12 minute read
Root Farming Pocket Gophers
Written by F.E. “Jack” Putz
Other than in Saturday morning cartoon carrot-pulling contests with denim overall-clad farmers and a more nefarious portrayal in the movie Caddyshack, few people ever catch a glimpse of a Southeastern pocket gopher (Geomys pinetis). Also appropriately known as ‘sandy mounders,’ gophers are fossorial root-eating rodents that individually excavate, maintain, and defend tunnel systems that can be up to three-hundred feet long. By turning over so much soil, they clearly qualify as ecosystem engineers, but it was previously not known why they work so hard to maintain such long tunnels. It is less clear what effects their tunneling, sand mounding, and root eating have on vegetation. Judging from the presence of sandy mounds on turf grass lawns it appears that this native herbivore is averse to exotics, but their species preferences remain to be determined.
While I am fascinated with gophers and happily recall the few times I witnessed them at work, the arsenal of available gopher eradication devices and techniques indicates that this appreciation is not widely shared. Perhaps gophers would garner more respect if people realized that the sand in those mounds was pushed and carried up from tunnels excavated two feet or so below the surface. Their tunnels are deep enough to not collapse even under horse hooves but are also far below the reach of the roots on which gophers depend for sustenance.
This gopher story starts when Veronica Selden, then an undergraduate at University of Florida, and I were in a field near my house sitting and sweating on the edge of a massive hole we’d excavated to expose a gopher tunnel. Although she’d taken two botany classes with me, she stubbornly persisted in being more interested in animals, which is why we were armed with long-handled spades in the quest for a pocket gopher-related research project. Various experts advised us that gopher tunnels were easily located by inserting a metal rod into the soil, but perhaps because the tunnels are so deep, we both failed to master that technique, hence all the digging.
Once our breaths were caught, our core temperatures had descended below the danger zone, and our higher brain activities somewhat recovered we both, seemingly simultaneously, experienced ‘eureka’ moments. I don’t recall exactly how either of us initially expressed our shared revelation, but the idea cooked down to the following hypothesis: Southeastern pocket gophers farm roots in their tunnels. I’m not sure of the source of Veronica’s inspiration but mine was clearly related to the not-so-pleasant task of having to deal with a root-clogged sewer: like sewers, gopher tunnel interiors are dark, moist, and nutrient-rich for the same reason (note that unlike some other species of gopher, ours scatters its feces rather than depositing them in dedicated fecal chambers).
Testing the root-farming gopher hypothesis required that we exclude root-munching gophers from tunnel segments for a few weeks, re-open the tunnels, and measure in-growing roots, if any. Gophers were plentiful where we conducted this experiment in abandoned pastures being restored to longleaf pine savanna on the common lands of Flamingo Hammock Land Trust near Gainesville, Florida. Stipulating that gophers could be inconvenienced but not injured, my neighbors were happy to host our research.
Our initial exclusionary efforts failed because the inconvenienced gophers dug around our barriers and refilled the previously blocked sections of tunnel. I must report that my neighbors and even family members seemed delighted that their resident scientists were outsmarted by a rodent. In our defense I pointed out that the outsmarter was a fossorial rodent not some common surface-dwelling alley rat.
After several energetically expensive failures we combined our intellectual strengths and came up with a way to outsmart and outmaneuver the gophers; as a gopher exclusion device (GED) we employed a fifty-five-gallon drum with its ends cut off. We also streamlined our excavatory efforts by upgrading from broad-bucketed analogue geotomes (which definitely sounds more scientific than ‘spades’ or ‘shovels’) to narrow-bladed trenching tools. Our refined experimental procedure consisted of digging a straight slit trench until we intercepted a gopher tunnel. After determining the tunnel’s direction, we dug a circular slit trench that matched the GED’s perimeter and penetrated well below the tunnel. Before sliding the barrel over the exposed monolith of undisturbed soil, Veronica used her fiber-optic-equipped cellphone to search for roots in the tunnel while I collected volumetric soil samples adjacent to the exposed tunnel to assess root availability; I also enumerated all plant species present within the GED. After that, we gently slid the GED down over the collapse-prone cylindrical soil monolith to block both ends of the tunnel, backfilled the trench, and waited seventeen to forty-four days before doing the opposite. If this all sounds easy, go ahead and try it yourself!
