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.
Parachuting red-eyed tree frog (Agalychnis saltator) [1]Sylvia’s leaf frog (Cruziohyla sylviae) [5]Agalychnis callidryas in amplexus, showing nictitating membranes over the eyes [2]So why the orange feet? One hypothesis is that a predator will hone in on the conspicuous coloration and if the frog is dropped when handled, the frog will adopt a compact resting posture that conceals its bright colors— making the search image of the predator unreliable which increases the frog’s chances of slipping under the radar [1]Mexican tree frog (Smilisca baudinii), another explosive breeder [1]; see the footage I took of an aggregation in 2016Canal Zone tree frog (Boana rufitela), a male showing off its turquoise gular sac and flanks. It is also known to fluorescence under UV light! [1]Olive-snouted tree frog (Scinax elaeochroa), one of the most abundant tree frogs, though its bright colors may certainly play a role in how easy it is to find them! For the majority of tree frogs, adults are far more common; however, the reverse is the case for this species [1]Boulenger’s long-snouted tree frog (Scinax boulengeri). Yellow and green thighs are a diagnostic character, but the overall morphology, fringes and all, make the species difficult to misidentify [2]San Carlos tree frog (Dendropsophus phlebodes), juvenile [2]. Generally they have thinner vein-like markings along the dorsum, in contrast to the more strongly dichromatic splotches of the hourglass tree frog (Dendropsophus ebreccatus)Juvenile tink frog (Diasporus diastema), the size and appearance of a jellybean [1]Diasporus diastema, a miniaturized arboreal eleutherodactylid with a high-pitched single note call [2]Chiriquí robber frog (Craugastoridae: Pristimantis cruentus) with an ant lodged in its mandible [1]Pristimantis cruentus, in a more typical body condition [1]Emerald glass frog (Centrolenidae: Espadarana prosoblepon) [3]. Parental care behaviors are diverse in glass frogs. In this species the female will remain with the clutch for a period of time, whereas in others (e.g., Hyalinobatrachium valerioi), males will actively guard the eggs from predators and keep the eggs moist by sitting on top.Two spurs on the front legs (humeral spines) are used in male-male combat to compete for perch sites and access to females [3]Strawberry poison frog (Dendrobatidae: Oophaga pumilio), Sarapiquí, Costa Rica [1]. Across populations of the Panamanian islands, O. pumilio has diversified into a greater variety of morphs than on the mainland. Lower numbers of predators on islands may have led to a weaker selective pressure for red aposematic coloration and allowed for the persistence of other color phenotypes.150 km southeast of Sarapiquí, an almost entirely red morph of Oophaga pumilio.. note the bluish digits! [1]Juvenile O. pumilio on a fallen bromeliad, the bright blue lateral patch will recede as it matures [3]Nurse frog (Aromobatidae: Allobates sp.), unusual coloration for A. talamancae (lacking a light brown dorsal stripe), likely due to variation between populations of different localities [1]Green and black poison frog (Dendrobates auratus) [3]Oophaga pumilio carrying tadpole [3]. Typically, the male will guard egg clutches in the leaf litter, and upon hatching, the tadpoles will be transported by the female to small pools of water in depressions, treeholes, and bromeliads. For over a month and a half, the female will continue to revisit her offspring and provision them with unfertilized eggs to supplement their growth.While photographing the frog, the tadpole changed its behavior by swishing the tail back and forth, probably due to its perception of the female’s more frequent hopping to escape and possibly helping it maintain a grip [3]Savage’s thin-toed frog (Leptodactylus savagei). Shrieking calls when captured are thought to attract secondary predators that may distract their primary aggressor enough to allow for an escape [1]Southern narrow-mouthed frog (Microhylidae: Hypopachuspictiventris). Species in this enigmatic group will breed in ephemeral pools after heavy rains, and the males have a distinct call that has earned them a second common name, sheep frogs. [4]Rain/robber frogs of the genus Craugastor are among the most diverse neotropical frogs. The most distinct of the group is an obligately terrestrial species, the broad-headed robber frog (Craugastor megacephalus) [1]Craugastor megacephalus[1]Unidentified Craugastor sp. [1]Noble’s robber frog (Craugastor noblei) [1]Almirante robber frog (Craugastor talamancae) [1]Craugastor talamancae[1]Fitzinger’s robber frog (Craugastor fitzingeri) [1]Craugastor cf. fitzingeri[1]Bransford’s robber frog (Craugastor bransfordii), one of the smallest of the genus! [1]
