Monday, May 8, 2017

The Snakes that a Snake-eating Snake Likes to Eat -- Guest Post --

  By Scott Goetz.   

     It shouldn’t bother me when people characterize the eating habits of Eastern Indigo Snakes by saying,  “they’ll eat anything they can fit in their mouth”. After all, Indigos do eat a wide variety of prey. Specifically, they’ve been documented to eat over 49 species: everything from rattlesnakes to rabbits. And let’s not forget some of the weirder prey items like baby sea turtles or hatchling invasive Burmese Pythons. 

    Remember that children’s book “Are you my mother”? It’s about a baby bird that gets knocked out of the nest and approaches one animal after the next asking (you guessed it), “are you my mother?”. After eventually finding its actual mom they both, presumably, live happily ever after. The end.  Classic book but, let’s just say the tale might have taken a darker turn had the baby bird in the book posed that question to an Indigo snake. 

    Sorry, I’m getting off topic, I just wanted to put that visual in your head. The purpose of this paragraph is to complain about people who say Indigos eat everything. Why? Because when this is the only thing you say about Indigo eating habits, I think it creates the perception that there’s nothing interesting going on. It makes me think of Indigos as slithering vacuum cleaners, just wandering around aimlessly eating whatever unfortunate animal they happen upon. But just because an animal eats a lot of different things doesn’t necessarily mean they don’t have preferences. So you might ask, why should we care if Indigos do in fact prefer certain prey types over others? Well, if an organism has strong prey preferences, it can greatly influence their movement patterns, the way they hunt prey, and general natural history. In short, it can influence an organism’s ecology.

 If the predator/prey relationship is strong (i.e. a predator really targets a specific species or type of prey) it may even drive evolutionary responses in both predators and prey. In ecology terms we refer to this as an evolutionary arms race or the Red Queen effect, named after a quote by the queen in “Alice in Wonderland”. Who knew this blog post on snakes would have so many references to children’s literature? I promise, I occasionally read books that are above an eighth-grade level. 

Anyway, some of the coolest (personal opinion) examples of biological arms races involve interactions with toxic or venomous species and these interactions may actually be what caused the development of the chemical defenses in the first place. My personal favorite involves Common Gartersnakes and Rough-Skinned Newts. The newts in this story are poisonous, however, some populations of gartersnakes have developed a higher tolerance or resistance to the newt toxins allowing them to eat these salamanders that are otherwise too toxic for most predators. The newts in these areas responded by evolving stronger toxins. In turn, the snakes evolved even greater resistance. You get the picture, evolution driven by predator/prey interactions. I should note, I’m only giving a cursory overview of what’s going on between gartersnakes and newts, for a more detailed description, I highly recommend this post from Life is Short, but Snakes are Long

Take home message here - dietary preferences are important because they can influence the physiology and ecology of the predator and possibly their prey. 

An Indigo Snake in Conecuh National Forest eating
a Copperhead. Photo courtesy of Mark Bailey.
If you’ve been paying attention so far, you’ve probably recognized that I’ve been setting myself up to eventually reveal that Indigo snakes do appear to have some prey preferences. Let’s do that now: the first indication arrived in a study by Dirk Stevenson and others in 2010 that compiled all known Indigo feeding observations coupled with the identification of prey items recovered from the guts of dead Indigos that were in museums. This study expanded the known list of prey species and it revealed that some prey types were dramatically more common than others. Importantly, snakes accounted for almost half (49.2%) of all known Indigo prey. Similarly, David Steen and colleagues (2016) observed that telemetered Indigos released in Alabama also primarily ate snakes (11 out of 14 obs.). Interestingly, almost half of the snakes eaten were pit vipers, including Copperheads (5/14) and a Pygmy Rattlesnake (1/14). However, the drawback to these two observational studies is that they can’t tell us about prey preferences because they provide no measure of prey availability. In other words, maybe Indigos are just eating prey in direct proportion to their availability and snakes just happen to be the most abundant prey type. 