I’m proud to report that this research collaboration between a young student and a very senior professor was a model in terms of shared duties. I supplied the geotomes, did a lot of digging, and convinced my neighbor Ken to provide the GEDs; I would also have covered any monetary expenses but there weren’t any. I also wandered around looking for the fresh gopher mounds that indicated the presence of potential (if unwitting) project participants. Perhaps my most important contribution was that throughout the research process, from hypothesis formulation through successful publication in a top-tier international peer-reviewed journal, I provided supervisory services as well as copious amounts of sage advice on a wide variety of topics not exclusively related to gopher biology. For her part, Veronica assumed the easier tasks of reading everything ever published about pocket gophers, studying mammalian energetics, curating and analyzing the data, creating the graphs, illustrating the story, and drafting the manuscript.
My painstaking efforts to document the plant species represented within the perimeters of the GEDs revealed few insights about gopher feeding preferences. Instead, the composition of the vegetation reflected what was found in the fields. Present were common natives, like elephant’s foot (Elephantopus elatus), yellow woodsorrel (Oxalis corniculata), and romerillo (Bidens alba), with too much cover by bahiagrass (Paspalum notatum), a persistent exotic species that seems to be thwarting our pine savanna restoration efforts.
Veronica searched the freshly opened tunnels for roots but never found any. What she did find in a single tunnel was evidence of some surface feeding and food caching in the form of bahiagrass stolons and some fragments of other species that I couldn’t identify. In contrast, re-opening of gopher-excluded tunnels revealed profuse proliferations of roots. Fresh white roots dangled from tunnel ceilings like stalactites and papered tunnel walls and floors (for a short video taken with Veronica’s fiber optics see: https://vimeo.com/786373819). Importantly, root masses in tunnel-adjacent soil samples were so low that a gopher who tried to feed itself by digging tunnels would soon starve. This conclusion derives from digging being 360-3,400 times more energetically costly than walking, a fact obvious to all shovel wielders. Fortunately, roots growing into their tunnels supply gophers with an extra 0.42 kJ of food energy per meter. That estimate is based on a fine root energy content of 18.39 kJ/g, fifty-six percent digestibility, and fifty-four percent assimilation efficiency (if a reader wants further explanation of these or any other numbers and calculations in this article, I could help but for purely pedagogical purposes would instead put you in contact with Veronica). Over the length of a one hundred forty-five-foot-long tunnel, in-growing roots supply your average gopher, one with a basal metabolic rate of 0.852 cm3O2/hour, with an average of twenty-one percent but up to sixty-two percent of its daily energetic needs.
We suspect that even sixty-two percent of daily energetic needs underestimates the contributions of farmed roots to gopher diets. One reason for this conclusion is that although we didn’t purposefully leave the GEDs in place for different lengths of time, we found that rates of root in-growth decreased with exclusion duration. It appears that constant cropping stimulates root growth to levels beyond what we measured. Another factor to consider is that despite not being able to detect roots through more than half an inch of soil, gophers are certainly smart enough to concentrate tunnel-making in root-rich areas. Furthermore, the succulent young roots that grow into tunnels are probably more digestible than the generic literature value of fifty-six percent.
Even if gophers satisfy half their energetic needs with farmed roots and derive an additional fraction during tunnel excavation, it remains a mystery where they obtain the rest of their food. Perhaps they pull entire plants down into their tunnels, as shown in the cartoons but also captured live by our colleague Norm Douglas: https://vimeo.com/786372221. We don’t even know whether they have preferences among the species that extend roots into their tunnels. For example, how important to their diets are deep-rooted species with starchy roots such as tread softlies (Cnidoscolus stimulosus)? When they encounter a tuber do our gophers behave like the naked mole rats of Africa (Heterocephalus glaber) and eat only the inside leaving the outer portions to resprout? Species with tubers and other sorts of underground storage organs abound in our pine savannas but their contributions to gopher diets remain to be assessed. Although gophers also forage a bit when they emerge above the surface to mound their tunnel spoils, fear-of-predation minimizes their use of this potential source of sustenance. Their eyesight is apparently not very good, but I also wonder whether they take the opportunity while aboveground to map out future tunnels so as to increase the likelihood of interception with preferred food species.
