What’s Going On Underground With Gophers?

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Did you cozy with a book or movie over the winter holidays? Maybe, as seems to be very popular right now, an intriguing whodunit. Well, we’re back with a different kind of mystery. And it’s all about gophers. Yes, there is a mystery about gophers. Here on the case is SciFri sleuth Christie Taylor. Hey there, Christie.

CHRISTIE TAYLOR: Hey there, Ira.

IRA FLATOW: You know what comes to mind when I think about gophers? Not a mystery, but that crazy gopher-filled movie, Caddyshack. You know what I mean?

[LAUGHTER]

CHRISTIE TAYLOR: That’s fair. A little slapstick, but it makes sense when you think of how these animals have been considered pests by so many people. But they’re also really impressive little animals, Ira. Their tunnel networks are absolutely massive, even for just one gopher, which is good for aerating soil. Ecologists consider them really important ecosystem engineers.

IRA FLATOW: Yes, and we like ecosystem engineers.

CHRISTIE TAYLOR: Yeah, we do. And gophers are also mysterious. We don’t know much about them because they almost never leave those tunnels, not even to eat.

IRA FLATOW: Well, wait a minute. If they don’t leave, then what do they eat? How do they eat?

CHRISTIE TAYLOR: That’s the million dollar question, Ira. The leading theory has long been that they simply eat roots while they dig tunnels. Need more roots? Dig another tunnel. But a team at the University of Florida looked into that, and they’re now questioning whether there’s more to the story. Their work was published in Current Biology this summer.

I talked to Veronica Selden. She was an undergraduate when she did this project, and it was her idea in the first place. I also talked to her advisor, biology professor Dr. Jack Putz. Here’s Veronica explaining why she started looking at gophers, also known as pocket gophers, more closely.

VERONICA SELDEN: Pocket gophers are fossorial rodents. They spend nearly their entire lives excavating underground tunnel networks. And that excavation is an energetically costly activity. But in terms of length, the tunnel systems can extend up to 160 meters long.

CHRISTIE TAYLOR: Wow.

VERONICA SELDEN: I was intrigued by the question of how they obtain enough food to sustain that activity. And we also wondered why they would invest so much energy into excavating and maintaining and defending such extensive tunnel systems when they’re solitary animals and theoretically don’t need that amount of space.

CHRISTIE TAYLOR: How do you go about figuring out, then, how much energy it takes to dig out a tunnel like that?

VERONICA SELDEN: We found and excavated into pocket gopher tunnels and took measurements of the tunnel radius and took soil samples from adjacent to the tunnel. And with that, we were able to calculate the soil bulk density. And using the dimensions of the tunnel, we were able to calculate using a preexisting equation how much energy it would take to excavate a tunnel of a certain volume.

CHRISTIE TAYLOR: Jack, as her advisor, as Veronica’s advisor, what made you interested in this question about pocket gophers and the energy they consume, or the energy that they need to do what they do?

JACK PUTZ: These are animals that are common in many places. You see their mounds, but you very seldom see them. I mean, anything that’s spending its life below ground is just intriguing. When we first went out to excavate a tunnel, we really had no idea what we were doing. And we were surprised at how deep we had to go before actually encountering a tunnel. And we noticed that at that depth, there are very few roots.

So they’re down below the volume of soil with dense roots, and so it made us think about, well, where are they getting their food if there’s so few roots down there? When they come to the surface to move out the material that they excavated from their tunnels, they’re subjected to predators. Birds and snakes and everything else eats pocket gophers. So they want to stay down low, but there’s not much food down there.

CHRISTIE TAYLOR: So at the end of the day, you dug out the tunnels. You did the math. Veronica, are they getting enough food from the roots that they eat when they’re excavating these tunnels?

VERONICA SELDEN: We found that on average they are not getting enough energy from excavation to sustain that excavation. There is an energy deficit there.

CHRISTIE TAYLOR: How big is the deficit?

VERONICA SELDEN: On average, nine kilojoules. So they’re using nine kilojoules more than they’re gaining.

