Sunday, September 27, 2009

You are what you eat

What does an animal eat? It’s perhaps the most basic question a biologist can ask. Knowing what a species consumes is critical to understanding to how relates to its habitat and the ecosystem. Sometimes, these secrets are readily revealed.

Many snakes, when disturbed, will be all too happy to divulge their last meal by presenting you with a fresh regurgitation of partially-digested creature. Maybe these snakes feel they will be able to flee more rapidly with an empty stomach. On the other hand, perhaps this encourages potential predators to find something else to eat, something that smells a little more appetizing.

Snakes are famous for constricting their prey, I’m sure many people have nightmares of a large specimen wrapped around them. But, lots of snakes in our area don’t constrict at all, they simply grab prey in their mouth and start swallowing. Their bottom jaw is made of two different bones which they manipulate to swallow large prey and work it down their throat. As you may imagine, this is likely an unpleasant experience for the hapless prey, which, as they’ve not been constricted, are swallowed alive.

On these occasions, a frightened snake could mean good news for its last meal. Hognose snakes are famous for their defensive displays, which often includes throwing up. They are also fearsome toad hunters, these amphibians are their primary prey. It is not unusual to find a bloated hognose snake that will freely cough up a live toad with only the slightest provocation. This toad, perhaps after a flustered moment, will eagerly take the opportunity to hop away, and perhaps will be more cautious the next time a serpentine shape approaches. I recently worked in a lab in southwest Georgia that kept a toad as a pet for years after a recently captured hognose snake gave him a second chance at life. A hognose snake I recently captured coughed up a live spadefoot toad that weighed more than it did!


Snakes don’t always make it that easy for you, however, and traditional methods of determining snake diet don’t present very appealing options. Typically, when a researcher finds a snake with a tell-tale bulge in its body, the snake can be coerced into regurgitating its prey by pressing a thumb against its body and working the prey item out of its mouth. In the best case scenario, the researcher now has a recently ingested prey item that allows for easy identification. What is more likely, however, is that an odorous warm glop will appear, a glop that defies classification. In both cases, the snake loses its meal. For an animal that doesn’t eat often, a forced regurgitation may have unintended consequences to its long-term health.

But, it’s better than the other typical method. If you were to peruse some classic snake ecology papers, you’ll quickly realize that thousands of snakes have given their lives so that we can better understand their natural history. One of my goals is to advance our understanding of snakes without sacrificing them in the process. If you’ve watched any of the number of crime shows on television, you know that researchers can often find DNA from the most unusual sources. Although the methods are often dramatized on the television, some of the concepts are grounded in real science.

You are what you eat, right? When I catch a snake, I hold it in a cage until it provides a scat sample and I preserve this scat sample in alcohol or acetone. The goal is to take these samples into the lab and subject them to a long and complicated process with the goal of extracting any present DNA. These small fragments of DNA are then exposed to other strands of DNA in the hopes of kickstarting their replication. Different species have different and unique strands of DNA; these strands are made of base pairs (if I mentioned adenine, thymine, cytosine, and guanine, does it bring you back to your high school biology class?). When the DNA has replicated, we can examine the unique sequence of base pairs in the product and determine what unlucky species had been eaten by the snake.


It’s expensive, time-consuming, and requires additional testing before it’s accepted as a legitimate methodology, but I hope this technique changes the way we study and learn about snakes. Ideally, it will be more accurate than sorting through a glop of vomit, less intrusive than forced regurgitation, and no snakes will be killed in the process.


Sometimes snakes make it easy for you. Here's a recently captured hognose snake that coughed up a partially digested frog. There was enough left to identify it as a southern leopard frog. Photo courtesy of Michelle Baragona.

7 comments:

Paul said...

Cool stuff! For food items in the stomach, would it be possible to go in the mouth to swab or somehow sample the stomach contents?

David Steen said...

Neat idea; I'd need some confirmation that I'm actually hitting a food item with the swab. A laboratory study might be required to test its feasibility. One of the benefits of the method I propose is that they're not required to have recently eaten.

It certainly would eliminate a layer of complexity if it weren't necessary to hold the snakes in the lab. Fortunately, they typically do their business within a couple of days.

Tucker L said...

Nice post. I have a picture of an unidentified tree frog that I took in western Iowa just a couple weeks ago and though you might be the best person to ask about it. If you could identify it that would be great. Here is a link to the post: http://birdingwithtucker.blogspot.com/2009/09/16th-birthday-birding.html

Thanks.

Anonymous said...

"What is more likely, however, is that an odorous warm glop will appear, "

Hmm. "Warm"? Shouldn't it be pretty close to environmental temperature since snakes are cold-blooded?

Tucker L said...

Thanks for the help with the frog identification!

David Steen said...

Good question. The term "cold-blooded" has fallen out of favor because it tends to be misleading. Scientists typically refer to these animals as ectothermic, meaning they cannot internally control their body temperature and/or poikilothermic, meaning their temperature varies depending on their surroundings. Cold-blooded can be a misnomer; some snakes, just by virtue of their behavior (i.e. basking) can maintain a higher temperature than we can with less fluctuation. A snake with a full meal basking in the sunlight to help digestion will be considerably warmer than environmental temperature.

Charles said...

Very interesting!