Recent conservation successes with Indigo Snakes

This article is part of a series highlighting new research in snake biology presented by herpetologists at the World Congress of Herpetology VII in Vancouver, British Columbia. If you want to learn more about the WCH, check out the June 2012 issue of Herpetological Review, or follow the Twitter hashtag #wch2012, with which I will tag all posts in this series.

Click here to view the Spanish translation of this article!

Haga clic aquí para ver la traducción al español de este artículo!

An Eastern Indigo I was very fortunate to see in 2010.

Ask anyone: the Eastern Indigo Snake (Drymarchon couperi) is one of the most commanding and majestic snakes anywhere. Once found throughout Florida and in the coastal plain of southern Georgia, extreme south Alabama, and extreme southeast Mississippi, today the Eastern Indigo survives in numbers only in peninsular Florida and southeast Georgia. Although the species persists in low numbers in the Florida panhandle, it has been extirpated from the rest of its range as a result of declines in and alterations to the longleaf pine (Pinus palustris) ecosystem that it inhabits and on which it critically depends.

One of the largest snakes native to North America, D. couperi is one of five species belonging to a genus that ranges from Georgia to Argentina. Closely related to Racers (Coluber), Patch-nosed Snakes (Salvadora), and Whipsnakes (Masticophis), Indigo Snakes are charismatic and harmless. Carl Kauffeld called them "truly handsome and impressive" in his classic 1957 book Snakes and Snake Hunting. My herpetology professor, Whit Gibbons, told us a story of an exotic dancer who called his lab asking to borrow one to use in her show. This was during the early 1970s, when Eastern Indigos were common in the pet trade, before their federal listing under the Endangered Species Act (one of the first and still one of the only snakes ever listed).

A Gopher Tortoise basks at the entrance of its burrow.
When fire suppression closes the canopy, their basking
and egg-laying microhabitat is lost.

Unfortunately, Indigo Snakes are one of North America's most endangered snake species, primarily as a result of habitat destruction and fragmentation. Research by Natalie Hyslop showed that male Indigo Snakes in southeastern Georgia have home ranges as large as 3,000 acres (nearly five square miles), and one male Indigo Snake moved a distance of about 13 miles (22 km) over two years. As anyone familiar with the southeastern United States knows, it is almost impossible to find five square miles without a road interrupting it, and, as a result, many Indigo Snakes are run over and killed as they cross busy highways and interstates. Conservation of such a highly mobile species is extremely difficult, and by the early 2000s, population strongholds in Georgia were limited to two military bases, Fort Benning and Fort Stewart, where large tracts of uninterrupted sandhill habitat still remain. Furthermore, the degradation of longleaf pine sandhills via fire suppression encourages the growth of hardwood deciduous trees that close the canopy and push out Gopher Tortoises (Gopherus polyphemus), the burrows of which are critical Indigo Snake microhabitats during the winter breeding season. Although habitat degradation is the most insidious factor contributing to Indigo Snake declines, over-collection for the pet trade and malicious killing (both intentional and collateral, as when gasoline fumes are pumped down a tortoise burrow to kill rattlesnakes) are also considerable threats.

In 2008, a non-profit group called The Orianne Society was founded with the purpose of saving the Eastern Indigo Snake from extinction, which seemed inevitable given the rate of land development and habitat degradation in the southeast. TOS has advanced Indigo Snake conservation in a myriad of ways, from acquiring and restoring land to captive breeding. At the Mopani Indigo Snake Preserve in south-central Georgia, TOS biologists are tracking Indigo Snakes using wildlife detector dogs, also used to track other elusive wildlife, from whales to bats to salamanders. Last winter, I was generously invited to witness firsthand the effectiveness of CJ and his handler, biologist Kiley Briggs, at tracking Indigo Snakes at Mopani.

