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The Crawfish Frog’s Jaw

My cell phone rang. Jen Heemeyer, who was radio tracking Crawfish Frogs exiting wetlands following their spring breeding efforts, was over-the-top excited: Frog 160 returned to her burrow, the same one she occupied all last summer. With these two facts—that Crawfish Frogs not only occupy a single crayfish burrow throughout the year but will return to that very same burrow following breeding the next year—Jen had discovered that each Crawfish Frog has a home, a place on the planet that is uniquely theirs. Jen also understood that for this to be true, there must be some special relationship that we did not yet understand between Crawfish Frogs and the abandoned crayfish burrows they occupy.

I had recruited Jen, Vanessa Kinney, and Nate Engbrecht to be the core grad student team of researchers responsible for the Crawfish Frog project; after they graduated, I brought in Rochelle Stiles to continue the project. Our funding came through a State Wildlife Grant (SWG) administered through the US Fish and Wildlife Service. The SWG program is designed to ameliorate local-level conservation issues before they grow to become national problems requiring Endangered Species Act attention. Biologists from the Indiana Department of Natural Resources had approached me about leading the Crawfish Frog effort because these secretive frogs had been recently listed as state endangered in Indiana; I had a long, successful track record in amphibian field research; and I had the expertise after recently editing the compendium Amphibian Declines: The Conservation Status of United States Species (Lannoo 2005).

Crawfish Frogs (Rana areolata) derive their common name from their tendency to inhabit crayfish-dug burrows. Jen had shown during her previous field season that once a Crawfish Frog settled into its burrow following breeding, it rarely left—it was usually in its burrow or on a small bare spot next to its burrow, its feeding platform. We would later discover that Crawfish Frogs will occupy the same burrow for up to five years.

Our first observation was Crawfish Frogs almost always face their burrow entrance—when they’re in their burrow they look up, toward the sky; when they’re on their feeding platform they face the burrow entrance. Their burrow entrance was clearly the center of their attention, if not their universe.

As Jen tracked her frogs, she became concerned because she never saw Frog 460. While a radio signal was coming from his burrow, Jen could not determine whether 460 was alive, dead, or had shed the transmitter and moved on. We purchased a wildlife camera, set up a surveillance on his burrow, and immediately realized 460 was fine, almost always out on his feeding platform except when Jen approached. More importantly, we discovered the camera followed 460’s activities with a high degree of resolution. We had a eureka moment as we realized that for as long as Crawfish Frogs inhabited identified burrows, cameras could track their aboveground behavior. We immediately bought a dozen units and deployed them facing the burrows of Jen’s telemetered animals.

The burrows Jen’s Crawfish Frogs inhabited were always dug by upland crayfish and therefore could be located hundreds of meters from wetlands. These burrows are deep (1–1.5 m)—crayfish keep digging until they reach groundwater— and narrow. After some time, crayfish will abandon these burrows and dig fresh ones; up to 40% of upland crayfish burrows across a landscape are vacant. These empty tunnels constitute the Crawfish Frog housing market.

Our second observation was astonishing and completely unexpected. We discovered that Crawfish Frogs in crayfish burrows survive snake attacks, even prolonged and determined assaults. We can think of no other North American frog species that can endure an aggressive attack by a big snake such as an adult Black Racer.

As we thought through the reasons for Crawfish Frog burrow habitation, we were assisted by Crystal Thompson’s 1915 publication. She noted that Crawfish Frogs so nearly approximate the size of the [entrance to their] holes that the rubbing of their soft bodies probably tends to smooth the walls; the exceptionally powerful hind limbs and the extent to which they can distend the body serving to secure them so firmly that they could be mutilated before being dislodged; and when alarmed the frogs do not ordinarily descend far into the burrows (where the burrow approximates their body size). Thompson’s observations, combined with our wildlife camera data, suggested that when frightened, Crawfish Frogs on their feeding platform employ a fixed-action pattern form of behavioral response comprising jumping into their burrow, turning around to face the entrance, puffing up their bodies, and lowering their heads. This scenario was confirmed when the strategy misfired.

Nate was covering for Jen. It was just after dawn, and Nate found Frog 520’s signal coming from a location away from her burrow and moving much faster than a Crawfish Frog could hop. Nate caught up, pounced, and came up with an Eastern Hognose Snake sporting a suspicious midgut bulge. The terrified snake simultaneously pooped and puked. And up came 520’s carcass.

It’s often what doesn’t fit that proves the most insightful. What didn’t fit here was that 520 was missing its head, and it was obvious from the lacerated tissues that her head had been ripped off her body. Unlike sharks and crocodiles, snakes do not macerate their prey, so what had happened here? Since 520 was attacked head on, she must have been in her burrow. A frog frightened on land will flee, and therefore be attacked at the rear of its body, usually bitten on and secured by its hind legs. Frog 520 had no hind limb punctures or lacerations. Our best guess was that during the attack, 520 was in her burrow, facing the entrance. She had lowered her head, inflated her body, wedged herself against her burrow wall, and put up a great resistance. At some point, though, we think she made a mistake— she must have lifted her head. The snake got purchase on her snout, began tugging on the inflated and wedged frog, and eventually tore off her head. The decapitated frog then deflated, the snake ingested its body, and a couple hours later Nate showed up. The lesson here is that crayfish burrows and the behaviors Crawfish Frogs employ while using them allow these frogs to withstand even the most aggressive attacks by snakes, as long as they keep their head down.

