By Dirk J. Stevenson
In 2018-2019, I surveyed for rare invertebrates on Alligator Creek Wildlife Management Area near Lumber City. The search included obligate commensals of gopher tortoise burrows, creatures found only in these burrows. Here’s what I found. (For more, see the post “Down the Tortoise Hole.”)
Lying on a massive gopher tortoise burrow apron with my head resting on the sand just outside the mouth of the hole, I strain and extend my arm as far into the tunnel as I can. I am using a hi-tech contraption: a 6-foot mop handle with a 24-ounce plastic soda mug secured by zip ties. The tool, an idea borrowed from a graduate student, is made to harvest scoops of sand from deep in the burrow.
At dawn, the humidity is near 100 percent. The gorgeous plants of this aeolian-dune “sugar-sand” sandhill – gopher apple, woody goldenrod, prickly pear and the wonderfully aromatic scarlet basil – are damp with condensation. After sampling seven large burrows and not finding my quarry, I am dripping with sweat, covered with sand and questioning my sanity.
The exuberant but unending vocalizations of a nearby yellow-breasted chat run through my brain like a bad Top-40 tune. (I recently light-trapped for moths here: The same loony-crooner sings at night).
Then at my eighth burrow, pay dirt. My scooper scoops up several sizeable wads of fresh tortoise dung!
The scat is fresh and as wet-slick as pan-cooked spinach, but not disagreeable in odor. Composed almost entirely of the broad-leaved grasses tortoises stalk and peck, the dung weathers into tight packets of dried vegetation similar to a plug of chewing tobacco. Some tortoise scholars call them “field cigars.”
Donning glasses, I eagerly pick through the dung with my fingers. And with an adrenalin rush, I find what I’m looking for: several individuals each of three different species of obligate commensal beetle species.
All are less than 10 millimeters long. There are two scarabs (Alloblackburneus troglodytes and Onthophagus polyphemi) and a histerid (Chelyoxenus xerobatis). The scientific names hint at their love of xeric habitats and even their penchant for the subterranean lairs of tortoises. (Full disclosure: My beetle identifications were confirmed by a real entomologist, Dr. Rick Hoebeke of UGA’s Museum of Natural History.)
Also, when I dropped my backpack on the ground near the burrow mouth, a cloud of small flies emerged like a puff of smoke. The anthyomiid flies were gopher tortoise burrow flies (Eutrichota gopheri). Growing no longer than 7 millimeters, they are an important prey, and maybe the main staple, for the gopher tortoise burrow robber fly (Machimus polyphemid).
Robber flies are the raptors of the insect world, darting from perches to capture live arthropod prey. Although a few western U.S. species inhabit burrows (one favors badger burrows), it is highly unusual for a robber fly to develop a strong attachment or dependence on an animal burrow.
I had long wanted to see this golden-haired species in the wild and was thrilled when one flew from a burrow at Alligator Creek as I withdrew my scooper. Apparently, the zip ties used to attach the cup tickled the burrow’s roof where the fly hides, perched on bare sand or tiny rootlets, causing it to leave.
The behavior of this robber, and four others I found at separate burrows, was just what you might expect from a true obligate commensal. Each ventured no farther than about 5 feet from the burrow mouth and after a few moments flew directly back to hole. They were clearly uncomfortable being outside the security of the burrow shaft for long.
As you might can guess, I have spent an inordinate amount of my life visiting tortoise burrows and pondering the wonderful forms of life, in addition to the gopher, that sometimes or always call the burrows home. When you are in the field, take time to pause on a tortoise burrow apron and drink in the milieu, of time, of sand.
Tortoises have been digging in south Georgia a lot longer than we have. Let’s do all we can to help conserve this endearing species and the creatures that depend on it.
Dirk J. Stevenson is a naturalist and owner of Altamaha Environmental Consulting.
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