Proposal (348) to South American Classification Committee
Incluir Pterocnemia dentro de Rhea
Antecedentes: El "Lesser Rhea" fue descripto en 1834 por d´Orbigny como Rhea pennata y así fue denominado por autores posteriores hasta 1871 cuando G. R. Gray crea el género Pterocnemia, basado principalmente en que la especie presenta la parte superior del tarso emplumado. Aparentemente, ese nombre no fue usado posteriormente hasta 1919 cuando R. Dabbene lo utiliza directamente para Pterocnemia tarapacensis sin aclaración alguna. Posteriormente, otros autores (Peters 1923, Wetmore 1926, Hudson 1927, Zotta 1935, Olrog 1959, y la mayoría de los autores modernos) usan este nombre.
Nueva información: Sibley y Ahlquist (1990), basados en su estudio de evolución molecular (DNA-DNA Hybridization), usa nuevamente el nombre Rhea pennata para esta especie. También otros autores o instituciones utilizan este nombre, por ejemplo: Animal Diversity Web del University of Michigan Museum of Zoology, Wikipedia, Rossi Fraire, H.J & Martella, M.B en British Poultry Science y ZipcodeZoo.com, etc.
Las especies en el campo son muy parecidas morfológicamente y en comportamiento y según parece, las diferencias entre ambos géneros, son sólo que Pterocnemia tiene la parte superior del tarso emplumado y tiene menor número de escutelaciones (restringidas a la parte inferior delantera del tarso).
Recomendaciones: teniendo en cuenta de que existe un análisis molecular que incluye Pterocnemia en Rhea y que las diferencias morfológicas que separan a ambos géneros son poco importantes (pensemos en otros géneros de aves como Tityra, Myrmeciza, etc. que las especies presentan áreas con o sin plumas), yo voto SI a esta propuesta.
Manuel Nores, May 2008
Comments from Cadena: "NO. I don't have a copy of Sibley and Ahlquist here, but since there is only one species of Rhea and one species of Pterocnemia, I do not understand how can their molecular analysis reveal that Pterocnemia is included in Rhea as described in the proposal. By definition, if you include two species of a single clade in a phylogeny as did Sibley and Ahlquist you recover two lineages, but since these two are sister to each other there is no reason to change the taxonomy. Manuel's points on the similarity of the two are well-taken, but as I said in the Pipromorpha-Mionectes proposal, unless traditional genera are not monophyletic, I think we should not modify classification at this level (and at higher ranks)."
Comments from Stiles: "A tentative YES. Although more recent sequence data would be desirable, I agree that the evidence for a generic distinction is not very convincing - more feathered tibia might be simply an adaptation to a cooler climate and tarsal scutellation is notoriously plastic in several other groups."
Comments from Remsen: "NO, but reluctantly. I agree with Manuel that the characters used to place these two species in separate genera are not very impressive, and I am struck by the overall superficial similarity between these two. The decision to use 1 or 2 genera in this case is largely arbitrary. Genetic distance might provide some guidance, but the problem is that as far as I can tell Sibley and Ahlquist (1990) did not sample Pterocnemia, and they started from the standpoint that Pterocnemia is included in Rhea without further discussion (that I can find). Sibley & Monroe (1990) implemented the merger without comment. Given that most other classifications, including Davies 2002 book on ratites and tinamous (Oxford's "Bird Families of the World" series), retain Pterocnemia, I would like to see additional analyses before adopting a merger. So many papers have been written on ratite classification that I suspect comparative data in terms of genetic and morphological distance are already in the literature, but I would like to see this synthesized before changing status quo. That said, I suspect that such a synthesis will show that a merger is warranted."
Comments from Pacheco: "YES. Creio que a proposição é perfeitamente pertinente. Há pelo menos uma evidência em favor publicada em:
Delsuc et al. (2007) Molecular evidence for hybridisation between the two living species of South American ratites: potential conservation implications. Conservation Genetics 8(2): 503-507.
Abstract: n a private wildlife research facility and rhea farm of Argentina, artificially incubated eggs of putative hybrid origin between the Greater Rhea (Rhea americana) and the Lesser Rhea (R. pennata) hatched and gave birth to healthy chicks. Molecular genotyping by the analysis of mitochondrial Cytochrome b (Cyt b) and nuclear Chromo-Helicase DNA binding (CHD1) gene sequences confirmed the hybrid origin of these chicks which were molecularly sexed as females. The possibility of hybridisation argues for careful management of captive populations of these species, especially if individuals are to be released in the wild for conservation purposes.
"I assume that the existence of hybrid among Greater Rhea and Darwin Rhea is a good indication of a closer relationship than admitted before."
Additional comments from Remsen: "On the basis of the paper and abstract that Fernando found, I change my vote to YES. Intergeneric hybrids are know, of course, in Trochilidae, Anatidae, and other families, and the instance above came from captive birds. But combine this with the absence of discrete morphological characters to distinguish them and I think burden-of-proof falls on maintaining separate genera. My subjective definition of 'genus', at least for non-passerines includes diagnosability at the skeletal/fossil level."
Comments from Stotz: "YES. While I tend to vote for the status quo in the absence of clear evidence to the contrary, I have to say that two monotypic Rhea genera seems like a taxonomic treatment that doesn't provide useful information. These are not very distinctive taxa, and there can't be any question that they are sister taxa. I can't see that there is any value to continuing to maintain two genera of Rheas."
Comments from Zimmer: "YES, for reasons perhaps best summarized by Doug."
Comments from Jaramillo: "YES - I think that separating these species into different genera is not all that informative. It feels a bit uncomfortable to merge these in the same genus because I have become so accustomed to "Pterocnemia" but concede that there is little reason for doing so. There may be stronger arguments for separating the two taxa of Pterocnemia as different species though, but with this question it would also be useful to see how separate the various lineages in Pterocnemia/Rhea are."
Comments from Schulenberg: "NO. That's a reluctant vote, since the differences between the two are slight. According to Blake 1977, the diagnosable differences simply are size and whether or not the tarsus is feathered are the tarsal scutellation. I'm not impressed by those differences in morphology, but who knows what a genetic perspective would reveal. Like Van, I have to wonder, are there no genetic data already available bearing on this question? Has anyone looked? Or have all the reams and reams of ratite relationship projects managed to overlook this central question?"