As far back as middle school we are taught the term symbiosis, a biological term that generally means a close, prolonged relationship between two different organisms living in close physical association in which each benefits from the other. A classic example of symbiosis is that which exists between clownfish and sea anemones. The clownfish secretes a substance that protects it from the anemone’s sting and provides it a safe haven as it swims among the anemone’s tentacles. In return, the clownfish attracts other fish that the anemone then kills and eats. Both species benefit.

Perhaps fewer of us are familiar with the term commensalism, a type of symbiotic relationship in which one species benefits and the other species is not affected. A good example of commensalism is a bird nesting in a tree. The bird benefits because it gets a safe place to raise its young, but it neither helps nor harms the tree.

In southern South Carolina, an interesting commensal relationship exists between gopher frogs and gopher tortoises. The gopher tortoise, Gopherus polyphemus, is the only turtle in the Southeast that digs its own burrow. These large, deep burrows then provide a safe haven for gopher frogs, Rana capito. Gopher frogs also use the burrows as part of a feeding strategy as they sit near the entrance to the hole, hunting for prey. The frog benefits; the tortoise does not.

Gopher tortoises are considered a keystone species, meaning their extinction would have a significant negative impact on the ecosystem, because gopher frogs are not the only animals to benefit from the tortoises’ burrows. As many as an estimated 250 other species — some claim as many as 350 species — have been documented using the tortoise tunnels, including several species of snakes and small mammals.

In South Carolina, gopher tortoises are found almost exclusively in Jasper, Hampton, and Aiken counties. However, they range throughout Florida and west to southwest Louisiana. They have grayish black domed shells that can grow up to 12 inches in length. Their front legs are wide and flat, featuring large, flat nails for digging. Their digging skills set them apart from other native tortoises. Gopher tortoise burrows can be more than 1 foot wide and 15 feet long. The tunnel angles downward as far as 6 feet below the surface. Gopher tortoises spend much of their time in their burrows, venturing out mainly to breed and eat.

Like many tortoise species, gopher tortoises are long-lived, with a lifespan reaching 40 to 60 years. Incredibly, a gopher tortoise now living in captivity is purported to be more than 100 years old. Sexual maturity is usually reached at 15 to 20 years of age. Gopher tortoises typically breed only once each year, usually in warmer months. Females may lay anywhere between one and 25 eggs, which hatch 70 to 100 days later. Like many reptiles, the sex of the eggs is determined by the temperature. Females are produced in warmer sand while males predominate in cooler sand. 

Gopher frogs are small, stout frogs, typically only 3 inches long with wide bodies, short legs, and pointy noses. Their coloring is quite variable, changing with their range and sex. Unlike the relatively small range of the gopher tortoise, gopher frogs can be found throughout the Coastal Plains from Alabama to North Carolina. Their preferred habitat is longleaf pine and scrub forests with sandy soil. Outside the range of gopher tortoises, gopher frogs may find shelter in the burrows of small mammals or any available tunnel. Their diet is comprised primarily of small insects, spiders, and worms. Gopher frogs breed in the spring. Females may lay up to 2,000 eggs in shallow water. Water temperature effects their metamorphosis — egg to tadpole to adult — usually in 4 to 7 months. They live approximately 7 years.

Gopher frogs are endangered in South Carolina, primarily due to the loss of wetlands. Riverbanks Zoo has partnered with the S.C. Department of Natural Resources in a “head start” program to raise and release gopher frogs in the Lowcountry. The program involves collecting portions of an egg mass in the wild, then taking them to Riverbanks Zoo where they are raised through metamorphosis. The young frogs are then released back into the wild where hopefully they will thrive.

Finally, I must spend a moment highlighting the commensal relationship between gopher tortoises and the beautiful Eastern indigo snake. These large, nonvenomous snakes can reach up to 8 feet in length, and while they cannot be found in South Carolina, they range throughout Florida and southeast Georgia. Gopher tortoise burrows provide the snakes with protection from predators and may even be used as a nesting site for female indigo snakes.

So now you know … commensalism is yet another piece of our incredible natural history fabric!