So, I have been trying to find sources that compare and contrast Gray and Eastern Ratsnakes. They both have identical habits, overlapping ranges in the Eastern TN and Western NC area, and similar appearances and sizes. They even share similar behavior in hunting. Does anyone have good sources on the comparisons of these two species?
I actually have a stake in this too, as it confounds me to watch my ratsnake observations get labeled as “complexes” when other observations with practically no difference in where they are or what’s shown in the photos are Research Grade species
East of the Mississippi River, where one Pantherophis ends and another begins is one of the most contentious debates in herpetology. The taxonomy of the group has fluctuated and names have changed and/or swapped around very frequently as we learn more about the relationships of these taxa through molecular phylogenetics and phylogenomics.
These taxonomic issues stem from two major confounding problems: 1 - these snakes exhibit high phenotypic variation (think about the greens, yellows, blacks, greys, and combinations of each that can be seen in these snakes); and 2 - there has been extensive gene flow/admixture (“hybridization” isn’t exactly the correct term but also works) between what is currently considered P. spiloides (Gray/Black/Central) and P. alleghaniensis (Black/Yellow/Green/Eastern). Common names also can confuse things as depending on where you are they’ll be referred to different (or the same) common name. I’ll also add that changes in taxonomy in field guides tend to lag behind the scientific community by several years, and so there is a bit of reliance on outdated taxonomy. Most people following field guides (especially Peterson’s guide) will say that spiloides stops at the Appalachians, and alleghaniensis is east to the coast. Unfortunately, it’s not that simple.
Without going into the entire Rat Snake Taxonomy argument, the most recent research (Burbrink et al. 2021. Evolution) showed that spiloides/alleghaniensis have admixed extensively. What we would say is “true” alleghaniensis is only found in Florida and up a sliver of the SE Coastal Plain. “True” spiloides on the other hand is east of the Mississippi, and appears to have “come down” from the Appalachians into the southeastern United States. There, alleghaniensis and spiloides have admixed extensively, such that “true” Central rat snakes are East of the Mississippi through to the Appalachians, true Eastern Rat Snakes are along the Southeastern Coast, and everything else in between has genes from both species (essentially meaning there is no clear way to define them to species, despite what field guides may say).
Here is a relevant sampling map from the paper mentioned above; at the top, if it isn’t dark green or dark blue, it’s an admixed population with evidence of genes from both species. What is kind of interesting is this extensive gene flow doesn’t seem to be the case with other Pantherophis - bairdi/obsoletus (Baird’s and Western) and obsoletus/spiloides (Western and Central) don’t show the same levels of introgression (see the second and third maps in the link above).
To add to the confusion, the taxonomy of these taxa is still in flux - check out Hillis and Wuster, 2021 (Herp. Review - opens a PDF then scroll down a page) for a short summary there. This is in part because the name “alleghaniensis” is assigned to a holotype that is from the area of admixture, and likely represents a “hybrid” individual itself. (This goes against the rules of zoological nomenclature, which is a whole separate discussion).
So, the short answer to your question is: if you aren’t right on the coast, or from Mississippi east to the Appalachians, there is essentially no way to definitively say that the rat snake you see is either a “true” Central ratsnake (Gray; spiloides) or a “true” Eastern ratsnake (alleghaniensis). You’ll find some with characteristics of both, and it’s likely that they are the result of extensive gene flow over hundreds of thousands/millions of years, without a clearly defined assignment. In these cases I ID them as “Complex Pantherophis alleghaniensis” as you cannot tell the species apart in hand, regardless of color pattern or anything else. However, lots of users ID anything east of the Appalachians as alleghaniensis based on field guides, so it’s a bit of a mess on iNaturalist as to what observations are assigned where. Occasionally you’ll then get an argument in the comments; you might even get someone still arguing for the validity of Elaphe in North America or that everything should be obsoletus. It’s kind of a mess, and depends more on who ID’d your image than what species the snake actually represents.
The ratsnakes in the mountains of TN and Western NC are more towards the “true” Central (spiloides) end of the spectrum.
EDIT: Based on Hillis and Wuster 2021, the name alleghaniensis should be assigned to the central lineage, quadrivittatus to the eastern lineage, and spiloides sunk in synonymy. Here is a much clearer map image with those updates. This change is not reflected on iNaturalist at this time.
That is fascinating. I had some comments pointing P. spiloides as between the Mississippi and the Appalachian range and P. alleghaniensis was east of the range like you had mentioned. Do you think a great deal of this may be small genetic change from a common ancestor due to slight geographic differences? After all, NE TN and Western NC can be quite cold for the Southeastern US.
