Proposal (155) to South American Classification Committee
Split Shy Albatross Thalassarche cauta into two or three species
Effect on South American CL: This proposal would split our Thalassarche cauta into two or three species, with possible recognition of T. cauta, T. salvini, and T. eremita as separate species. All three have been documented for our region, cauta on the Falklands (southern Argentina also??), T. salvini in Peru, Chile and Argentina, and T. eremita in Peru and Chile. I do not recall if there are well documented records of cauta from Peru, but it has been reported at least. I have not uncovered a good record of cauta from Chile yet.
Background: Nunn et al. (1996) suggested a reorganization of albatross genera based on morphology and mtDNA. This has been accepted by this committee and the AOU committee. More contentious has been the proposal to split the albatrosses into as many as 24 species based partly on unpublished mtDNA data and a rather extreme PSC based definition of species (Robertson and Nunn 1998). Apparently New Zealand biologists have quickly adopted these splits, but they have been controversial enough elsewhere that they have not been taken seriously by many. There may have been an issue of throwing the baby out with the bathwater, in addition to a dearth of published data. Fortunately, or unfortunately depending on how you view procellariiform systematics, more is coming out in print recently that is beginning to clarify some aspects, and muddy others.
One of the suggestions made by Robertson and Nunn (1998) is that the four subspecies of the polytypic Shy Albatross (Thalassarche cauta) be split into four species, named Shy (Thalassarche cauta), White-capped (T. steadi), Salvin's (T. salvini), and Chatham (T. eremita) albatrosses.
Nunn and Stanley (1998) in a paper on variation in rate of molecular evolution related to body size (metabolic rate effect) sampled the complete cytochrome b sequence from 85 species of tubenoses. They found that larger taxa, such as albatrosses, have shorter terminal branches in their phylogeny than smaller taxa, suggesting a slower rate of evolution in mitochondrial DNA in larger taxa. Most important for this analysis is that of the three taxa sampled from this complex (eremita, salvini, cauta), their phylogeny shows eremita and salvini to be sisters and these in turn are sister to cauta. Thus, eremita and salvini are more closely related to each other than either is to cauta. Plumage details (dark hood, dark-tipped bill, yellowish adult bill colour, black undersides to primaries, darker underwing of immatures, etc.) also unite eremita and salvini.
Using the same dataset Penhallurick and Wink (2004) find a difference of 1.05% between cauta and eremita; 0.96% between cauta and salvini; and 0.26% between eremita and salvini. They choose to maintain the complex lumped since they consider that 1.05% is well below the percent difference of traditionally recognized species in Thalassarche (1.66% - 3.55%). However, there is circularity in this reasoning, as the taxonomic questions that are troubling are these that involve taxa that are allopatric and have not traditionally been considered good species.
Abbott and Double (2003a). Their work focused on assessing the species status of cauta versus steadi, rather than the morphologically more divergent salvini and eremita. They sampled a 3000 bp fragment of mtDNA containing the control region. They found the following:
- 1.8% average pairwise sequence divergence between cauta and steadi.
- 2.9% between salvini and eremita.
- 7.0% between the salvini/eremita group and the cauta/steadi group.
- Monophyly of cauta/steadi group upheld, but not of the salvini/eremita group. The latter may be paraphyletic, or more data is needed to confirm monophyly.
Double et al. (2003) compared the morphology of cauta and steadi, which are extremely similar taxa as adults, and impossible to separate as immature specimens. They discovered that several measurements differed significantly, but overlapped a great deal. A multivariate approach identified 84% of specimens previously identified with a DNA based test.
Abbott and Double (2003b) compared data from six variable microsatellite loci, to study genetic structure in cauta and steadi. They found high levels of genetic structuring and detected many unshared alleles between the taxa, providing strong evidence against any contemporary gene flow between them. Different populations of cauta were found to be genetically distinct, whereas populations of steadi were not, implying that dispersal is rarer in the former than the latter. There is lower genetic diversity within cauta than steadi.
Analysis: Some general notes on breeding range and times are needed here. The two white headed taxa, as adults, are cauta and steadi. This pair is the most similar morphologically, and genetically, of any pairwise comparison of four taxa in question. The form cauta breeds on islands off Tasmania (Albatross, Mewstone and Pedra Blanca). The form steadi breeds off islands south of New Zealand, three of the Auckland Island group, and in the Antipodes group. These two white-headed taxa tend to head west from after breeding is done, with many wintering off South Africa and in the southern Atlantic as well as the Australia-New Zealand region. The form salvini breeds on Crozet (4 pairs), Snares, and Bounties (main breeding islands), the latter two south of New Zealand. It should be pointed out that the few nesting on the Crozets (Indian Ocean) are highly unusual and illustrate the great potential for long distance colonization of these albatrosses; the fact that the four different taxa in this complex breed in a relatively small part of the world (New Zealand/Australia) is worthy of note. The outpost on the Crozets is more than twice as distant from the regular breeding islands of salvini; this is a greater distance than the regular breeding range of the entire complex! A great proportion of the population of salvini heads to the Humboldt Current during the non-breeding period, although some also head west to South Africa and the southern Atlantic. Finally, eremita breeds on pyramid Rock, Chatham group (New Zealand sector again), and many head to the Humboldt Current in the non-breeding season; no westward movement is known for this taxon.
Breeding times differ somewhat between taxa
- cauta begins breeding in early
- steadi begins breeding in November.
- salvini breeds Oct-mid Nov.
- eremita breeds Aug _ Sept.
