Glass frog

The frog whose heart you can see

A glass frog's belly is transparent — look from below and you can see its heart beating, its stomach and its bones! It is tiny, barely bigger than a fingernail. By day it sleeps under a leaf, almost invisible — its see-through body hides it. The father frog guards the eggs and kicks away wasps that come near.

Glass frogs (Centrolenidae, over 150 species) live in the mountain forests of Central and South America on leaves above streams; 2–3 cm long. The belly skin is transparent: heart, liver and intestines are visible. A 2022 study found that while sleeping they 'hide' 90% of their red blood cells in the liver — transparency rises 2–3 fold, and the blood returns on waking (in humans such clumping would cause a clot — of interest to medicine). In many species the male guards the clutch, keeps it moist and kicks away parasitic wasps. Tadpoles drop into the stream and develop in the mud. Their dependence on clean water means pollution and deforestation threaten many species.

Fun facts

• A sleeping glass frog hides 90% of its blood in its liver — and turns more transparent.
• You can watch its heart beating with your own eyes.
• The father frog kicks wasps away from the eggs.