Pumpkin toadlet

An adult and two young under normal light (left) and UV light (right), showing that fluorescence varies with age

B. ephippium is a very small frog with a snout–to–vent length of 12.5–19.7 mm (0.49–0.78 in) in adults,[5] but it is among the largest in its genus together with species like B. darkside, B. garbeanus and B. margaritatus.[6][7] Females tend to be larger than males.[3] When newly hatched B. ephippium typically measure just 5.25–5.45 mm (0.207–0.215 in).[4]

B. ephippium is overall bright yellow-orange and this is considered aposematic (warning colours) since its skin and organs contain tetrodotoxin and similar toxins.[8] Newly hatched B. ephippium are well-camouflaged and brown overall.[4] 11-oxoTTX (11-oxotetrodotoxin), an isolated analogue is extremely rare to be found in other animals, even marine animals, this analogue is considered four to five times more potent than the tetrodotoxin itself.[9] Other analogues isolated of this toad include the tetrodonic acid, 4-epipetrodotoxin, 4.9 anhydrotetrodotoxin and 11-nortetrodotoxin.[10]

In 2019, scientists discovered that the head and back of this toadlet and the closely related red pumpkin toadlet (B. pitanga) glowed under ultraviolet light, due to their fluorescent skeletons.[11][12] Young that have gained the bright yellow-orange adult colours still lack their fluorescence. It was initially speculated that the fluorescent colour also is aposematic or that it is related to mate choice (species recognition or determining fitness of a potential partner),[12] but later studies indicate that the former explanation is unlikely, as predation attempts on the toadlets appear to be unaffected by the presence/absence of fluorescence.[13]

Peculiarly, this species and the closely related red pumpkin toadlet are unable to hear the frequency of their own advertising calls, as their ears are underdeveloped. Instead their communication appears to rely on certain movements like the vocal sac that inflates when calling, mouth gaping and waving of their arms.[14][15] It is speculated that their calling is a vestigiality from the ancestral form of the genus, whereas their reduced hearing ability (they do have some hearing ability in frequencies outside their call) is a novel change in these species. Sounds make them more vulnerable to predators, but there has likely been little direct evolutionary pressure to lose it because of their toxicity.[14][15]

• Polka-dot tree frog (Hypsiboas punctatus) — the first frog discovered to be fluorescent, in 2017