New World snakes are "mimics until proven otherwise"

Henry Walter Bates spent more than a decade living in the Amazon, having the sort of adventures that inspired generations of naturalists. His most famous and lasting contributions to natural history are his foundational descriptions of mimicry among species. The type of mimicry that carries his name, Batesian Mimicry, involves a harmful species (the model) and a species that copies the appearance of the model while not actually sharing the harmful phenotype (the mimic). In vertebrates, the primary examples of this type of mimicry are brightly-banded snakes that mimic venomous species:

Dramatization of Henry Walter Bates thinking about mimicry in snakes. I'm not saying this actually happened this way.

Dramatization of Henry Walter Bates thinking about mimicry in snakes. I’m not saying it actually happened this way, but yeah, it probably did.

However, the mimicry among New World snakes has turned out to not be as clear-cut of a case of Batesian Mimicry as first thought. Venomous coral snakes and the multitude of brightly-banded species that are putative mimics break a couple of key assumptions about mimicry that have been well supported in other examples of mimics. First, snake mimics can occur way outside the geographic range of the model species. Second, the model species are not necessarily more abundant than the mimics.
A new paper in Nature Communications led by Alison Davis Rabosky uses distributional, phenotypic, and phylogenetic data to take a fresh look at these key assumptions.
While there are mimic species that are allopatric to venomous coral snakes, once distributional information in adjusted based on total snake richness and phylogenetic non-independence, coral snake presence is a strong predictor for the number of mimetic species. Additionally, the 19 independent origins of mimetic coloration occur after the temporal/spatial overlap with coral snakes, meaning that in general, the mimicry has not appeared before the model appears in time or space.
Figure 1 from Rabosky et al. (2016) describing the spatial distribution of mimic and model New World snakes

Figure 1 from Rabosky et al. (2016) describing the spatial distribution of mimic and model New World snakes

Interestingly, the authors identify high rates for the gain or loss of brightly-banded coloration in mimics, even in geographic locations with the highest richness of model species. Both the loss of mimetic condition and the instances of this loss happening within stable regions of the model species’ range are unique to New World snakes. The loss of mimetic coloration is extremely rare in other Batesian systems within butterflies, hoverflies, and spiders.
So what makes snake mimics special? The answer is still out there, but the authors suggest several alternatives including less predation pressure on allopatric snake mimics, drastic range shifts among mimics and their models, or a unique genomic architecture for these alleles. Regardless of these losses and gains of mimetic coloration, this paper provides strong evidence that the phenotypic variation in the putative snake mimics is driven by the presence of coral snakes, or as the authors put it:

We propose that future research is best served by a working hypothesis in which as New World RBB [Red-Black Banded] snake species are considered mimics until proven otherwise..”

Rabosky, A. R. D., Cox, C. L., Rabosky, D. L., Title, P. O., Holmes, I. A., Feldman, A., & McGuire, J. A. (2016). Coral snakes predict the evolution of mimicry across New World snakes. Nature communications, 7. doi:10.1038/ncomms11484

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