Proposal (587) to South American Classification Committee
Split Gymnopithys leucaspis into two species
Effect on SACC: This proposal, if passed,
would split Gymnopithys leucaspis
into two species, a trans-Andean G.
bicolor and a cis-Andean G. leucaspis.
Background: Gymnopithys bicolor is a humid lowland forest antbird distributed
both east and west of the Andes. Nine subspecies are currently recognized: five
in Central America and western Colombia/Ecuador (the bicolor group, hereafter bicolor)
and four in the northwest Amazon basin (the leucaspis
group, hereafter leucaspis). The
status quo classification followed by SACC (tagged as “proposal badly needed”)
lumps bicolor and leucaspis following the rationale
outlined by Zimmer (1937a). However, various authors have followed the
alternate treatment of splitting bicolor and
leucaspis as separate species (Willis
1967, Hilty & Brown 1986, Sibley & Monroe 1990). There is now
sufficient data describing patterns of genetic, vocal, and plumage variation
within this complex for SACC to vote on these alternatives.
Genetic data: Hackett (1993) found significant (~5%) allozyme
divergence between trans-Andean bicolor and
Amazonian leucaspis, though refrained
from making a taxonomic recommendation and suggested analyzing these
populations with more sensitive molecular markers. More recently, Brumfield et
al. (2007) included single samples of both leucaspis
and bicolor (as well as single
samples of the other three recognized Gymnopithys
species) in a broader phylogeny of ant-following antbirds. This study used
both mitochondrial (cyt b, ND2, ND3) and nuclear (f5) markers, and found
strong support for a sister relationship between leucaspis and G. rufigula,
a congeneric species with an allopatric Amazonian distribution (G. rufigula is a Guianan Shield species,
leucaspis present in northwest
Amazonia), with trans-Andean bicolor sister
to the combined leucaspis and G. rufigula group (see snapshot of their
tree below).
The relevant portion of the maximum-likelihood tree
presented by Brumfield et al (2007). Bayesian (before slash) and bootstrap
(after slash) support values are given. This tree was inferred from the concatenated
data matrix of three mitochondrial and one nuclear gene sequences.
Vocal data:
Statistical differences in antbird vocalizations have been used to justify
splitting of antbird species. However, there is no published analysis of
vocalizations within G. leucaspis (as
currently defined, including bicolor).
The species account in Handbook of Birds of the World (Zimmer and Isler 2003) provides a detailed verbal description of leucaspis and bicolor loudsongs, stating that bicolor
song “starts with long, upslurred whistles that shorten rapidly and gain in
intensity, followed by shorter notes that drop in pitch and intensity before
becoming harsh” while that of leucaspis “begins
with upslurred whistles at an even pitch that shorten into rather abrupt notes
dropping in frequency and intensity, then lengthen and increase again in
intensity, finally decreasing in intensity and becoming harsh.” Additionally,
the loudsong of bicolor is reported
to be ten notes, compared to 20 for leucaspis,
though the HBW account (Zimmer and Isler 2003)
also notes that loudsongs are “quite variable in length”.
Plumage data: For antbirds, plumage is
rather divergent within Gymnopithys: G. rufigula is entirely brown with
patches of cinnamon, and G. salvini and
lunulata are sexually dichromatic,
with gray males and brown females. In contrast, plumage variation in bicolor and leucaspis is relatively slight, with subspecific plumage variation
in head/side coloration and overall darkness. Nevertheless, there appears to be
diagnostic plumage differences between these two groups: the bicolor group has two plumage traits – a
black subocular area and blue-gray plumage behind the eye – that the leucaspis group lacks.
Taxonomic possibilities:
There are two possible treatments at this time.
1. Maintain the status quo, leaving all taxa within both bicolor and leucaspis groups in a broadly defined G. leucaspis.
2. Split
G. bicolor from G. leucaspis.
Recommendation: I suggest that current evidence supports splitting bicolor from leucaspis. The strongest data supporting this split is Brumfield et
al.’s (2007) finding that the Amazonian leucaspis is sister to Amazonian G. rufigula and not trans-Andean bicolor. As the species status of G. rufigula has not been questioned,
these genetic relationships strongly argue bicolor
and leucaspis should be treated
as different species.
Vocal and plumage data supporting this split are less
conclusive. Loudsongs may differ (Zimmer and Isler 2003), but have not yet been
subjected to quantitative analysis or behavioral playback experiments. Plumage
is likewise similar between bicolor and
leucaspis, though there are
diagnostic differences in multiple plumage patches, providing weak support for
the proposed split.
