Amphibians
Glass Frog
Hyalinobatrachium fleischmanni · also called Fleischmann's glass frog, northern glass frog
The glass frog is a small green tree frog that turns part of itself see-through. Look at one from above and it is an ordinary leaf-green frog; turn it over and you can watch its heart beat through the skin.
Overview
Glass frogs are a family of roughly 160 small species living in the wet forests of Central and South America, almost always along fast, clear streams. Hyalinobatrachium fleischmanni, Fleischmann’s glass frog, is one of the most widespread. They are nocturnal and arboreal, spending the day asleep on the underside of a leaf and the night hunting insects and, for males, calling from streamside vegetation.
The transparency
A glass frog’s back is opaque leaf-green, but its underside is genuinely translucent: the belly skin has no pigment, and the tissues beneath transmit light, so the heart, liver, and coiled intestine are visible, usually wrapped in a white sheath, and a female’s eggs can be seen developing inside her.
Transparency is difficult for an animal because blood is bright red and highly visible. Glass frogs solve this in two ways. Their translucent limb and body edges blur the frog’s silhouette against a bright leaf, a form of camouflage called edge diffusion. And while sleeping, a glass frog hides most of its red blood cells in its liver, draining the colour out of its circulation and roughly doubling its transparency, then re-releasing the cells when it needs to be active.
Diet
Glass frogs are small insectivores, picking tiny insects, spiders, and other invertebrates off leaves and bark at night. Their modest size keeps them tied to small prey and makes them, in turn, food for snakes, birds, and larger frogs.
Reproduction
Breeding centres on streams. Males call from leaves overhanging the water and defend their calling sites, sometimes with wrestling matches. A female lays a gelatinous clutch of eggs on the underside of a leaf, and in most glass frogs the male provides care — guarding the eggs against predatory wasps and flies and keeping them hydrated by absorbing water and voiding it over the clutch. On hatching, the tadpoles fall into the stream, burrow into leaf litter and sediment, and develop there.
Conservation
Fleischmann’s glass frog is common and listed as Least Concern. The family as a whole is more mixed: many glass frogs have tiny ranges tied to particular streams and cloud-forest patches, and are threatened by deforestation, water pollution, and the amphibian chytrid fungus. Because they depend on both intact forest and clean flowing water, glass frogs are useful indicators of a healthy stream ecosystem.
Glass Frog: common questions
Can you really see a glass frog's organs?
Through the belly, yes. The skin on a glass frog's underside has no pigment and is translucent, and the muscle and connective tissue beneath are only partly opaque, so from below you can see the heart beating, blood moving through vessels, the looping intestine, and often the eggs inside a female. The back, by contrast, is leaf-green — the transparency is mostly on the parts pressed against the leaf.
Why is a glass frog transparent?
It is camouflage of an unusual kind. Rather than matching the leaf exactly, the translucent edges of the frog let light pass through, softening its outline and the shadow it casts so a predator's eye slides over it. The effect works best when the frog is motionless and flattened against a bright green leaf, which is how it spends the day.
How does the glass frog hide its blood?
Researchers found in 2022 that a sleeping glass frog packs roughly 90% of its red blood cells into its liver, which is lined with reflective crystals, removing the red colour from circulation and making the animal significantly more transparent. When it wakes and needs to move, it releases the cells back into the bloodstream. Concentrating blood like that would cause dangerous clots in most animals, and how the frog avoids this is still being studied.
Do glass frogs look after their eggs?
In many species, yes — and it is usually the male. Females lay a jelly clutch of a few dozen eggs on the underside of a leaf hanging over a stream. The male then stays with the clutch, sitting on it or nearby for days or weeks, urinating on it to keep it moist, and driving off egg-eating wasps and other predators. When the tadpoles hatch they wriggle free and fall into the water to continue developing.