagen study were not considered candidates for possessing potent poisons.
The discovery of toxic birds is itself recent. Thirty years ago, members of the genus Pitohuis – also located in Papua New Guinea – were the first bird species documented as poisonous.
Birds of a feather, shock together
Regent whistlers, rufous-naped bellbirds and select Pitohuis cousins have one of the world’s most potent, naturally occurring neurotoxins in their feathers.
They likely obtain these alkaloids in their diet by eating Melyrid beetles, which contain high batrachotoxin concentrations.
Ingesting these toxins eventually manifests in the plumage of these bird species, which are considered poor-eating by indigenous populations in New Guinea due to the burning and stinging sensations experienced when handling them.
It’s the same class of toxin found in the skin of poison dart frogs in South America. These frogs secrete the poison from their brightly patterned skin, which serves as a warning indicator to potential predators. Scientists believe that, like the New Guinean birds, these frogs obtain their toxic skin by feasting on related Melyrid beetle species.
The study’s co-lead researcher, Kasun Bodawatta, had to explain the potent effect to his colleague when obtaining feather samples on location.
“[They] thought I was sad and having a rough time on the trip when they found me with a runny nose and tears in my eyes,” Bodawatta says.
“In fact, I was just sitting there taking feather samples from a Pitohui.
“Removing birds from the net isn’t bad, but when samples need to be taken in a confined environment, you can feel something in your eyes and nose. It’s a bit like cutting onions – but with a nerve agent.”
If birds eat the toxins, why aren’t they dead?
Batrachotoxins attack the body almost at once. An animal – or human – exposed to the poison of South American dart frogs will experience muscle spasms, cramps and cardiac arrest.
Bodawatta, fortunately, is able to handle toxic birds like Pitohui as the neurotoxins in their feathers occur at much lower levels than South American frogs.
But if these animals are eating highly toxic beetles, why aren’t they dying?
The answer is buried in their DNA.
The catastrophic symptoms experienced by toxin exposure by other animals is resisted by these birds and frogs because of mutations in the genes that code for functional sodium channel proteins in their bodies.
Sodium channels function as gateways, helping regulate the flow of sodium ions into key cells. But when batrachotoxins bind to these channels in most species’ muscle cells, they fail to close.
“By forcing sodium channels in skeletal muscle tissue to remain open, [batrachotoxins] can cause violent convulsions and ultimately death,” says Bodawatta.
Being able to survive the toxin would therefore require changes in an animal’s protein-coding genes. Fortunately, the two birds appear to have comparable adaptations to the frogs: an example of convergent evolution where totally different species develop separate, but similar beneficial anatomic variations.
Both the birds and frogs have mutations to the SCN4A gene which codes for the NAV1.4 sodium channel. But while the variations are different, they appear to confer the same advantage.
“It was natural to investigate whether the birds had mutations in the same genes [as the frogs],” says Bodawatta.
“Interestingly enough, the answer is yes and no. The birds have mutations in the area that regulates sodium channels, and which we expect gives them this ability to tolerate the toxin, but not in the exact same places as the frogs.”
“Finding these mutations that can reduce the binding affinity of Batrathotoxin, in poisonous birds in similar places as in poison dart frogs, is quite cool, and it showed that in order to adapt to this Batrachotoxin lifestyle, you need some sort of adaptation in these sodium channels.”