At the same time Jen was conducting her telemetry study, Vanessa was monitoring drift fence arrays at Crawfish Frog breeding ponds. Vanessa noticed that the smallest breeding Crawfish Frogs were about as heavy in grams as they were long in millimeters—for example a frog might weigh 94 g and be 95 mm long or weigh 102 g and measure 100 mm. The numbers of the largest Crawfish Frogs, however, were always mismatched, with weight values consistently exceeding lengths—for example, 122 g and 110 mm. Further, these frogs often looked beat up. We couldn’t have known it at the time, but a few of these animals were likely close to a decade old. For example, during our main study, which lasted from 2009 through 2016, Frog 53 bred every year. Crawfish Frogs mature at 2 or 3 years old, which would have made 53 at least 10 years old in 2016. We have similar data on other frogs. In his book The Ecology and Behavior of Amphibians (Wells 2007), Kent Wells points out that among frogs and toads, longevity is generally proportional to body size (large frogs live longer), but his data also suggest that the longest-lived ranids are aquatic—able to submerge to avoid predators. In a sense then, descending into crayfish burrows provides cover for Crawfish Frogs in the same way that submerging does for big aquatic ranids. For this to work, though, Crawfish Frogs had to add the behaviors of facing the burrow entrance, inflating their bodies, and lowering their heads. This is a pretty interesting narrative, and at this point we considered the story completed. Then, as they always seem to, Crawfish Frogs sprung another surprise.

In 2011, Nate led a morphological study of Crawfish Frogs. He wanted to know whether these bulky frogs, derived from more graceful ancestors, possessed morphological specializations attributable to burrow dwelling. He knew he needed to examine both adult morphology and developmental patterns among the four species in the subgenus Nenirana, which includes Crawfish Frogs and their closest relatives, Pickerel Frogs, Carolina Gopher Frogs, and Dusky Gopher Frogs. Nate’s results showed that Pickerel Frogs have the smallest bodies, while Crawfish Frogs, especially those in the northern portion of their range, have the largest. His data also showed that the two gopher frog species and Crawfish Frogs were thicker and had more rounded heads and shorter hind limbs than Pickerel Frogs.

The developmental shifts Nate measured were more illuminating. With growth, Pickerel Frog heads get proportionately smaller (length and width) compared with their bodies (this happens in humans, also). Similarly, in both gopher frog species head length gets smaller with growth, but head width doesn’t—it remains proportional to body size. In Crawfish Frogs, head length again gets smaller with growth, but, crucially, Crawfish Frogs exhibit a marked increase in head width with growth. That is, contrary to most vertebrates, as a Crawfish Frog grows, its head width gets proportionally larger as its head length gets shorter. This differential growth reshapes a Crawfish Frog’s jawline such that as it matures, when viewed from above, its snout approximates a semicircle.

Consistent with Nate’s data, in 1953, Arthur Bragg observed that in Crawfish Frogs the larger the size, the rounder the snout. In The Call of the Crawfish Frog (Lannoo and Stiles 2020), Rochelle and I suggest that a rounded snout minimizes the gap between the burrow wall and the jawline of the frog, providing fewer opportunities for predators to gain purchase. In effect, when a Crawfish Frog is in its burrow facing up, toward the opening of the burrow, and lowers its head at the approach of a predator, its rounded snout acts as a hatch that protects the softer parts of its body from a predator’s teeth. This arrangement works only when a Crawfish Frog occupies a burrow with a diameter similar to the girth of its body. And it works only if a Crawfish Frog keeps its head down. Jen’s intuition that a Crawfish Frog’s burrow must mean much more to it than a simple hole in the ground was spot on.

As mentioned above, Crawfish Frogs are a species of conservation concern. There is a strong tendency by humans to blame victims for the crimes committed against them. This type of thinking has often underpinned informal discussions of Crawfish Frog conservation, and it goes like this: If Crawfish Frogs were not so slow, clumsy, and stupid they might muster enough wherewithal to pull themselves up by the bootstraps and save themselves from extinction; Crawfish Frogs must be held at least partially accountable for their declines.

Before our work, biologists could be excused for coming to this conclusion. Considering Crawfish Frog behaviors from the perspective of other frogs, they are slow. When we were processing frogs at Vanessa’s drift fences, we would remove a frog from a bucket, take a measurement, set the frog down beside us, record the measurement, pick the frog up, take another measurement, set the frog down and record, and so on. This didn’t work with Southern Leopard Frogs: release them and they bound off never to be seen again. But remember, unlike Southern Leopard Frogs that flee in response to predators, we now know that Crawfish Frogs are adapted, both behaviorally and morphologically, to hold their ground when threatened. The frog Nate found in the belly of a Hognose Snake didn’t die because she remained still with her body inflated and her head down. She died because she lifted her head—she moved.