Analyzing lizard stomach contents from Greece
Well, it's been a few months now since we got back to New Haven and the whole team is pounding away at data analysis and writing. The data is looking great, and I'm hoping to have the first paper together by the end of the month. I won't spoil the surprise just yet - I'll post all about that first paper once it's finished and together, but just to give you a taste of the other work we've been doing, here's an update Kat wrote about her lizard stomach content analysis. It's been a lot of work but she's finding some really exciting results!
Hi, Kat here! Colin’s made me entomologist in chief of the
lizard gang. For my senior thesis, I’m focusing on the insect aspect of Colin’s project. The main thrust of my thesis is comparing the available prey community of P. erhardii to the diet composition of the lizard. As Colin described in a previous post on his blog we collected stomach content samples from most of the lizards that we caught in Greece in order to see what they were eating. It was all fun and games when we were pumping the lizards, but now we’re up to our ears in vials of vomit. Each vial contains the content of one lizard’s stomach, preserved in ethanol. So far, I’ve analyzed almost 300 samples…
To analyze the samples, I put the contents of each vial on a petri dish and examine it under a dissecting microscope. Sometimes the stomach contents have formed a bolus where all of the food particles are clumped together and wrapped in a layer of mucous. In order to see what’s inside, I peel off the mucous and carefully dissect the bolus. You can see the undissected bolus below in the photo on the left, which I took using an ipod camera held up to the microscope eyepiece. The sample on the right has been carefully pulled apart to reveal the insect parts inside.
Next, I sort the contents into piles of similar body parts and identify the order of arthropods that each body part belongs to. Each vial of stomach contents can contain several different orders of arthropods. In the sample below, you can see two spiders (Araneae, top left and middle left), a cranefly (Diptera, top row second to the left), a snail (Gastropoda, top row second to the right), an ant (Hymenoptera, top right), a webspinner (Embioptera, middle row) and several leafhoppers (Hemiptera, bottom row).
So far, the hungriest lizard that we stomach flushed was an insatiable fellow caught on Aspronisi, who ate and subsequently vomited a total of 93 insects: a combination of beetles, flies, ants, and leafhoppers. Most of the stomach contents are much easier to analyze; in fact, only about 15% of the stomach contents I’ve analyzed have contained more than 10 individual arthropods.
However, analyzing stomach contents isn’t as simple as just counting bugs. As you could imagine, the stomach of a lizard isn’t the best preservative. Most of the organisms I find in the stomach contents have been broken, pulled apart, discolored, and partially digested. For example, isolated spider legs are pretty easy to find in stomach contents because they easily break off from the body. They’re pretty easy to recognize: long, symmetrical cylinders that don’t taper at the ends, often covered with fine hairs. Even if I can’t find a single spider body in a sample, I can count the legs by multiples of eight and determine how many spiders the lizard ate.
Spider legs are pretty easy to recognize, but I often find confusing body parts in the stomach samples that require some detective work to identify. One of my most frustrating mysteries involved this strange element, which I found in several different lizards’ stomachs:
This bugger confused me for a little while. Due to other clues within the stomach contents, such as a triangular scutellum (a panel on the dorsal side of the thorax), I guessed that this element was part of a hemipteran (“true bug”). At first I thought that the brown tube might be some sort of sheath for the mouthparts: hemipterans have characteristic sucking mouthparts that look like a needle. However, I couldn’t quite figure out how the oval base of the element would attach to the head. To figure out what exactly I was looking at, I did some sleuthing and looked through the pinned specimens in a teaching collection at Yale. I was finally able to find the element I was looking for…but at the other end of the organism! You can see the mystery element here on the left, at the rear of the insect (this is a leafhopper):
It was an ovipositor! Ovipositors are tubular projections that extend from the tip of the abdomen in females, used to lay eggs. When not in use, the ovipositor is folded under the body. They’re pretty easy to recognize when they’re attached to the organism, but they look really weird when their separated and floating in a pile of lizard vomit.
Not everything in the lizards’ stomachs is ground to a pulp. Many of the smaller organisms are swallowed whole, and tend to remain intact during the process of stomach flushing. I really enjoy finding these organisms in the samples, not only because they’re a whole lot easier to identify, but also because they’re often pretty cool looking:
The arthropod on the left is a tick with brushes on the ends of two of its legs, used for helping the tick attach itself to animals’ fur. The arthropod on the right is a pseudoscorpion, which has the same pincer appendages as scorpions but lacks a tail with a stinger. Organisms like these are what makes analyzing stomach contents interesting; I never know what I’m going to find in the next one!