In sum, genetic divergence and the sister relationship of leucaspis with G. rufigula support splitting bicolor
from leucaspis. This split is
weakly supported by vocal and plumage divergence. This proposed treatment is
also consistent with the commonly found pattern of divergence between cis- and
trans-Andean populations of widely distributed lowland forest taxa.
Vernacular
Names:
If passed, this proposal would require new English names for bicolor and leucaspis. Ridgeley and Greenfield (2001) suggested “White-cheeked
Antbird” for leucaspis and retaining
“Bicolored Antbird” for bicolor. This
treatment emphasizes the most prominent plumage difference between the two
taxa, the white “cheek” of leucaspis.
These English names therefore seem appropriate, though the committee could also
consider alternatives.
Literature Cited:
Other papers are cited in
the SACC bibliography
Ben Freeman, September 2013
______________________________________________________________________________________________
Comments
from Stiles: “YES,
although a more comprehensive analysis of vocalizations, including use of
playbacks, would be nice. Because both species are common, vocal birds, surely
enough recordings exist in Cornell and Xenocanto to perform such an analysis.
However, the genetic data seem convincing and the slight but diagnostic plumage
differences seem on a par with several other recent splits in the
Thamnophilidae.”
Comments
from Pacheco: “YES.
O somatório de evidências, sobretudo
o mais recente estudo de Brumfield et al.
(2007), dão suporte apropriado à adoção da proposta aqui apresentada.”
Comments from Zimmer: “YES. I think that the only reason that the
proposed split “is weakly supported by vocal and plumage divergence” is that
there has been no quantitative vocal analysis of the two taxa. I suspect that such an analysis would show
that loudsongs of leucaspis and bicolor differ at least as does either
of them from rufigula. Vocal differentiation between allospecies in
the obligate ant-following genera (particularly within this species-group of Gymnopithys and the entire genus of Rhegmatorhina) appears to be pretty
conservative from an evolutionary standpoint – perhaps there is some conferred
advantage in signal recognition between different species that are all in the
mode of constantly searching for the same moving target (ant swarms), and
having to coexist at swarms while competing for prey. At any rate, the genetic data revealing the
sister relationship between leucaspis
and rufigula pretty much dictates
either splitting off bicolor, or
lumping all three groups (rufigula, leucaspis, bicolor) into one very bizarre (given the striking plumage
differences between rufigula and the
other two) species, something that I don’t think anyone is advocating.”
Comments
from Jaramillo: “YES – leucaspis sister to rufigula I find odd and disconcerting although vocally these three
forms are very similar. Still, the genetic separation between leucaspis and bicolor is strong, and although it would be great to have a more
formal analysis of voice, what is available suggests diagnosable differences in
voice, although slight. But rather than lumping rufigula into this species, I think the option of separating bicolor and leucaspis is the better one.”
Comments
from Robbins: “YES, for
splitting Gymnopithys leucaspis into
two species, as genetic data show that Amazonian leucaspis is more similar to the morphologically distinct G. rufigula than leucaspis is to trans-Andean bicolor.”
Comments
from Pérez-Emán: “A
tentative YES based on Brumfield et al. (2007) data, as the alternative
possibility to lump rufigula, bicolor and leucaspis is not in agreement with plumage differences between rufigula and bicolor/leucaspis.”
Comments
solicited from Mort Isler:
“Although I
have no new vocal analysis to add, I agree with Kevin Zimmer's remarks in his
response to Proposal 587 that it is highly likely that quantitative vocal
analysis of Gymnopithys taxa will
show that ‘loudsongs of leucaspis and
bicolor differ at least as does
either of them from rufigula.’ In
this case, for reasons described in the proposal, it seems reasonable to make
the split without thorough vocal analysis.”
Comments
from Remsen: “YES, with my thoughts exactly as stated by
Jorge.
“As for
English names, there will be those who go ballistic that we don’t come up with
new names for both daughter species. As
much as I appreciate that logic and urge that we do so in most cases, there are
extenuating circumstances. First, in
this case, the two are not sister species – that’s the rationale for the split
– so any sort of compound name is completely out unless rufigula also gets one, which would concoct and create a novel name
for a long-established one. In this
case, “White-cheeked Antbird” has a long history, going back to at least
Ridgely’s (1976) Panama book and used by Sibley & Monroe (1990), so we
already have a good name with a historical track record for leucaspis.”