A very happy Orianne Society volunteer holds an Indigo Snake

Indigo Snakes are known to feed primarily on other snakes, lizards, turtles, small mammals, frogs, and birds. Juveniles might feed on fish in the wild, because they spend the early part of their lives in mesic lowland areas and readily consume fishes in captivity. Unusual food items, in comparison to that of other snakes, include small Gopher Tortoises and all venomous snake species native to the Southeastern US (including Copperheads, Cottonmouths, Coral Snakes, and several rattlesnakes). For this last reason, Indigo Snakes generally have a more positive reputation than other snake species among rural residents of the southeast.

Sign alerting motorists to the presence of Indigo Snakes

At the WCH7, Jim Godwin, a zoologist with the Alabama Natural Heritage Program, and Jimmy Stiles, a student with  herpetologist Craig Guyer at Auburn University, brought us good news regarding the Eastern Indigo Snake in Alabama. Due to the collaborative efforts of over a dozen institutions and organizations, including the Alabama Department of Conservation and Natural Resources and TOS, captive-reared Eastern Indigo Snakes have recently been released into Covington County, Alabama's Conecuh National Forest. These snakes were born in captivity from wild females caught in Georgia and head-started at Zoo Atlanta. The plan was to test the effects of a hard (unpenned) or soft (penned) release on snake survival by following snakes with radio telemetry, but the 1 hectare pens built to contain the soft release animals "are just a suggestion to the snakes", according to Godwin. Instead of waiting the intended 90 day soft release period, many of the soft-release snakes released themselves 5-90 days after their initial release, by going under the fence. Because the snakes were implanted with radios, their progress could be followed. Fortunately, the team found that there were no significant differences in survival between snakes that had been hard and soft released, and that hard and soft release snakes had similar sized home ranges. Significantly, the percent overlap between male and female home ranges was higher for soft  release snakes, and this effect increased with time spent in the enclosure. In terms of management implications, releasing snakes in pens does not seem to have a negative effect on snake survival and probably ultimately has beneficial effects on the structure of the established population. Earlier attempts to reestablish Eastern Indigo Snakes in Alabama were unsuccessful, possibly both as a result of the hard release techniques used and the release of too few snakes in too many locations. In the two years since the initial release in 2010, most of the Conecuh Indigos have survived, although several have been killed by predators and several more run over by cars. Improvements in the fire management regime in the Conecuh and continued research on the reintroduced snake population should mean a bright future for the Eastern Indigo Snake in Alabama.

Clearly I could write about Indigo Snakes all day, but if you want to learn more, check out The Orianne Society's website or read some of the papers linked in the References section below.

ACKNOWLEDGMENTS

Thanks to Mark Wallace for his photo of the happy volunteer.

REFERENCES

Bauder JM, Macey JN, Wallace MP, Snow F, Safer AB, Stevenson DJ (2012) Drymarchon couperi (Eastern Indigo Snake). Juvenile observations. Herpetological Review 43:343

Breininger D, Bolt ML, ML, Drese J, Stolen E (2011) Factors influencing home-range sizes of Eastern Indigo Snakes in central Florida. Journal of Herpetology 45:484-490 <link>

Breininger DR, Mazerolle MJ, Bolt MR, Legare ML, Drese JH, Hines JE (2012) Habitat fragmentation effects on annual survival of the federally protected eastern indigo snake. Animal Conservation 15:361-368 <link>

Godwin J, Wines M, Stiles J, Stiles S, Guyer C, Rush EM (2011) Reintroduction of the Eastern Indigo Snake (Drymarchon couperi) into Conecuh National Forest. State Wildlife Action Grant Report. <link>

Hyslop NL, Cooper RJ, Meyers JM (2009) Seasonal shifts in shelter and microhabitat use of Drymarchon couperi (Eastern Indigo Snake) in Georgia. Copeia 2009:458-464 <link>

Stevenson DJ et al. (2010) Prey records for the Eastern Indigo Snake (Drymarchon couperi). Southeastern Naturalist 9:1-18 <link>

Stevenson DJ, Ravenscroft KR, Zappalorti RT, Ravenscroft MD, Weigley SW, Jenkins CL (2010) Using a wildlife detector dog for locating Eastern Indigo Snakes (Drymarchon couperi). Herpetological Review 41:437-442

