Of the >125 frogs species in Costa Rica, over 40 can be observed at a single locality in the lowland rainforests. For people like myself who are not adept at pinpointing sounds, the cacophony of tinks and trills make it a challenge to locate and identify who is responsible, but nonetheless make up a quintessential part of the tropical experience. The astounding diversity of frogs in every aspect of their biology, ecology, and life history is far too ambitious for a single blog post, but here I’ll talk a little bit about tree frogs, then share a selection of other species from the Caribbean slopes.
Speciose groups of animals that occupy the same environment often diverge in resource use, including microhabitat choice, diet, and activity patterns, which alleviates competition and increases resource availability to those so specialized. Even for species that appear near identical in niche, subtle differences can sometimes be uncovered. For instance, the iconic red-eyed tree frogs (Agalychnis callidryas) and their gliding relatives (Agalychnis spurrelli) both emerge in mass during the first heavy rains of the wet season in the Osa Peninsula. For A. callidryas, breeding occurs continuously throughout the next several months, but in A. spurrelli it is rather explosive and abrupt. They descend from the canopy in extraordinary numbers, males calling to females in hopes of securing amplexus and fertilizing the eggs once deposited. Witnessing such an event is jaw-dropping. The sheer number of frogs can seem almost more abundant than the greenery they lie atop of, and the substrates suitable for egg development quickly become limited.
To circumvent this problem, the two species have evolved differences in reproductive behavior, with A. callidryas preferentially depositing egg masses on branches and leaves that extend upwards to the canopy and A. spurrelli laying on vegetation that protrudes from the waters below. Such fine scale differences in a specific life history stage may seem inconsequential, but there can be significant repercussions for physiology and behavior. Being accessible from the canopy, A. callidryas eggs are far more susceptible to predation by arboreal snakes (e.g., Imantodes & Leptodeira), whereas A. spurrelli experiences a greater threat from submergence. These different mortality risks are thought to contribute to premature hatching of embryos, in which A. callidryas is stimulated by the vibrations of a snake’s predatory approach and A. spurrelli more readily exit their gooey homes when the water level rises. However, evidence to support this pattern is mixed, and many phyllomedusines (including Agalychnis & Cruziohyla) hatch early in response to various environmental stressors to a certain degree.