At this point we’re in a situation that I think is one of the most enjoyable facets of being a biologist. We have some observational data leading us to believe a phenomena is occurring and now we get to sit down, come up with some hypotheses, and figure out how to test them. First, we predicted Indigos would prefer snakes over other prey types. Second, we predicted that Indigos would prefer pit vipers over other types of snakes. This second prediction was based, in part, on the high number of pit viper-feeding observations in Alabama and the idea that, aside from being venomous, pit vipers might make ideal prey for Indigos. Pit vipers are generally heavy-bodied ambush predators that are unlikely to be capable of “out-running” a hungry Indigo. Pit vipers also rely on visual crypsis for defense, a good defense against a hawk, but maybe not so effective against a snake predator able to locate prey via chemosensory cues. To test these predictions, we chose to use a behavioral assay known as a tongue-flick attack score (TFAS).

The TFAS is a measure of the responsiveness of a snake to different prey cues; the idea is that snakes flick their tongues at a higher rate when encountering scents that interest them. To test the snakes, we rubbed wetted, cotton-tipped wooden applicators against the body of potential prey items, presented the cotton tips to Indigo snakes, and recorded the number of tongue-flicks within 60 seconds. The more tongue-flicks, the greater the interest. If the snake bit the applicator (i.e. attacked) that signified really strong interest. We started by testing the snakes using three prey cues or “scents” and a control (deionized water). The prey scents we tested were mouse, Ratsnake, and Copperhead. 

So, where does one find a bunch of Indigo Snakes for a study like this? Luckily we were able to collaborate with the folks at the Orianne Center for Indigo Conservation, part of the Central Florida Zoo. This group is breeding and head-starting Indigos for two reintroduction programs in Alabama and Florida. We ended up testing 55 hatchling Indigos, amounting to 220 one-minute trials. 

We got some cool results: Both the Ratsnake and the Copperhead scents were more interesting to Indigos than the mouse scent. So far so good regarding our first hypothesis: Indigos do seem to prefer to eat snakes. Further, we found that Indigos responded more strongly to Copperhead scent than Ratsnake scent. This finding might support our second hypothesis that Indigos prefer pit vipers. However, because we only tested one species of pit viper (Copperheads) at this point we couldn’t really say whether Indigos prefer native pit vipers in general or they just really like to eat Copperheads (stay tuned for our follow-up study to figure this out!).

So these results were pretty cool (not my opinion, this one’s just an objective truth). But from an academic point of view, these findings add to our knowledge of a large, beautiful, and charismatic snake species. We now have a better idea of how Indigos fit in the larger ecology of southeastern ecosystems and the results hint at potential interspecific interactions (i.e., maybe Indigos and pit vipers have a coevolutionary relationship).  Practically speaking, we’ve learned some potentially valuable life-history information on an imperiled species. 

     Knowing what an animal eats or prefers to eat is, in some sense, pretty basic information, but it could change how we monitor the health of natural populations. It may also change what we feed captive snakes that will eventually be released into the wild, and help inform us as to which areas might be good locations for future reintroductions (i.e. sites with lots of snakes). David, myself, and growing list of collaborators have done some follow-up experiments related to Indigo dietary preferences and interactions with pit vipers that I hope to share in future posts.

This work is made possible through the support of the Alabama Department of Conservation and Natural Resources.

Author’s note: The use of an image, such as, a book cover is permissible under fair use copyright laws if you are reviewing a product. To keep David’s blog on the up and up, here is my review of “Are you my mother?”. I give this P.D. Eastman classic four stars, despite its super predictable ending which I spoiled anyway at the beginning of this post.

About the Author: I’m currently at Auburn University finishing my doctoral research focused on the invasion of Cuban Treefrogs in Florida. Beyond invasive ecology of reptiles and amphibians, I’m also interested in interspecific interactions, particularly predator/prey dynamics. I’m a fan of all amphibians and reptiles, but I’ve always been most captivated by snakes. 

Steen, D. A., J. A. Stiles, S. H. Stiles, J. C. Godwin, and C. Guyer. 2016. Observations of feeding behavior by reintroduced Indigo Snakes in southern Alabama. Herpetological Review 47:11–13.

Stevenson, D. J., M. R. Bolt, D. J. Smith, K. M. Enge, N. L. Hyslop, T. M. Norton, and D. J. Dyer. 2010. Prey records for the Eastern Indigo Snake (Drymarchon couperi).Southeastern naturalist 9:1–18.

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