When Jack Putz and his student decided to study root growth in southeastern pocket gopher (Geomys pinetis) tunnels, they didn’t realize how difficult it would be to isolate sections of the underground structures. First, they tried to block two ends off with aluminum plates. “And then we got outsmarted by a pocket gopher,” Putz, a biologist at University of Florida, tells The Scientist.
The pocket gopher, immortalized as Carl Spackler’s fossorial nemesis in the movie Caddyshack, dug around the researcher’s barricades, filled in the section isolated for study, blocked it off from surrounding tunnel connections, and proceeded to burrow elsewhere. In the end, to isolate short tunnel segments for study, Putz and his student Veronica Selden had to carefully dig in rings and bury hollowed-out cylindrical barrels that served as 360-degree blockades around 57 cm-long sections of tunnel.
Their hard work paid off, leading to the insight that significant root ingrowth into the tunnels provides a food source that the pocket gophers seem to cultivate, according to research published today (July 11) in Current Biology. “Pocket gophers . . . are actually farming roots in the extensive tunnel systems that they maintain presumably for that purpose,” says Putz.
The paper cites research showing that some species of ants, termites, and beetles engage in farming activities but notes that while experts may disagree on the semantics of what qualifies as farming, the pocket gopher is the first nonhuman mammal found to farm.
Pocket gophers dig tunnel networks with lengths totaling up to 160 meters, the authors write in their paper, and they remain within them for most of their lives to avoid surface predators. Digging this underground system requires hundreds to perhaps thousands of times more energy than walking over land, raising questions about how the rodents sustain themselves. When the researchers estimated the energy gophers expend dislodging and pushing soil out of a tunnel, they found it far exceeded the caloric value estimated from the mass of roots the mammals would initially encounter in the tunnels. “If pocket gophers depended on the root food that they encountered while [first] excavating, they would quickly starve to death,” says Putz.
The study took place on a longleaf pine savanna in Florida, where Putz says that roots don’t typically grow down to the depths of the gopher tunnels, which averaged about 40 cm under the surface in this study. However, after isolating 12 tunnel sections from gopher activity for up to 44 days each, roots from other surface plants such as sorrel (Rumex hastulatum) and common beggarticks (Bidens alba) filled the empty tubes. Putz says that the tunnels aerate the soil while the excrement that gophers spread within them fertilizes the soil, both of which likely foster deeper root growth. The initial absence of roots in the tunnel branches along with the lack of root detritus in the sandy mounds aboveground suggests the gophers had been cropping and eating them, the researchers say.
From the densities and growth rates they observed across all the tunnel sections, the researchers estimate that the roots provide enough biomass across mean tunnel lengths of 44 meters to satisfy, on average, 21 percent of the gophers’ daily caloric needs. Nonetheless, root ingrowth in each segment was highly variable, says Putz. For example, the high root growth in one tunnel was estimated to yield 60 percent of the calories a typical gopher would need to consume in a day.
The authors write that to compensate for the average energy deficit predicted from their root cultivation, gophers may employ three tactics: focused cropping in areas with high root densities, relying on high-calorie tubers, and seeking aboveground food sources. Elizabeth Parsons, a wildlife ecologist at the University of Georgia who was not associated with this research, says the gophers would tend to avoid the risk of surface foraging, and instead favor pulling entire plants directly into their tunnels from below, as they’re well-known to do.
Parsons points to the small sample size as one of this study’s main limitations. Southeastern pocket gophers tend to live in sandier soils like those of this savanna, but she says that’s not the case across their entire range of Florida, Georgia, and Alabama. These results, she says, might not apply in southern Alabama, for example, because the higher clay content in the soil there may change the energy dynamics.
Putz agrees that sample size was a limitation of the study, and says it could actually have resulted in underestimating the extent of the gophers’ root cultivation because he and Selden never found what likely comprises a high-calorie portion of the animals’ diet: Many plants in savannas have underground storage organs—called tap roots, tubers, or rhizome bulbs—that contain energy for plants that need to resprout frequently due to grazing or fire activity. “But I’m sure the pocket gophers are finding them, and when they find [what are basically] carrots or potatoes, that’s a goldmine for them.”
As to whether this research demonstrates that gophers farm, Parsons isn’t yet convinced. “I think for them to get the farming label there needs to be some intention and specificity—things that need to be worked out with future research.” For example, she says it would be good to know whether the animals specifically fertilize tunnels used for foraging as opposed to those where they tend to live. Parsons says it’s possible that pocket gophers are farmers, but that may depend on whether their activity fosters root growth or whether their cultivation targets specific plant species they prefer to eat—factors that are not yet known.
Putz agrees that the word farming carries connotations that may not apply, saying that “it’s controversial whether this constitutes farming or not because it really depends on how you define farming.” The authors write that gophers may be disqualified from this distinction simply by the fact that they don’t seed or weed their crops.
Regardless of whether gophers pass the farmer test, both Parsons and Putz agree these rodents are ecosystem engineers, alongside beavers and earthworms, and that they are wrongly vilified as pests. Instead, they say, their underground tunnels benefit the surrounding area: The aeration and fertilization from their tunnels creates healthy soils for plants. Parsons adds that pocket gophers create habitat for several beetle species that are only found within their tunnels, and she and colleagues have found evidence that their aboveground dirt mounds serve as cooling zones for several species of lizards and frogs.
Putz notes that the rodents’ populations are dwindling due to human-caused habitat loss. “I’m hoping that when the public hears about this research, they will appreciate pocket gophers in a new way.”