Proposal (565) to South American Classification Committee
Merge Mitu into Pauxi
Effect on SACC: This would merge the cracid genus Mitu into Pauxi.
Background: Our current footnote summarizes the situation:
22. Generic limits in the curassows are controversial. Vuilleumier (1965) merged Mitu, Pauxi, and even Nothocrax into Crax, and this was followed by Delacour & Amadon (1973). Vaurie (1967d), however, outlined rationale for maintaining the four genera as separate, and also pointed out that Nothocrax was a strong outlier in the group, a prediction subsequently verified by genetic data Pereira & Baker (2004). Most subsequent treatments have followed Vaurie (1967d). However, Frank-Hoeflich et al. (2007) presented morphological and genetic data to support the merger of Mitu into Pauxi. Proposal badly needed.
New information: Frank-Hoeflich et al. (2007) analyzed mtDNA sequence data and an impressive matrix of phenotypic characters to generate trees (one using parsimony, one Bayesian) based on combined analyses that indicated that Mitu + Pauxi together formed a strongly supported monophyletic group with respect to Crax, but that the two genera were intermingled and the two Pauxi species were not sisters. The same pattern is evident in their mtDNA (661 bp cyt-b) tree alone. In the tree based only on osteological characters, only one species of Pauxi and three Mitu were available; they formed a monophyletic group but within the group, there was virtually no resolution. Their tree based on external morphological and behavioral characters showed that Pauxi might form a monophyletic group but that it was inside Mitu. Based on these analyses, Frank-Hoeflich et al. (2007) proposed a new classification that merged Mitu into Pauxi.
Pereira & Baker (2004) also previously found the same basic pattern with a much larger data set of mtDNA (6900+ bps, 6 genetic regions).
Therefore, the data would seem overwhelmingly in favor of the merger – no data set confirms the monophyly of either genus.
Discussion: Pereira & Baker (2004) did not recommend the merger of Mitu and Pauxi because they considered the result probably due to former hybridization in the wild between Pauxi unicornis and its “sister” taxon in the phylogeny, M. tuberosum, which is also the species of Mitu that is parapatric with P. unicornis:
“Given the relatively long period since Mitu and Pauxi diverged (6.5 mya, on the basis of P. pauxi), incomplete lineage-sorting seems an unlikely explanation.
“Conversely, it is well known that cracids can hybridize very easily in captivity (Nogueira- Neto 1973, Pereira et al. 1996, Grau et al. 2003), though natural hybridization has not been reported in the wild (Vaurie 1968). However, the restricted geographic distribution of P. unicornis just south of the range of M. tuberosa is consistent with the hybridization hypothesis, because they are the most likely taxa to have come in contact. Sequence divergence between those taxa rules out the recent transfer of mtDNA genomes in captive birds, and indicates instead that an ancient transfer in the wild might have occurred. Because only one specimen of each taxon was sequenced, caution is warranted in making conclusions about the generality of that transfer of the mtDNA genome. Ultimately, the hybridization hypothesis needs to be checked with nuclear markers on bigger samples of P. unicornis from both disjunct populations before any recommendations can be made for revising generic limits.”
Although Frank-Hoeflich et al. (2007) cited Pereira & Baker (2004) for their support for their findings, they did not discuss Pereira & Baker’s interpretation. In part this is understandable because Pereira & Baker only had tissue of P. unicornis, whereas Frank-Hoeflich et al. (2007) had samples of both subspecies of P. pauxi, and these did not come out as sister to P. unicornis.
Nonetheless, I tentatively support the cautionary view of Pereira & Baker for the following reasons. First, it seems unlikely that the bizarre bony casque on the foreheads of the two Pauxi species would have arisen independently, especially since the two are also so similar in plumage that some authors have treated them as conspecific. In fact, Vaurie (1967) devoted a section to enumerating the subtle differences between the two species to defend their treatment as separate species. The two Pauxi (each with two subspecies) also are the only curassows that share a lower montane distribution – all four taxa are isolated in Andean foothills or outlying ranges, whereas Mitu are lowland tropical. These shared features are evidently overwhelmed or underweighted in the analysis of morphology in Frank-Hoeflich et al. (2007). Convergence or parallelism is not out of the question, of course, but I would like to see stronger genetic data to verify this.