Potential sympatry: One pair of steadi has bred on the Forty-Fours (Chatham Island group); this island is only a few km. from the Pyramid, which is the breeding site of practically all eremita. A steadi has visited colonies of salvini on the Snares. A single salvini made a nest in the main colony of eremita; I think it was by itself, with no evidence of hybridization. One pair of eremita has nested on the Snares, a breeding site for salvini. Some of these sympatric breeding events have occurred for various years in a row. There is no evidence of hybridization in any of these events, although hybridization in albatrosses is not unknown (Laysan and Black-footed interbreed regularly on Midway Island, Hawaii) (Tickell 2000).
Recommendation: There are two clear groups (I do think that eremita and salvini are sisters, and form a monophyletic group), one is the eremita/salvini pair, and the other is the cauta/steadi pair, these two groups are consistently different in morphology, genetics, breeding, and at-sea distribution, and there is no sign of hybridization although at least one pair of steadi has bred practically within sight of the main colony of eremita. It is one data point, but a data point nonetheless.
The two white headed taxa (steadi and cauta) are so similar morphologically and genetically that I do not think they can be thought of as anything other than subspecies. They are probably in the early stages of speciation, particularly as microsatellite evidence points to little or no current gene flow, but personally I do not think that the evidence points to them as being more than two good subspecies, two early lineages. Some may disagree, and suggest that these two should be separated as good species, but given their morphological similarity and the relatively low genetic divergence of this pair, as compared to the others, they may be best considered part of one species.
But what about the other pair? The differences both morphologically and genetically between eremita and salvini are much greater than between steadi and cauta. Even so, at least genetically these two are much more similar to each other than other albatross species pairs. However, there have been historical sympatric nestings with no evidence of hybridization that I know of.
I think there is a strong case to be made in splitting this species into two (steadi/cauta vs. eremita/salvini). The case may be weaker to also split eremita and salvini, but the two differ consistently morphologically, genetically, in their distribution and have not interbred although sympatry has occurred. I feel comfortable in splitting this pair out as well, but understand that this may be contentious, particularly since the original work done that suggested dividing every single terminal taxon in the albatrosses left a bad taste in the mouths of many. But remember: do not throw out the baby with the bathwater!
I would suggest separating this species into three, using the following English Names: cauta/steadi (use White-capped Albatross even though it has been used recently for only one of the component taxa?); salvini (Salvin's Albatross); eremita (Chatham Albatross).
[Remsen insert: To do this dichotomously, let's make a "NO" vote to retain status quo (1 species), and a "YES" vote to split into 2 or more species, with 2 vs. 3 to be decided subsequently].
Abbott, C.L. and M.C. Double (2003a) Phylogeography of shy and white-capped albatrosses inferred from mitochondrial DNA sequences: implications for population history and taxonomy. Molecular Ecology 12: 2747-2758.
Abbott, C.L. and M.C. Double (2003b) Genetic structure, conservation genetics and evidence of speciation by range expansion in shy and white-capped albatrosses. Molecular Ecology 12: 2953-2962.
Double, M.C., R. Gales, T. Reid. N. Brothers, and C.L. Abbott. 2003. Morphometric comparison of Australian Shy and New Zealand White-capped Albatross. Emu 10: 287-294.
Nunn, G.B., Cooper, J., Jouventin, J., Robertson, C.J.R., and Robertson, G.G. 1996. Evolutionary relationships among extant albatrosses (Procellariiformes: Diomedeidae) established from complete Cytochrome-b gene sequences. Auk, 113: 784-801.
Penhallurick, J. and M. Wink (2004). Analysis of the taxonomy and nomenclature of the Procellariiformes based on complete nucleotide sequences of the mitochondrial cytochrome b gene. Emu 104:125-147.
Robertson, C.J.R. and G.B. Nunn 1998. Towards a new taxonomy for albatrosses. In Albatross Biology and Conservation (Robertson, G. and Gales, R., eds.). Pp.13-19.
Tickell, W.L.N. 2000. Albatrosses. Yale University Press, New Haven.
Alvaro Jaramillo, Dec. 2004
Remsen comments: "YES [to recognize 2 or more species]. What convinces me that at least one split is necessary are those reports of pairs of one nesting within range of the other. They may indeed be isolated events, but they signal that those taxa have reached the level of differentiation associated with species rank in albatrosses, or at least hey put the burden of proof on those who would consider them conspecific under BSC. The typical case in such situations (when a few individuals of one taxa find there way into the breeding range of a closely related taxon) is for hybridization to occur, and we usually dismiss these as irrelevant (to BSC) because of the dramatic asymmetry in population sizes prevents them from being true tests of mate choice. When the outcome is the opposite, as in the steadi/eremita case, this reveals in my opinion something fundamental about comparative levels of differentiation."
Comments from Robbins: "YES. Genetics and plumage morphology indicate that there should be at least two species recognized within this complex, thus I vote "yes" for recognizing steadi/cauta as specifically distinct from eremita/salvini."
Comments from Stiles: "YES. I agree with Van: when an isolated pair or two retain their fidelity in the face of hordes of potential mates of the other form, this constitutes a pretty rigorous test of isolating mechanisms. Also, since intrinsically it seems logical that individuals (especially young ones, possibly unpaired) might wander to the "wrong" island more often than pairs of adults, the fact that no hybrids or mixed pairs have been recorded constitutes an even stronger indication that isolating mechanisms are effective. Three species seems the best choice in this case."
Comments from Pacheco: "YES. Reconheo que h razes suficientes para considerar a existncia de, pelo menos, duas espcies; mesmo que, sob a tica do BSC. Voto pelo reconhecimento especfico entre os pares de txons steadi/cauta e eremita/salvini."
Comments from Silva: "YES, I think there are good reasons to recognize at least two species in this group."