Are All Animals Really Omnivores?

An alligator eats a pond apple (courtesy of the Everglades NPS)

I was teaching one of my online biology courses yesterday and discussed something virtually every biology student learns: classifying organisms as producers (who make their own food), consumers (who eat other organisms for food), or decomposers (who decompose dead organisms for food). I then mentioned that consumers can be further classified as herbivores (eating only plants), carnivores (eating only animals), or omnivores (eating both plants and animals). I then asked the students how they would classify an alligator. Of course, they classified alligators as carnivores. I then showed them the picture above. That alligator is eating a fruit (a pond apple) on purpose.

It has long been known that alligators and crocodiles ingest plant material, but it was originally thought to be accidental. Perhaps the alligator was biting for a fish, missed, and took in some plant material that was floating in the water. However, recent research shows that in most species, the ingestion is probably not by accident. It is a part of the dietary strategy.

After class, I was looking at the scientific literature and ran across an incredible report about a similar phenomenon in bonnethead sharks. Once again, it has been well known that these sharks ingest seagrass, but it was thought to be accidental. Furthermore, since a carnivore’s digestive system is tuned towards breaking down meat, it was thought that the sharks gained no nutrition from the accidentally-ingested grass. We now know that this is definitely not the case.

The authors of the study fed bonnethead sharks a diet that was mostly seagrass with just a bit of squid. The seagrass had been labeled with a specific isotope of carbon (carbon-13), which makes up only about 1% of naturally-occurring carbon. This allowed the them to identify the chemicals from the seagrass and figure out what happened to them after the seagrass had passed through the sharks. They found that the sharks were actually digesting the seagrass and using it for nutrition. In fact, even though their diet was 90% seagrass, the sharks gained weight! Finally, the authors found that the sharks’ digestive tracts showed the activity of enzymes which are designed to break down plant matter. They write:

We show that a coastal shark is digesting seagrass with at least moderate efficiency, which has ecological implications due to the stabilizing role of omnivory and nutrient transport within fragile seagrass ecosystems.

If sharks and alligators can eat and digest plant matter, probably all animals we think of as carnivores are at least capable of eating and digesting plant material. Combine that with the fact that animals thought to be strict herbivores have been found deliberately eating other animals, and we come to the strong possibility that all animals are really omnivores.

Of course, one “take home” message from all this is that creation is marvelously complex, and our attempts to categorize it are incomplete, at best. However, it also has implications when it comes to the issue of origins. Most young-earth creationists (including myself) think that before the Fall, all animals were herbivores. We also believe in a global Flood, where Noah and his family had to care for different kinds of animals on the ark for a bit more than a year. Some of those animals were carnivores, but they could not have been fed other animals (except perhaps some sea creatures from time to time). Creationists critics often say both situations are impossible, because some carnivores must eat meat, or they will die.

If a species of shark can gain weight on a diet of mostly plants, it is at least conceivable that prior to the Fall and for a year on the ark, the animals that gave rise to the “carnivores” we see today could have lived on a diet of only plants.

19 thoughts on “Are All Animals Really Omnivores?”

  1. This is intriguing. However, this –

    If sharks and alligators can eat and digest plant matter, probably all animals we think of as carnivores are at least capable of eating and digesting plant material.

    seems like a bit of a logical leap to me. If bonnethead sharks and alligators have been long known to ingest plant material, then it is not *as* surprising to learn that such ingestion is purposeful. But are all animals we call carnivores known to ingest plant material? If not, then that’s an important difference between the examples above and the other carnivores that would make me hesitant to make such a claim about the other carnivores unless further evidence was uncovered.

    Given that a literal reading of Genesis specifies when land animals and humans were given plants to eat, and when humans were given meat to eat, but nowhere specifies when land animals were given meat to eat, and nowhere specifies when sea animals were given anything to eat at all, it seems to me to intentionally leave room for multiple possibilities.

