A Very Hoppy Spring

On a rainy warm spring night on Big Oaks National Wildlife Refuge the conditions are just right for a listener stationed along the road to hear a symphony of sound. The frogs are calling, filling the air with peeps and croaks and for one particular species, a drawn out snore that echoes through the grassland. That would be the call of the Crawfish frog, endangered in the state of Indiana and a species of importance at Big Oaks. Nicknamed the “Hoosier Frog”, their presence here extends the range of the species by 56 miles further than previously thought. The Big Oaks population is small, with less than 500 individuals estimated. Small populations are vulnerable to extinction because there is little genetic diversity to withstand a drastic change. Crawfish frogs get their name due to their use of abandoned crayfish burrows which they live in for most of the year. During the spring, they travel up to half a mile to the breeding ponds then return back to the very same burrows. Crawfish frogs prefer ephemeral ponds which fill during the rainy season. This keeps predators such as fish out of the ponds and the waters warm for egg development.

Big Oaks manages for Crawfish frogs in several ways. Crawfish frogs are only in the breeding ponds for about one month a year but that month is vital to the success of the species. These frogs differ from other species in their preference and success in ephemeral ponds. The leftover bomb craters from Jefferson Proving Ground munitions testing are often selected by the frogs. Ponds that are sometimes as small as a bathtub are chosen by the frogs likely because they mimic the water filled bison wallows that used to dot the landscape before European settlers. Additionally, refuge staff have installed water control structures on many ponds to artificially drain and fill them each year for the breeding season. During March and April nocturnal surveys are conducted where staff listen at night to hear which ponds the frogs are calling at and during the day ponds with active calling are checked for egg masses. Each call is representative of one male and each egg mass represents a female so combining the two numbers gives a population estimate for adult frogs on the refuge.

Each egg mass contains 5,000 – 7,000 eggs. As the tadpoles develop, success in each phase of their life is documented until they hopefully return to those ponds as adults to breed and carry on the population. Once the eggs hatch they float around the egg mass they hatched from, feeding on the algae and leftover nutrients from the egg mass. Once they are free swimming tadpoles it is easier to escape predators and they become less vulnerable. From tadpoles to adult frogs there is a steep decline in individuals that survive. Despite the high number of eggs laid, only about 3 or 4 per mass are estimated to make it to breeding age (2-3 years old).

As an intern at Big Oaks, around 70% of my time is devoted to Crawfish frog monitoring and management. I conducted nocturnal call surveys, walked around the ponds daily during the height of egg mass laying to keep track of which ponds had eggs, and continue to monitor tadpoles. We are head starting tadpoles at a select number of ponds and I feed them algae tablets and monitor their growth and progress. Before breeding season, we captured frogs at some of the ponds and installed PIT tags (essentially a microchip that can be scanned when a frog is recaptured). Handling frogs and performing the PIT tag insertion was an incredible experience and is a skill I can now put on my resume. Amphibian work requires one to be careful and thorough especially with such a threatened species. Diseases spread easily to amphibians and the field team carefully decontaminates our equipment between ponds to minimize the risk.

Management tools for Crawfish frogs are varied and supported by data and communication with state and nationwide scientists. The draining and filling of ponds has a positive effect on egg laying by Crawfish Frogs because predators such as fish or salamanders don’t stick around in ponds that go completely dry. In ponds that aren’t able to completely drain, occasionally egg masses are placed in protective containment to develop and hatch out of the reach of predators. Burning the grasslands where Crawfish frogs burrow keeps them open and clear of shrubs. Burning is done selectively before breeding season or after an area is done and there is no calling so the frogs have ample time to return to their burrows safely. Refuge staff constantly evolve and test management tools and the efforts ensure the success of this species at Big Oaks.

Written by Gretel Baur, biology intern spring 2022

Images: Crawfish frog egg mass, egg masses in a container, tadpoles in shallow water, and adult crawfish frog (handled under proper permitting)