Issue 15, Fall 2019

You are what you eat!

Why study diet?

When scientists use the word “diet” they aren’t referring to Atkins or Paleo. Instead, scientists are describing the items an animal eats, and even the items it prefers to eat (yes, animals can be picky too!). As researchers, knowing what an animal eats is essential to our understanding of its biology, behavior, and role(s) in the food web. Prey availability is often responsible for large-scale animal migrations — like the southward migration of birds prior to food-scarce winter conditions. Understanding an animal’s diet can also have implications for human health, particularly when it comes to seafood.

Ok that sounds pretty important…but how do we study diet?

Typically, the best way to understand what an animal eats is simply to open its stomach and examine the contents. However, this method is not without caveats. For example, consider the following scenario: you examine a blacktip shark’s stomach and find parts of a crab, a heavily digested fish, and other various bones and hard parts. What does this tell you? Did that shark eat one or multiple crabs? What species of fish did the shark eat? What are the various bones and hard parts from? As you can imagine, examining stomach contents sometimes generates more questions than answers! That’s why scientists have developed newer techniques to help answer some of these questions. One of these techniques is a process called DNA barcoding, which allows us to extract DNA from partially digested tissue to determine which species of prey the tissue is from.

Do you always have to euthanize an animal to examine diet?

No! In fact, we strive to use as many non-lethal and minimally invasive techniques as possible. One of these methods is Stable Isotope Analysis, or SIA. Briefly, we can examine the natural chemical tracers in an animal’s tissue (typically muscle), which we collect in a swift and relatively painless manner, to obtain information about the diet of that animal. Unlike stomach content analyses, SIA doesn’t provide us with species-level dietary information. However, we can get a general idea of the animal’s food source location (e.g. far offshore in the open ocean or close to shore in a seagrass bed), and the animal’s “trophic level” in the food web (e.g. top predator). Another non-lethal technique we can use is known as gastric lavage – read on for more information about this method!

Which technique is best?

Well, that depends on your study animal and your question. Say you’re investigating a rare and potentially endangered species. In this case, you would likely choose a non-lethal technique like SIA or gastric lavage. Either of these techniques would allow for quick and minimally invasive sampling, without having to sacrifice the animal. Conversely, if you’re examining the diet of a species that has relatively healthy population levels, you may choose to investigate the stomach contents through DNA barcoding to obtain a more thorough interpretation of the species’ diet. The best approach to answering questions about diet is to use a combination of techniques, wherever possible. Studying diet is a huge part of understanding the biology of an animal and is a major aspect of our research at the Mississippi State University Coastal Research and Extension Center. Read on to learn about several studies we’ve conducted to investigate the diets of various fish and shark species off the coasts of Mississippi and Alabama!

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