Tree falls play an integral role in structuring tropical forest communities. The collapse of a single tree— whether that be a thin spindly Cecropia or a towering Ceiba— breaks the homogeneity of the dim green ambience of the canopy and causes a cascade of dramatic changes to the immediate surrounding environment. The high intensity of sunlight permits seed germination and sapling survival, and a variety of fauna are attracted to the novel microenvironment, reaping the rewards of diverse plant growth, decomposing organic matter, or simply an ideal refuge and spot to thermoregulate. Even when the canopy inevitably closes, the vegetation composition remains unusual and indicative of what had transpired, and I often devote a bit more time to searching within these patchy areas.
Many vertebrates make use of the spaces within the tree itself to complete their life histories, including one of the most elegant frogs of Costa Rica, Sylvia’s leaf frog (Cruziohyla sylviae). Within the cavernous hollow of one fallen tree, cloudy masses of Cruziohyla eggs overhung a stagnant murky pool of water. Almost the entirety of the clutches had been vacated, but after careful inspection, two tadpoles remained. It’s always fascinating to observe the anatomy of developing terrestrial anuran embryos. In the tadpoles pictured above, you can see functional eyes, red root-like extensions of the gills, and a long tail that extends around the circumference of the globular pocket. Another two weeks or so and their yellow yolk sacs will be depleted, and the tadpoles will fall into the water beneath to feed on small aquatic invertebrates and plant matter along with their siblings. Young C. sylviae face a high risk of predation from egg eaters like cat-eyed snakes (Leptodeira septentrionalis) and carnivorous tadpoles such as the spiny-headed tree frog (Triprion spinosa). When still suspended in a gooey clutch, embryos of many phyllomedusines (Cruziohyla included!) have evolved a strong antipredator response to tactile disturbance, dropping to the water prematurely to escape. Embryos and tadpoles lack the same chemical defense possessed in adults, which may have driven these behavioral, rather than physiological, strategies to reduce predation.
Although conservation efforts have been successful in attracting wild Cruziohyla to breed in artificial nest cavities, observations in situ are rather rare. An ideal nest site such as this one will potentially be used by the same species (and even the same individuals) for years to come, so if I’m lucky, I may be able to hike up the cordillera again to this spot to document future generations of this species.
With exception of their egg-laying habits, adult Cruziohyla exclusively inhabit the canopy. Males call faintly from great heights with a singular low note, and females purportedly respond with a trill… so I’ve been told. Every time I’ve spotted this species, I was looking down a steep slope to find one perched on a large Welfia palm leaf— no doubt a déjà vu. Their striking orange color on the flanks is unmistakeable and unlikely to confuse with Agalychnis tree frogs, even from the perspective of the frogs themselves! The dorsum, in contrast, is nodulous with light green moss-like specks. Fringes on the thighs can retract neatly against the main body, hiding any evidence of their flamboyance. When trying to retrieve them from up high, this behavior makes them blend in with the shades of green, while their sticky toepads secure them in position. Curiously, the eyes of C. sylviae transition from pale in the center to yellow near the rim and dark purple at the periphery. Why frogs have such high diversity in iris color difficult to explain, but it may be a mechanism for species and mate recognition.