Evolutionary biologist Jerry Coyne (author of Why Evolution is True and coauthor of Speciation) kindly posted my photos of the Plate-billed Mountain-Toucan (Andigena laminirostris) on his website today. Earlier today I wrote a post about the striking similarity of color patterns in the Plate-billed Mountain-Toucan and the Toucan-Barbet (Semnornis ramphastinus), and I mentioned this mimicry in the comments on Jerry’s site. Reader Bruce Lyon responded to my comment by reminding me that there are other remarkable examples of plumage mimicry among the toucans.
For example, here in the lowlands of Amazonian Ecuador two large Ramphastos toucans co-occur, the White-throated Toucan (Ramphastos tucanus) and the Channel-billed Toucan (R. vitellinus). They have very different songs: the larger White-throated Toucan is a “yelper” while the slightly smaller Channel-billed Toucan is a “croaker”. However, both species have virtually identical plumages. If they are not singing it can be very difficult to tell them apart, except by the bill proportions (the Channel-billed Toucan has a smaller bill relative to its head).
On the other side of the Andes, in the western Choco lowlands, there is another pair of co-occurring Ramphastos toucans: a “yelper”, the Chestnut-mandibled Toucan (R. swainsonii), and a slightly smaller “croaker” Choco Toucan (R. brevis). These also have plumages that are identical to each other, but they are completely different from the plumages of the Channel-billed and White-throated Toucans on the other side of the Andes.
Judging only by their appearances, anyone would think that the identical-looking Chestnut-mandibled and Choco Toucans of the western lowlands were each other’s closest relatives. Anyone would also think that the White-throated and Channel-billed Toucans of the eastern (i.e Amazonian) lowlands were also each other’s closest relatives, since they are almost identical to each other and very different-looking from the western pair of toucans.
However, bill shape and song link the big western species to the big eastern species, and the smaller western species to the smaller eastern species, in spite of their very different colors. DNA analysis (Weckstein 2005) conclusively proves that bill shape and song, not the birds’ colors, are the phylogenetically informative characters in this case. (One of the most exciting things about DNA analysis is that it teaches taxonomists which morphological characters indicate evolutionary history.) The croakers on each side of the Andes are each other’s closest relatives, and the yelpers on each side of the Andes are each other’s closest relatives. This means the nearly perfect match between plumages of the coexisting species has come about through natural selection rather than inheritance from a common ancestor. There must be a very strong survival advantage to looking as they do, for one or both coexisting species.
What could the advantage be? One possibility, proposed especially by Prum (2014), is to reduce interspecific social dominance interactions. If larger toucans drive away smaller species of toucans feeding in the same tree, then it is advantageous for the smaller species to resemble the larger species. This might defuse the larger species’ instinctive aggression against non-conspecifics. An alternative theory relies on the observation that toucans are nest raiders. Most birds fear the largest toucans and do not put up a fight when they raid a nest. Perhaps if the smaller toucan looks like the larger and more fearsome species, other birds will mistake the smaller species for the larger one and flee more often.
Note added July 25: It remains possible that both species in each region have evolved these colors because they are regionally advantageous to both species. However, it is difficult to imagine what regional selective pressures, other than pressure for mimicry, would make the two co-occurring species so precisely similar to each other in each region. In the comments on my last post, in response to a question by Peter Tobias, I suggest some experimental tests of the mimicry hypothesis. End of note.
This kind of near-perfect apparent mimicry is rather rare in birds. It is probably facilitated in this case by the genetic similarity of the species involved. A pattern that is easy to evolve in one member of the pair should also be easy to evolve in the other member of the pair, since their genetic architectures are so similar. Perfection would be harder to achieve in more distantly-related mimicry pairs, like the Plate-billed Mountain-Toucan / Toucan-Barbet pair I wrote about in the previous post.
There are other apparent cases of mimicry in Neotropical birds. The Kiskadee-like flycatchers, those nemeses of beginner birdwatchers, are possible examples. However, it can be hard to separate phylogenetic constraints (inherited genetic architecture) from actual mimicry. The toucan case seems nearly ironclad, now that we know their phylogeny. Now that we
know are pretty sure mimicry happens in birds, we should be on the lookout for other examples, and I think the Toucan-Barbet case I mentioned in my last post is best interpreted this way.
Prum, R. (2014) Interspecific social dominance mimicry in birds. Zoological J of the Linnean Society 172: 910-941.
Lou Jost (www.loujost.com)