On a small island in the Caribbean, ancestrally known as Borinkén, a demon lives in the Guajonales mountains on the southeast of the island. Or so says the folklore legends told by those who were haunted by the echoing calls  of the coquí guajón (Eleutherodactylus cooki) of Puerto Rico. This evil spirit of the mountains is the second largest species of the 17 species of Eleutherodactylus that can be found in Puerto Rico.
This species is also known as the Puerto Rican rock frog , a reference to its endemic habitat, the Guajonales. The Guajonales are a unique rock formation in the southeast of Puerto Rico. Some believe that the name of the rock formation comes from a Taino word meaning piles of rocks with small crevices . This is a fitting name, as those crevices are home to the coquí guajón where they help amplify male’s matting calls and solidify this little frog’s place in legend [2,4]. Guajón are well-adapted to their unique cave homes. Their eyes are the largest reported of the species found in Puerto Rico, which helps them see in the especially dark caves they call home [2,3]. To aid them in climbing, they have especially large distal discs (or finger tips) and the longest arms known to the genus [2,3].
Like the rest of its genus, coquí guajón has exclusive male parental care . This means only the male takes care of the clutches of eggs. Also, like the rest of its genus, the young go through direct development . Contrary to most other frogs, coquís don’t go through a tadpole phase, but are instead born fully formed. What makes the coquí guajón unique in these aspects, is that one male can take care of up to four egg clutches from up to four different females ! The females know which male is going to make the best dad, because the guajón has a distinct difference between the males and females that no other coquí has. The best dads have more brightly colored yellow bellies in comparison to other males of the species . And good dads they are; many researchers that study the guajón have stories about how this toothless amphibian does his best to bite the fingers of anyone who comes near their eggs . Likewise, the males protect their clutches against predators and competing males, which I think makes them good candidates for the “Dad of the Year” award.
Of course before the female has the opportunity to assess the male’s coloration, she has to find him. The males sing to make themselves easier to find, but what makes the guajón song unique is its haunting reverb that gets amplified by the caves. On top of that, the guajón has a relatively quiet time all to himself to sing. Unlike most frogs native to the island, the male of the guajón sings the most during the late afternoon while the sun is still out .
Sadly, our little demon of the mountains is endangered . Their limited native range, land development, climate change, and diseases have done a number on the species. But not all hope is lost. Various organizations like Projecto Coquí and Para la Naturaleza have made strides in bringing consciousness of the species to the public and protecting its land. Additionally, local scientists are bringing attention to the importance of the guajón both as a biological indicator to the health of the forest  and a pivotal predator to control insect populations . Only time can tell if these efforts will be successful in preserving the coquí guajón. But in the meantime, those of us that live in Borinkén should feel privileged to be haunted by this beautiful spirit of the mountains.
Boricua born and raised, Sofía Meléndez Cartagena is a PhD student at UC Davis who specializes in running after bees and observing their beehaviours. She believes that computational thinking is not just for computer scientists, and that many biological problems have computational solutions. Currently, she’s become especially obsessed with observing the origins of social organizations in insects.
 Burrowes, Patricia A. (2000). Parental care and sexual selection in the Puerto Rican cave-dwelling frog, Eleutherodactylus cooki. Herpetologica, 56(3), 375-386.
 Barrios González, Marla M. & López Colón, Jonathan Alfredo (2019). Coquí guajón o demonio de Puerto Rico (Eleuctherodactylus cooki). Perspectivas en Asuntos Ambientales, 7, 10-21.
 Joglar, Rafael L. “Coqui Guajon.” Proyecto Coqui, https://proyectocoqui.com/es/coqui-guajon/. Accessed 6 October 2021.
 U.S Fish and Wildlife Services. “Puerto Rican rock frog or coquí guajón Eleutherodactylus cooki.” U.S Fish and Wildlife Services, https://www.fws.gov/southeast/wildlife/amphibians/puerto-rican-rock-frog/. Accessed 11 November 2021.
 Villanueva, Luis J. “Coquí guajón (Eleutherodactylus cooki).” CoquiPR, https://coquipr.com/coqui-guajon.html. Accessed 4 October 2021.
Main image source