Much remains to be learned about our pocket gophers. Firstly, do they eschew worms? Other herbivores comfortably slip over into carnivory, such as arthropod-snapping hummingbirds and egg- enjoying cows. I’ve also witnessed ardent vegetarians suffering severe bacon envy. Furthermore, the worms, grubs, and burrowing crickets that undoubtedly wiggle, slither, stumble, and fall into gopher tunnels would add diversity and protein to a gopher’s root diet. Our common moles (Scalopus aquaticus), confirmed carnivores with high metabolic rates and voracious appetites, dig prey but also listen for those that drop into their much more superficial tunnels. I also wonder whether gophers eat the fungal hyphae that undoubtedly proliferate in their dark, moist, temperature controlled, and fertile tunnels. Whatever it is that they eat, someone should determine whether their tunneling activity varies with their reproductive status, food availability, or seasonal temperature changes.
Our claim that Southeastern pocket gophers are the first described non-human mammalian farmers stirred up a great deal of controversy. Although species of fungus-growing ants, termites, and beetles are generally accepted as farmers, they all plant their crops whereas gophers don’t. Gophers do till and fertilize the soil and their pruning seems to stimulate root growth, but they are not propagators. To my mind their failure to sow should not keep them from the ranks of the world’s farmers, but my definition of agriculture includes forest farming and not just the cultivation of annual grains and other crops. Seed cropping originated multiple times in human societies around the world where seasonal cold or dry periods favored annual plants over perennials. In the tropics and wherever else trees reign, forest farming typically involves land-use practices much more subtle than plowing and planting. Instead, long-lived plants including trees, shrubs, and lianas, are gently tended where they are found; this tending may continue for decades or even centuries. The resulting domesticated forests are enriched in species that provide people with food, fiber, glue, building materials, and medicines. The taxonomically unwary might mistake these anthropogenic forests as untouched by humans, but they would be sorely mistaken.
We hope that our research on Southeastern pocket gophers stimulates others to study these amazing native mammals. Insightful and patient naturalists can learn a great deal about how they influence our ecosystems and our native and not so native plant species. I also hope that researchers elsewhere in the world will determine whether any of the dozens of other fossorial root-eating mammal species around the world farm. Included among the suspected root crop cultivators are the eleven other species of North American Geomys, African naked mole rats, Tibetan zokors (Myospalax fontanierii), and Australian wombats (Vombatus ursinus and Lasiorhinus spp.), although I expect the latter are too lazy to farm. Finally, hopefully by highlighting some aspects of their natural history, respect will be engendered for Southeastern pocket gophers, a much-maligned native species with which we should willingly share our landscapes.
References and Further Reading
Selden, Veronica, and Francis E. Putz. 2022. Root cropping by pocket gophers. Current Biology 32:R734-735. https://doi.org/10.1016/j.cub.2022.06.003.
Zhang, Yanming, Zhibin Zhang, and Jike Liu. 2003. Burrowing rodents as ecosystem engineers: the ecology and management of plateau zokors Myospalax fontanierii in alpine meadow ecosystems on the Tibetan Plateau. Mammal Review 33: 284-294. https://doi.org/10.1046/j.1365-2907.2003.00020.x.
Reichman, O. J., and Eric W. Seabloom. 2002. The role of pocket gophers as subterranean ecosystem engineers. Trends in Ecology & Evolution 17: 44-49. https://doi.org/10.1016/S0169-5347(01)02329-1.
Diaz-Torribo, Milton, and Francis E. Putz. 2022. Time to dig: Under-ground storage organs on plants in fire-maintained pine savannas. The Palmetto 23: 10-12.
About the Author
F.E. “Jack” Putz is a Distinguished Professor of Biology at the University of Florida and author of several books including Finding Home in the Sandy Lands of the South, Yaupon Wins: An Ethnobotanical Novella, From One Old Dog to Another: Poems, and (as Juan Camilo Moro) Borneo Dammed: A Very Family Affair. He’s also published hundreds of technical articles about plant ecology, savanna restoration, and tropical forestry, but they are much less fun to read.