Stevenson DJ et al. (2009) An Eastern Indigo Snake (Drymarchon couperi) mark-recapture study in southeastern Georgia. Herpetological Conservation and Biology 4:30-42 <link>

Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

Snakes that can see without eyes

This article is part of a series highlighting new research in snake biology presented by herpetologists at the World Congress of Herpetology VII in Vancouver, British Columbia. If you want to learn more about the WCH, check out the June 2012 issue of Herpetological Review, or follow the Twitter hashtag #wch2012, with which I will tag all posts in this series. 

Click here to view the Spanish translation of this article!
Haga clic aquí para ver la traducción al español de este artículo!

Close-up of pit organ of Tropidolaemus subannulatus

Pit vipers have an amazing and little-known ability to see infrared light. They do this by means of their eponymous pits, which are essentially a second pair of eyes located in the loreal region of their face, between the normal (visible light or "lateral") eye and the nostril. Some snakes, such as Emerald Tree Boas (Corallus caninus), have up to forty pits, meaning that in effect they have forty-two 'eyes': two lateral eyes and forty infrared eyes. Pit vipers have just two, but these organs are among the most exquisitely sensitive sensory organs in the animal kingdom. Other animals can also see wavelengths outside of the spectrum of light visible to humans. For example, bees and many birds can see ultraviolet wavelengths, and the complex eyes of mantis shrimp possess at least 16 different photoreceptor types, allowing them to see visible and ultraviolet light with fine sensitivity, as well as polarized light, thought to allows them to see their transparent prey.

Figure from Goris 2011
a) Boa constrictor b) Corallus caninus
c) Python molurus d) Gloydius blomhoffii

Because pit organs are found in snakes as distantly related as boas, pythons, and rattlesnakes, they must have evolved at least three times over the last 125 million years (boas and pythons, believe it or not, are fairly distant relatives). As you can see, the morphology of the pit organ is very different in these three snake lineages. In pit vipers, it is most sophisticated. The pit viper pit organ is made up of three parts: an inner and an outer chamber, separated by a thin membrane. This membrane functions as an "infrared retina", detecting infrared radiation that enters the inner chamber. The inner chamber cannot be seen from outside of the snake's body, but it communicates with the exterior air via a pore located between the eye and the pit. Because the exterior opening of the outer chamber is smaller than the membrane, infrared light sources cast a shadow on the membrane, which are detected as an image by the nervous system. It works a lot like a pinhole camera. The information is processed by the nervous system separately from that gained using the lateral eyes, but all four (in the case of pit vipers) images are integrated in the brain to produce one single coherent image of the environment. It isn't so different from what your brain does when it integrates two slightly different images of the world, each collected by one of your eyes, to produce an integrated image with depth. The neurology of this process in infrared snakes is relatively well understood, although it is hard to imagine processing visual information from more than two sources.

Rather than thinking of the pits as a "sixth sense", what they actually do is to improve the vision of the snake by making use of parts of the electromagnetic spectrum for which there are no color pigments. To envision this, imaging seeing heat (which is the most common source of infrared radiation) as an additional color. In fact, pit vipers can see differences in temperature in both directions - so an object that is colder than its surroundings also become more visible to the pit organ. It's like the image of a person holding a caterpillar to the left, except with real colors added also. Check out this site for more infrared images.

Innervaton of the crotaline pit organ.
Figure from Goris 2011

During the World Congress of Herpetology's venomous snake evolution session, Bruce Young of the University of Massachusetts at Lowell presented amazing new results revealing directional asymmetry of the thermal image. It was known that, depending on the habitat of the species, there was some difference in the configuration of the pit, but Young's recent work showed that the area of maximum focus (analogous to the fovea of the visible-spectrum eye) is above and behind the head in terrestrial species, and below and behind the head in arboreal species. Because the many uses of the pit organ include enabling snakes to better see predators and prey in great detail in the dark, including those that are partially concealed to the lateral eyes, it could be inferred that these differences in pit organ morphology are determined partially by ecology. Much more work needs to be done on this fascinating system, especially cataloging the diversity of the pit organs of boas (53 species, not all of which have such organs), pythons (41 species), and other pit vipers (216 species).

