Wednesday, 14 January 2009

Poisonous Pitohuis

This is a first for me; I am blogging about a research paper I have recently read. One of the perks of working in the greatest library in the world, ever, is the freedom to access any book or journal I want or need to. I was browsing the latest Molecular Phylogenetics and Evolution, and the title of this paper leapt out at me: Phylogeny of the avian genus Pitohui and the evolution of toxicity in birds. Toxicity in birds? I had known that the pitohui and some other birds have been known to have poisonous skin, but wanted to know more.

Above illustration by Mo Hassan, January 2009

Top left:
Hooded pitohui
Pitohui dichrous Bonaparte, 1850
Pachycephalidae; Passeriformes; Aves; Chordata

Bottom left:
Variable pitohui
Pitohui kirhocephalus (Lesson & Garnot, 1827)
Pachycephalidae; Passeriformes; Aves; Chordata

Top right:
Crested pitohui
"Pitohui" cristatus (Salvadori, 1875)
Pachycephalidae; Passeriformes; Aves; Chordata

Middle right:
Rusty pitohui
"Pitohui" ferrugineus (Bonaparte, 1850)
Pachycephalidae; Passeriformes; Aves; Chordata

Bottom centre:
Blue-capped ifrita
Ifrita kowaldi (De Vis, 1890)
Pachycephalidae; Passeriformes; Aves; Chordata

Bottom right:
Little shrike-thrush
Colluricincla megarhyncha (Quoy & Gaimard, 1830)
Pachycephalidae; Passeriformes; Aves; Chordata

The paper (Dumbacher et al., 2008) begins by introducing the pitohuis and the family Pachycephalidae, jay-like birds from Indonesia and Australasia. The authors then mentions that the pitohuis were found to be poisonous, secreting batrachotoxins (the same compound that is found in the toxins of poison-dart frogs and the fugu) through their skin. The toxins affect the nervous system by disabling the pathways of nervous signals. The birds have this compound probably to protect them from both predators and external parasites.

It isn’t just the pitohuis that are toxic; the blue-capped ifrita* was also found to be poisonous, but was considered only distantly related to the pitohuis; both are members of the same group that includes such different-looking birds as crows, drongos and birds-of-paradise, the Corvoidea. The ifrita had already proved difficult to pigeonhole, as various taxonomic studies placed the bird with such disparate families as the babblers, quail-thrushes, logrunners or fairy-wrens. The question was raised whether the ifrita and the pitohuis evolved the ability to secrete batrachotoxins independently, or whether their common ancestor at the base of the Corvoidea had this ability. This hypothesis was tested in this study, as well as the idea that toxicity is coupled with bright coloration.

*See this post from Darren Naish’s Tetrapod Zoology for a bit of background on the ifrita.

The genus Pitohui has been considered monophyletic (of one origin) since at least 1881, and all but one of them have been found to possess the toxins. They were known for being the “leaders” in mixed-species flocks, choosing where to feed, whilst being followed by other birds of different types. The possibility of Batesian or Müllerian mimicry taking place in these flocks is considered; Batesian mimicry is the mimicry of a poisonous species by an innocuous one (as in the monarch and viceroy butterflies); Müllerian mimicry is the co-mimicry of two unrelated poisonous species (like Heliconius butterflies).

The authors mention previous studies on the Pachycephalidae, but they note that none of them have been thorough, sampling few species, and none of the members of monotypic genera (those with only one species), none of which were known to be toxic. The complication is exacerbated further by the fact that other related families, such as the monarch flycatchers, have been difficult to analyse using phylogenetics.

A bird called the greater melampitta (Melampitta gigantea) was also analysed in this study, to find its place in the Passeriformes. Other corvoid species that were analysed include taxa which act as an outgroup, helping the phylogenetic tree to take shape. What the taxa all have in common is that they have all been considered, at one time or another, to belong to the Pachycephalidae, or to be close to it.

The aims of the study are then mentioned: to identify the nature of the toxicity, whether it is basal or derived independently; to evaluate the evolution of bright coloration in link with toxicity; and to verify the monophyly of Pitohui to test for mimicry.