This leads to my second concern, namely the underlying genetic data. Pereira & Baker themselves advocated a follow-up study that included nuclear loci. (See Carling and Brumfield  for an important lesson in potential problems in accepting results from mtDNA-based phylogenies when they seem to conflict with common sense: in this case, for North Americans, the mtDNA trees said Lazuli Bunting was sister to Blue Grosbeak, not to its parapatric ecological equivalent, Indigo Bunting, but nDNA supported the traditional view, i.e. Lazuli and Indigo are sisters). Also, all samples are single individuals from aviaries, none with vouchers. The authors in both studies took utmost care, of course, to insure proper identifications, so I’m probably being overly picky. But with hybridization among curassows evidently frequent in captivity, however, I worry about backcrosses. The same aviary that produced all the samples in these studies has already produced a bogus taxon that is certainly an aviary hybrid despite claims that the bird was trapped as a chick in Bolivia; see Joseph et al. (1999).
Recommendation: A tentative NO on this merger. I will change my recommendation and my vote if other’s comments persuade me that my view is too conservative.
Literature cited (if anyone needs pdfs, just let me know)
CARLING, M. D. & R. T. BRUMFIELD. 2008. Integrating phylogenetic and population genetic analyses of multiple loci to test species divergence hypotheses in Passerina buntings. Genetics 178: 363-377.
EO, S. H., O. R. P. BININDA-EMONDS, AND J. P. CARROLL. 2009. A phylogenetic supertree of the fowls (Galloanserae, Aves). Zoologica Scripta 38: 465-481.
FRANK-HOEFLICH, K., L. F. SILVEIRA, J. ESTUDILLO-LÓPEZ, A. M. GARCÍA-KOCH, L. ONGAY-LARIOS, AND D. PINERO. 2007. Increased taxon and character sampling reveals novel intergeneric relationships in the Cracidae (Aves: Galliformes). J. Zool. Syst. Evol. Res. 45: 242-254.
GRAU, E. T., S. L. PEREIRA, L. F. SILVEIRA, E. HÖFLING, AND A. WAJNTAL. 2005. Molecular phylogenetics and biogeography of Neotropical piping guans (Aves: Galliformes): Pipile Bonaparte, 1856 is synonym of Aburria Reichenbach, 1853. Molecular Phylogenetics & Evolution 35: 637-645.
JOSEPH, L., B. SLIKAS, K. RANKIN-BARANSKY, B. BAZARTSEREN, D. ALPERS, AND A. E. GILBERT. 1999. DNA evidence concerning the identities of Crax viridirostris Sclater, 1875, and C. estudilloi Allen, 1977. Ornitología Neotropical 10: 129-144.
PEREIRA, S. L., A. J. BAKER, AND A. WAJNTAL. 2002. Combined nuclear and mitochondrial DNA sequences resolve generic relationships within the Cracidae (Galliformes, Aves). Systematic Biology 51: 946-958.
VAURIE, C. 1967. Systematic notes on the bird family Cracidae. No. 10. The genera Mitu and Pauxi and the generic relationships of the Cracini. American Museum Novitates 2307: 1-20.
Van Remsen, November 2012
Comments from Nores: “NO. Although both genetic studies (Frank-Hoeflich et al. 2007, Eo et al. 2009) indicated that the two Pauxi species were not sisters, this requires corroboration. The two species are so similar that it seems unlikely, as indicated by Remsen, that the bizarre bony casque on the foreheads of the two Pauxi species would have arisen independently. In addition, I do not agree with sampling animals from aviaries, especially cracids, which hybridize regularly in captivity. For this reason I prefer to wait for new information before merging these genera.”
Comments from Robbins: “NO. For the same reasons that I stated in the Aburria proposal, I’m on the fence with this subjective decision. In the case of both, I’m fine with our current treatment if for no other reason than to just to have some stability and keep from flopping back and forth.”
Comments from Jacob Socolar: “In making these comments, I do not claim or pretend to be an expert on the matter. However, I have spent a good deal of time thinking about this puzzle ever since the Pereira and Frank-Hoeflich papers crossed my desk. In that spirit, I have two sets of observations to share that are not treated in the literature thus far:
1. A brown color morph is known in Pauxi pauxi, P. unicornis, and (recently) P. koepckeae, but not to my knowledge in any species currently treated as Mitu. If this is an ancestral character, then the suggestion that Pauxi is nested inside Mitu seems strange. But of course this is a single character compared to many morphological characters evaluated by Frank-Hoeflich.
