Reptile & Amphibian Forums

I've seen at least a couple of pics of this cross. What I want to know is why it happens. Bairdi seem pretty far removed from lindys, as far as I can tell, most noticeably in size.
Please correct me if I'm wrong, but a normal sized bairdi is around 3', while an average lindy is pushing 5'. Head size is another big difference, as well as preferential habitat. Lindys prefer arboreal repasts, too, while as far as I know, bairdi are predominately terrestrial mammal eaters.
Can somebody clue me in as to why the intergrade even happens?
All I can think of is similar female ovulation scents or rarity of ratsnakes in the overlap regions.
Thanks,
Jack
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...and I think to myself, "What a wonderful world."

Interesting thing I read about the bairdi, is that according to a DNA study done recently of the old and new world ratsnakes, there are some obsoleta (probably the lindheimeri) which are closer to the bairdi than they are to certain other obsoleta. This sort of reinforces the idea that obsoleta from different parts of the country are actually closely related species and not one species. It seems strange but who knows? Maybe the bairdi and lindheimeri are close enough to make it interesting for each other when others of their own type aren't around.

John

The study I have said (I think, it's not right beside me now) that the western strain of obsoletus are closer to bairds than to other black rats, but that they are not hybridising in the wild.

Jack,

At the eastern edges of their range, bairdii overlaps with obsoleta. And while they do have differences in their "prime" habitat in those areas (bairdii on the rocky hillsides, obsoleta along the river valleys), both species wander from those "prime" habitats (they are, after all, just ratsnakes and are therefore somewhat of a generalist).

In my limited experience, neither species is particularly abundant in the areas where they overlap. This seems strange as just a few miles east or west, they are among the most common and conspicuous snakes.

As for the size difference, I have seen a few bairdii that were in excess of five feet, and some quite a distance from any possible areas of intergradation (e.g. Pandale road). I think they average larger than most field guides imply.
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Chris Harrison

Texas rat snakes follow the river bottoms in the Texas Hillcountry. In this area the Texas rat snake moves into the range of the Bairds rat snake. Generally, they exist in their own habitat, but they overlap occasionally. There is natural hybridizing in this overlap area. There are some that still argue Bairdi is a subspecies of obsoleta and not its own species. Bairdi was taxonomically in the obsoleta complex. The scientific name used to be E.o.bairdi. It seems to me that with gene flow between Texas rat snakes and Texas bairds there would be a good case for Texas Bairds to be placed under the obsoleta species.

>>I've seen at least a couple of pics of this cross. What I want to know is why it happens. Bairdi seem pretty far removed from lindys, as far as I can tell, most noticeably in size.
>>Please correct me if I'm wrong, but a normal sized bairdi is around 3', while an average lindy is pushing 5'. Head size is another big difference, as well as preferential habitat. Lindys prefer arboreal repasts, too, while as far as I know, bairdi are predominately terrestrial mammal eaters.
>>Can somebody clue me in as to why the intergrade even happens?
>>All I can think of is similar female ovulation scents or rarity of ratsnakes in the overlap regions.
>>Thanks,
>>Jack
>>-----
>>...and I think to myself, "What a wonderful world."

Although they do come into contact with each other, and they do occasionally hybridize, hybrids are not common.

Compare the zone of contact in obsoleta and bairdii to other obsoleta ssp. In the other subspecies, the populations intergrade over many (hundreds of) miles. This is why they are regarded as subspecies (there is evidence of lots of gene flow).

bairdii and obsoleta come together over a very small area and generally are diagnosible within the limited contact zone. There are records of hybrids, but most snakes in the zone maintain their genetic identities. This suggests that there are isolating mechanisms that usually prevent hybridization.

Clearly it is not the same case as you see in the other obsoleta ssp.
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Chris Harrison

bairdi was once considered E.o bairdi. I was trying to figure why there was evidence of natural hybridizing and yet bairdi retained specie status. There just isn't very much gene flow. It really doesn't flow.

>>Although they do come into contact with each other, and they do occasionally hybridize, hybrids are not common.
>>
>>Compare the zone of contact in obsoleta and bairdii to other obsoleta ssp. In the other subspecies, the populations intergrade over many (hundreds of) miles. This is why they are regarded as subspecies (there is evidence of lots of gene flow).
>>
>>bairdii and obsoleta come together over a very small area and generally are diagnosible within the limited contact zone. There are records of hybrids, but most snakes in the zone maintain their genetic identities. This suggests that there are isolating mechanisms that usually prevent hybridization.
>>
>>Clearly it is not the same case as you see in the other obsoleta ssp.
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>>Chris Harrison

in a very small limited zone be more of a point they will readily breed? Just a thought. The amount of hybrids that exist is not the measurement of a species or subspecies. If they will readily breed or do breed can be indicative of species.

>>Although they do come into contact with each other, and they do occasionally hybridize, hybrids are not common.
>>
>>Compare the zone of contact in obsoleta and bairdii to other obsoleta ssp. In the other subspecies, the populations intergrade over many (hundreds of) miles. This is why they are regarded as subspecies (there is evidence of lots of gene flow).
>>
>>bairdii and obsoleta come together over a very small area and generally are diagnosible within the limited contact zone. There are records of hybrids, but most snakes in the zone maintain their genetic identities. This suggests that there are isolating mechanisms that usually prevent hybridization.
>>
>>Clearly it is not the same case as you see in the other obsoleta ssp.
>>-----
>>Chris Harrison

is that they rarely hybridize, even though they come into close contact. There is no "zone" of intergradation between these species. You find one, or the other. Very few documented hybrids are known.
If they were truly the same species, intergrades would be common and there would be evidence of genetic introgression of bairdii genes into the obsoleta populations that are just east of the contact zone. There isn't.
There must be some sort of reproductive isolating mechanism preventing more common hybridization.
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Chris Harrison

The amount of hybrids that exist is not the measurement of a species or subspecies. If they will readily breed or do breed can be indicative of species.

The fact that individuals may or may not interbreed is not informative (people have found hybrid corn x yellow rats, several hybrid rattlesnakes, etc). What is informative is the sharpness of the demarcation between the two species where they contact. Bairdii characteristics so no evidence of spreading into the obsoleta populations in central Texas, even a mile or less from bairdii range.
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Chris Harrison

would be, "What does a bairdi X lindheimeri hybrid look like?"

While I have collected both topotypical bairdi and lindheimeri over the years from the southern edge of the Edwards Plateau, I have also found quite a few that didn't quite look "right" for either taxon. This is particularly so in snakes from the upper end of the Sabinal Canyon (N of Vanderpool). Initially I dismissed all of these as simply melanistic lindheimeri until I saw the photo you posted of the "hybrid" from Lost Maples. All of the snakes I had found in Sabinal Canyon looked very much like that (a half-dozen or so, total). Most of these snakes were too dark for an accurate blotch count.

On the other hand, I have never found what I would call a "pure" bairdi in that particular area (not surprising, given that the road follows the river along the very bottom of the canyon for the most part).

As far as evidence of bairdi genes in adjacent lindheimeri, I can't speak for eastern part of the Plateau, but to the south (in Bexar County, below the Balcones Escarpment, in the Medina and Leon drainages) most lindheimeri consistently sport faint, dusky longitudinal stripes along with their blotches. I once found a snake in this area that superficially resembled bairdi more than lindheimeri: gray ground color, stripes more prominent than blotches, but it had the typical lindheimeri body form and demeanor, and rose-colored skin rather than orange (this was not the "hybrid" I posted old B&W photos of last year).

Tom Lott