55 specimens from various museums around the world were analysed in this study. The authors used standard procedures used in molecular systematics to gather the DNA from the specimens, then sequence and align the genes. The necessary phylogenetic reconstruction was then carried out using specialised software, including such techniques as Bayesian analysis (a way of modelling the likelihood of the evolution of the genes). A single maximum-likelihood tree was produced as the output, which looks very interesting.

It is evident from this tree that the pitohuis are not monophyletic, but polyphyletic. Pitohui dichrous and P. kirhocephalus form a monophyletic clade, “P.” ferrugineus and “P.” incertus seem to belong with the shrike-thrushes (Colluricincla spp.), “P.” tenebrosus, endemic to the Pacific island of Palau, is nested very neatly within Pachycephalus, the whistlers, with “P.” nigrescens as another member of this group, and “P.” cristatus fits in at the outside of this large group. Other monotypic genera of the Pachycephalidae are outside this group, with only the expected outgroups outside this. The ifrita, by the way, pairs with the greater melampitta, close to the outgroup of the birds-of-paradise.

So, what to make of this? The discussion section of the paper starts by mentioning other generic names given to some of the members of the genus “Pitohui”. The authors then admit to the polyphyly of the genus, and that they will present the results of this elsewhere in the future. I predict that P. dichrous and P. kirhocephalus will stay as they are, but the other species will have to be placed in different genera.

It is no coincidence, then, that the two species of pitohui that do form a true clade are the most vividly coloured, and the most toxic. P. kirhocephalus, the variable pitohui, is so called because of the variety of plumage forms, some brighter than others; the analysis seemed to support the monophyly of the different subspecies of P. kirhocephalus, but the data may not have been strong enough, as some of the specimens were “ancient”.

The fact remains, however, that the other birds known as pitohuis, not to mention the ifrita, are also toxic; could any other birds in the group also be toxic? Of those birds, only one, the little shrike-thrush, Colluricincla megarhyncha, tested positive, but only slightly, and perhaps the other members of that genus. Natives of New Guinea were surveyed to further test this idea, but no additional toxic birds were identified. The authors then state that, from their studies, it is likely that toxicity evolved multiple times within the Corvoidea, and it is not a basal trait. It is also likely that the pitohuis are not affected by the toxins in their beetle prey; instead, they sequester it and modify it for their own use.

The similarity of P. dichrous and P. kirhocephalus to the monarch butterfly is noted, being an example of aposematic coloration, where colours and patterns serve as a warning to potential predators. This idea was mentioned in their introduction, but they go into some more detail in their discussion. It appears that there is no clear link between coloration and toxicity, as many of the duller birds are toxic, and many of the brightly coloured ones (male birds-of-paradise included) are not. Sexual selection is cited as another possible reason for the coloration, however, this fails for the same reasons as above.

The polyphyletic pitohuis have many common characteristics; firstly, most possess some degree of toxicity while their relatives do not; secondly, they participate in mixed-species flocks and are noisy and gregarious birds, their sister taxa notably being quite contrary in this way; thirdly, they outwardly appear to be morphologically similar, whilst apparently being unrelated. Convergence is apparent in not just toxicity then, but appearance, behaviour and ecology too!

The authors admit the speculations about Batesian or Müllerian mimicry need more testing, but that there is a definite presence of some sort of mimicry, otherwise unknown amongst birds.

They conclude by mentioning the status of the morningbird, “P.” tenebrosa from Palau, putting its remarkable external difference from the Pachycephalus genus it now belongs to down to increased rate of evolution from being on a remote island.

Dumbacher, J. P., K. Deiner, L. Thompson, & R. C. Fleischer (2008). Phylogeny of the avian genus Pitohui and the evolution of toxicity in birds. Molecular Phylogenetics and Evolution 49:774-81.


Raptor Lewis said...

Interesting post, Moe. I, actually, find that Taxonomy, like most scientific fields seem to have, sometimes more, and sometimes less, debates, than othe fields. The controversial issues that reflect the field of taxonomy is that they expand into the controversial issues of other fields such as the Bird-Dino Theory debates over the taxonomy of birds and dinosaurs. So, in my opinion, Moe, you've chosen an interesting field of study.

Poison Arrow Dart Frogs said...

Fascinating post! I had no idea birds had nervous system toxins like dart frogs do!

dart frogs for sale said...

I was unaware of how fascinating birds were. I didn't know they had the types of intricate systems that they do. Great field of study. I'm going to have to take the time to learn more about this!