There was certainly a common ancestor and are sister taxa, but what appears to have happened here is diversification from that common ancestor, isolation (with some areas of continued overlap), and then the species/populations coming back into contact for these areas of confusion. The Mississippi River is the barrier responsible for the oldest break between any of the rat snake species (Western and Central) and then diversification happened through some other biogeographic breaks and climate effects. I’ll quote from Burbrink et al. (2021):
For [Eastern and Central rat snakes], divergence occurred at the Appalachian Mountains and Apalachicola/Chattahoochee River System. Pantherophis alleghaniensis may have diverged in an isolated Florida, due to sea-level changes during interglacials and subsequently expanded north and west. This isolation is reflected in the unique morphology in this region showing a high concentration of the yellow, striped color pattern in the range of P. alleghaniensis in Florida and parts of the southeastern coast of the US (Schultz 1996; Burbrink 2001). This suture zone between the Florida peninsula and the continental US and along the Fall Line has been found for many taxa (Remington 1968; Swenson and Howard 2005; Burbrink et al. 2008). While these two forms can be delimited with genetic data and occupy distinctly different geographic regions, reproductive isolation may not be complete. Although, individual loci likely show varying levels of intogression and selection. Niches are also different between these taxa; P. alleghaniensis is typically found in subtropical habitats in the Florida peninsula and coastal plains environments (Bailey 1995; Burbrink 2001), whereas P. spiloides is found throughout the remainder of the forested habitats east of the Mississippi River including the ecoregions defined as interior river valleys and hills, interior plateau, Appalachian habitats, southeastern and southcentral plains, and the Mississippi River Valley. Uncertainty in the identity of species using morphology where these taxa
meet was likely due to extensive gene flow within the hybrid zone (Burbrink 2001). Finer-scale testing is necessary to tease apart the effects of these river systems, ancient embayments, and connections to high elevation areas. Although the timing of divergence predates the last interglacial, it is possible that repeated 100,000 year glacial cycles isolated these taxa into refugia, as has been suggested previously for ratsnakes and other organisms (Fig. 3; Waltari et al. 2007; Noss et al. 2015).
I’ll also note that based on the Hillis and Wuster (2021) paper I mentioned above, the names for Eastern and Central rat snakes have changed again! The name P. alleghaniensis is assigned to the Central lineage (not the Eastern anymore), P. quadrivittatus is resurrected for the eastern lineage, and P. spiloides is sunk into synonymy. This is based on nomenclatural rules, principle of priority, and type localities of the original taxa. Here is an updated, clearer range map that shows this, but know that this change is not reflected on iNaturalist at this time. I edited my original post to add that figure. This is sure to add a bit more confusion as the names are changing places yet again, but hopefully in time it’ll catch on.
That’s an understatement!
Just a note that the paper that the Hillis and Wuster (2021) paper references is the 2020 , Resolving spatial complexities of hybridization in the context of the gray zone of speciation in North American ratsnakes (Pantherophis obsoletus complex).
Frank T. Burbrink, Marcelo Gehara, Edward A. Myers
If accepted, it really should simplify most confusion east of the Mississippi River.
There will still be “hybridization” areas between the Midland origin of stock, to be named P. alleghaniensis, and the coastal plain, P. quadrivittatus, but at least differentiating Easterns from Midlands should not be an issue.
On the human communication front, I just hope all parties avoid the common name “Gray Ratsnake” because this implies color to many folks on INAT that this is like the older Ratsnake ID, where you must choose amongst Black Ratsnake, Yellow Ratsnake, Deckert’s Ratsnake, Everglades Ratsnake, Gulfhammock Ratsnake, and any older "obsoleta " subspecies.
The 2021 Burbrink et al. paper I referenced above is the fully peer-reviewed, accepted, final version of that bioRxiv 2020 preprint you linked. This final accepted version was first posted online Dec 2020, but formally published in the February 2021 issue of Evolution.
Agreed completely on the use of common names. Given how variable (and unreliable) color is for identification, the use of “Eastern/Central/Western” makes way more sense.
Thanks for the update on the link, Erich.
I’m optimistic about this Ratsnake work, and I hope INAT adopts it soon.
Back when the first Evolutionary Species Concept (ESC) (as opposed to Biological Species Concept (BSC)) first challenged the long standing Ratsnake taxonomy, I, like many older folks, reacted badly to it; too many years of existing paradigms.
I kept thinking with more research and sampling, the two would come together, and I feel like that is happening now.
Even back then, morphologically, there were too many inconsistencies with the subspecies defined.
Another example is the P. emoryii/P. guttatus relationship:
I’ll be interested to see when iNat accepts these changes; generally they follow The Reptile Database updates but there are still some things that haven’t caught up.
I definitely understand the resistance to change though; it’s made worse by inconsistent usage in the secondary literature which perpetuates a seeming decade-long lag between when something happens in the scientific community and when the general public starts to utilize that change. I think the rise of phylogenomics (trees built on full genomes or thousands and thousands of loci, rather than just a couple of loci) really has and will continue to clarify a lot of relationships.
The resurgence of the subspecies argument (mainly driven by Hillis) is interesting. Whereas the last ~two decades has seen a move away from subspecies concepts, Hillis is really pushing for their reintroduction and continued use (as evidenced by the guttatus paper). Lots of resistance there though, as most subspecies are very poorly defined and don’t always align with the genetic/genomic data.