CHRISTIE TAYLOR: So how do we explain, then– they’re digging their tunnel, they’re chomping roots while they dig the tunnel, and that’s not providing enough food for them. How do we begin to explain, then, where they’re getting the rest of the energy they need to stay alive?

VERONICA SELDEN: We knew that roots would grow into sewer systems, a situation similar to a pocket gopher tunnel. So we looked into whether roots would grow into the gopher tunnels and perhaps if that could sustain their lifestyle. So to test that, we isolated sections of tunnel, preventing gophers from entering by blocking all sides. Before the isolation period, we confirmed that there were no roots visible in the tunnel by filming inside with a borescope.

After the period of isolation, we filmed inside the tunnel again, and we were greeted with a vision of many roots growing into the tunnel. And we found that on average, this root in growth, daily root in growth, can account for 21% of their daily basal energetic needs. That’s obviously not covering the entirety of their needs. So there is still an unsolved question of where are they getting the rest of their energy. But it does paint a more complete picture of how they’re sustaining themselves.

CHRISTIE TAYLOR: They’re not just eating the roots once when they dig out the tunnel, but also more roots would grow into the tunnel for them to sort of snack on as they’re going about their days.

VERONICA SELDEN: Yes.

CHRISTIE TAYLOR: Jack and Veronica, you also, in your paper, suggest that this behavior could be considered farming by the gophers. They’re creating conditions that allow these roots, their food, to grow better than it might otherwise. Do other biologists agree with this definition?

JACK PUTZ: That’s been so interesting to see the range of reactions that basically go back to how you define farming. Some people restrict farming to annual cropping of grains, but there’s other kinds of farming that I would consider legitimate uses of that term, where people who live in forested ecosystems farm forests by promoting the growth of trees and vines and shrubs that they then harvest. In the case of pocket gophers, as you said, they’re creating conditions appropriate for the growth of routes that favor the growth of roots, and then they’re cropping those roots.

And actually, in the cropping process, they may be stimulating the production of new roots and new roots that are more digestible than the ones that they excavated because they’re younger, and they’d be more succulent. And so that, together, for me and for about half the people who have contacted us, that does constitute a form of farming.

But what’s exciting is to think about an animal that’s accepted as an ecosystem engineer. I mean, they are turning over soil. They are having a big effect on the ecosystem. But considering that animal as actively farming roots, I just find exciting.

CHRISTIE TAYLOR: Veronica, how are you feeling about this research? You got it published before you’d even finished your undergraduate? I mean, that must be exciting.

VERONICA SELDEN: That was definitely very exciting. I enjoyed so much getting to learn about these gophers. As Jack said, they are considered ecosystem engineers. I hope that even if we don’t sufficiently convince everybody that they are farming, we can at least promote appreciation for the complex life cycles of these little animals underground.

CHRISTIE TAYLOR: What kind of research would maybe further clarify what’s going on with what they’re eating?

JACK PUTZ: I would like to see people doing more detailed work that would allow assessment of what roots are they preferring. We knew which species there were above ground, but not which species they’re eating. We also don’t know whether other fossorial burial mammals like pocket gophers are doing something that’s very similar, but we suspect that they are. And almost everywhere in the world, there’s an animal like this. But as far as I’m aware, no one has studied this aspect of their natural history.

Having cameras, getting ideas of activity cycles, and we don’t know, for example, why they’re digging when they’re digging, whether the amount of tunnel excavation varies seasonally, there’s so many mysteries about these animals. It’s just hard to study an animal that is below ground, especially that far below ground. But I think there’s a lot of research that could be done and should be done. And we hope that this project stimulates that kind of activity.

CHRISTIE TAYLOR: Thank you both so much for joining me today.

JACK PUTZ: You’re very welcome.

VERONICA SELDEN: Thank you again for having us.

CHRISTIE TAYLOR: That was Veronica Selden and the University of Florida’s Dr. Jack Putz. Their research on pocket gophers appeared in Current Biology in July. I’m Christie Taylor.

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