2. There is (barely) enough online material now to evaluate similarities and differences in the vocalizations made by members of the currently recognized genera Mitu and Pauxi. Based on the recordings in the Macaulay Library and on Xeno-Canto (as well as written descriptions of the song of P. unicornis sensu stricto, the following observations:
--The song of Pauxi unicornis sensu stricto is an outlier with respect to other Pauxi (including its presumed sister P. koepckeae) in that it has the sharp terminal BMM! note typical of Mitu tuberosa. While on the one hand this could be construed as support for a sister relationship between P. unicornis and Mitu, on the other hand it makes the hybridization hypothesis especially plausible.
One thing is clear: If, based on the booming songs, it would appear that if P. unicornis is more closely affiliated to M. tuberosa than to P. pauxi, then P. koepckeae should fall out with P. pauxi and NOT with P. unicornis as suggested by Weske and Terborgh (1971) based on morphological characters in their description of the taxon koepckeae.
--The 'barking' calls of P. pauxi, P. koepckeae, and P. unicornis are available online from the Macaulay Library, Xeno-Canto, and the Macaulay Library, respectively. These vocalizations are similar to one another, and (unless I overlooked something) there is no example in any of these collections of a Pauxi producing the whistling calls typical of Mitu tuberosa (and, I gather, other Mitu spp.)
However, Birds of Peru describes a 'sharp popping pseet' for P. unicornis, and there are a few examples of Mitu tuberosa and Mitu tomentosa producing barking calls similar to Pauxi on XC and ML.
In summary, I think that the vocal evidence, while utterly inconclusive, lends weight to the hybridization hypothesis and the maintenance of separate genera:
1. The similarity in booming songs of M. tuberosa and P. unicornis sensu stricto lends additional plausibility to the hybridization hypothesis.
2. P. koepckeae has a booming song similar to P. pauxi (and not Mitu), but has morphological characters more closely resembling P. unicornis (at least according to Weske and Terborgh). It is hard to believe that P. koepckeae and P. pauxi are not reciprocally monophyletic with respect to Mitu based on vocal characters. It is hard to believe that P. koepckeae and P. unicornis are not reciprocally monophyletic with respect to Mitu based on morphological characters.
3. Pauxi as a group appears to be distinctive in that the commonly given call may be (based on limited sample size, for sure) barks rather than whistles. But Mitu can each give both barks and whistles, and perhaps Pauxi can give the occasional whistle.
Comments from Stiles: NO, at least until we have nuclear data from an adequate sample of known-locality, well-identified birds. There is just enough possibility of hybridization to justify a conservative stance here, which is also in accord with the ecological-biogeographical information, as per Van.”
Comments from Pacheco: “NO. Given the concerns raised by colleagues, I prefer to be conservative and keep for now the two separate genera.”
Comments from Zimmer: “NO, for reasons outlined by Van, Manuel and Mark. I share concerns about the depth of the sampling and the use of aviary birds, and Van’s comments regarding the montane distribution of Pauxi also give pause. Like Mark, I would just as soon wait for more conclusive evidence before making a change that we could end up changing again down the line.”
Comments from Pérez-Emán: “NO. I think molecular evidence is not that strong yet to make such changes in our current taxonomic treatment of these genera. Morphology, vocalizations and distribution all argue in favor of retaining both genera. It is worth to notice that phylogenetic hypotheses resulting from Pereira & Baker (2004) and Frank-Hoeflich et al. (2007)’s studies are not congruent. Whereas in the first study Pauxi pauxi was sister to all Mitu + P. unicornis, in the second study there was support for a polytomy consisting of Mitu mitu, the two subspecies of P. pauxi and a monophyletic group including the rest of Mitu species + P. unicornis. This points toward the need of including nuclear characters to evaluate phylogenetic relationships of these species. The hybridization hypothesis cannot be discarded for reasons indicated by Pereira & Baker (2004) and Jacob Socolar, mainly their potential parapatric distribution (currently or in the past) and their similar vocalizations. Thus, before we have more conclusive evidence, I would rather keep both genera.”