    1. I don’t think it’s a leap at all, Joshua. The major argument is that carnivore digestive systems aren’t “tuned” to digest plant matter. However, clearly some carnivore digestive systems are. What would be the reason that some carnivore digestive systems aren’t? Remember, I just said “capable of” eating and digesting plant material. I didn’t say they necessarily do it. Thus, whether or not they are known to ingest plant material doesn’t matter. The question is whether or not their digestive systems could handle it if they did. The answer, based on studies like this one, seems to be “yes.”

      I agree that there are multiple possibilities for pre-Fall diets. However, it seems to me that the most straightforward reading of Genesis 1:29-30 is that all animals were herbivores pre-Fall. That’s not a necessary interpretation. It just seems to me to be the most straightforward one.

      1. “If sharks and alligators can eat and digest plant matter, probably all animals we think of as carnivores are at least capable of eating and digesting plant material.”

        It’s one thing if they are biologically capable but if it was seen that they all actually do this in nature, even if just occasionally, now that would be REALLY interesting.

        I remember being in a discussion with someone once who said that “Behemoth” of the bible could not refer to Sauropods because their jaw structure was not suited to mastication, as might seem necessary for the “grazing in the tall grass” description. I pointed out that my cat eats tons of grass with no masticating necessary. I also pointed out that many evolutionists once stated (quite definitively) that the idea of Behemoth being a Sauropod and eating grass was absurd as grasses hadn’t evolved yet. They were wrong. Grass was found to exist via fossil records. They then said that Sauropods couldn’t eat it. They were wrong. Excrement consisting of 4 different grasses was found alongside Sauropods in the fossil record. They ate LOTS of grass.

        On a side note, one of our neighbors started growing lemongrass recently and he’s had to squirt our cat with the hose more than a couple times to get him outta there! He loves that stuff. haha.

    1. Thats interesting you say that Dana. We have a Bison farmer at our local farmers market that can’t keep that cut of meat in stock. I guess it’s not really a cut of meat but he was explaining to me that it was noted by countrymen that whenever animals killed a big grazer, they would actually fight over the “grass sack”. So this farmer started selling it for people with pets (mostly dogs) and like I said.. has a hard time keeping up with demand!

      1. Dogs also like plain old undigested grass. And cats are attracted to nepetalactone oil, which is found in catnip (which is a close relative to mint), maybe they’re also attracted to the essential oils found in lemongrass.

        1. Oh, this is funny: I was just helping my mom with some gardening stuff, and I picked some broccoli and set it on the ground, and my mom’s little dog started chewing on it!

  2. Some would take issue with this (like Greg Koukl) and raise the objection that organisms seem to have designed parts to specifically catch and eat other animals i.e., spiders. Did God change these organisms at a specific time in the past? Was there some sort of mutation? Was there some sort of HGT? God’s foresight in the beginning? Something else?

    1. I would speculate that the things we interpret as designed for carnivory were probably used for something else pre-Fall. A spider’s web can be used to collect drops of water for the spider. The silk is used to help some spiders travel, and other spiders use it to wrap their eggs. Sharp teeth could have been used for thick fruits (like some monkeys use their sharp teeth for today), etc.

  3. Just recently a friend of mine on Facebook posted an article about how changing an animal natural diet could be detrimental to their health. The article discussed how dangerous it is for domesticated cats to be fed a vegan diet. It has long been stated that cats are carnivorous animals. Shouldn’t this study prove that statement as inaccurate?

    1. Well, this study didn’t deal with cats, so we can’t say anything conclusive. However, it certainly implies that the statement is inaccurate.

        1. I don’t know of any study that indicates cats get nutrition from catnip. As I understand it, they are ingesting it for the “high” they get. Do they actually get nutrition from it? That’s this issue.

  4. Actually, carnivorous animals on the ark could have eaten meat besides sea creatures, because God commanded Noah to take seven pairs of every clean animal and bird on the ark (Genesis 7:2-3)

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