Cottonmouth (Agkistrodon piscivorus)

When I wrote the title for this post, I realized that it could also apply to two other groups of snakes that get along just fine without eyes. One is the blindsnakes, or scolecophidians, a primitive radiation of snakes about which many fascinating posts are forthcoming. Also worth mention is the population of Tiger Snakes (Notechis scutatus) on Carnac Island in Western Australia. Seabirds, especially Silver Gulls, peck out the eyes of these snakes while defending their nests from predation by the snakes. In a 1999 study published in the journal Behavioral Ecology and Sociobiology, Xavier Bonnet and colleagues found that tiger snakes that had lost their eyes suffered no loss of body condition, growth rate, mating opportunities, or survival. This is especially remarkable because it means these snakes are getting by using tactile and chemosensory information only, since elapids have no pit organ and cannot see infrared light. The late biologist and author extraordinaire Charles Wharton also documented eyeless Cottonmouths on Sea Horse Key in Florida in 1969, which, being vipers, could continue to rely on their pit organs, the function of which was poorly understood at the time.

Tyson's diagram of the head of a rattlesnake;
the pit, which he called the foramen, is at B

Older theories for the purpose of the pits included that they were ears, extra nostrils, organs of smell, secretory organs to wash the cornea, tactile sensors, part of a lateral line system such as that in fishes, or sensory organs of a completely unknown "sixth sense". It wasn't until 1935 that Margarete Ros first associated the pit organs of an African Rock Python with infrared radiation by observing differences in its attentiveness to warm objects before and after she occluded its pits with petrolatum jelly. This was more than 250 years after Edward Tyson first mentioned snake pits at a scientific meeting of the Royal Society of London in 1683, during which he dissected a rattlesnake from Virginia that he called Vipera caudisona (almost certainly a Timber Rattlesnake, Crotalus horridus).

ACKNOWLEDGMENTS

Thanks to Kurt (orionmystery) for his photo of Tropidolaemus subannulatus, and to Pierson Hill for his photo of Agkistrodon piscivorus.

REFERENCES

Bakken GS, Krochmal AR (2007) The imaging properties and sensitivity of the facial pits of pitvipers as determined by optical and heat-transfer analysis. Journal of Experimental Biology 210:2801-2810 <link>

Bonnet X, Bradshaw D, Shine R, Pearson D (1999) Why do snakes have eyes? The (non-) effect of blindness in island tiger snakes (Notechis scutatus). Behavioral Ecology and Sociobiology 46:267-272 <link>

Goris RC (2011) Infrared organs of snakes: an integral part of vision. Journal of Herpetology 45:2-14. <link>

Kohl T, Colayori SE, Westhoff G, Bakken GS, Young BA (2012) Directional sensitivity in the thermal response of the facial pit in western diamondback rattlesnakes (Crotalus atrox). The Journal of Experimental Biology 215:2630-2636 <link>

Safer AB, Grace MS (2004) Infrared imaging in vipers: differential responses of crotaline and viperine snakes to paired thermal targets. Behavioural Brain Research 154:55-61 <link>

Tyson E (1683) Vipera Caudi-Sona Americana, Or the Anatomy of a Rattle-Snake, Dissected at the Repository of the Royal Society in January 1682/3 by Edw. Tyson MD Coll. Med. Lond. Cand. & RS Soc. Philosophical Transactions (1683-1775) 13:25-46 <link>

Van Dyke JU, Grace MS (2010) The role of thermal contrast in infrared-based defensive targeting by the copperhead, Agkistrodon contortrix. Animal Behaviour 79:993-999 <link>

Wharton CH (1969) The cottonmouth moccasin on Sea Horse Key, Florida. Bulletin of the Florida State Museum of Biological Sciences 14:227-272 <link>

Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.