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The verdant rainforests of Costa Rica are home to many species of tiny glass frogs, named for their semi-transparent skin and translucent bellies, which can reveal their organs from underneath. The elusive amphibians pictured here, known commonly as La Palma glass frogs (Hyalinobatrachium valerioi), measure just a couple of centimeters and sport a mottled coat that mimics the lime-green coloration of their leafy surroundings. As arboreal and nocturnal frogs, La Palmas spend most of their lives hopping between trees and disappearing under leaves at night, perfectly camouflaged.
During the mating season, from May to late October, male glass frogs become increasingly territorial as they search for mates, emitting high-pitched squeaks, brays, and other calls to entice females and defend their space. This mating ritual is known as amplexus, Latin for “embrace.”
Photographer Emanuele Biggi captured a male La Palma glass frog mating with a female, grasping her in an intimate embrace and preparing to fertilize her eggs as she lays them. Males often mate with several females in the same location; egg clutches from previous matings sit near the pair on the same leaf. The act of consummation shown in this photo is rarely seen by humans, as these frogs usually lay their clutches in the dead of night and on the underside of leaves that dangle over streams and rivers.
You can easily see their hearts beating or their organs digesting a meal.
Though the glass frogs’ translucence creates a curious visual effect—you can easily see their hearts beating or their organs digesting a meal—this camouflaging characteristic helps to confuse predators in the aftermath of reproduction. After laying about 35 eggs in a single gelatinous clutch on the underside of a leaf, female La Palma glass frogs flee the scene and leave the males to guard and protect up to seven broods at a time, day and night, until the eggs hatch. The frogs’ spotted coloration mimics the pattern of a clutch of eggs, allowing male La Palmas to blend in as they lay their bodies over their frogspawn in a protective and moisture-preserving stance. Any intruders, frog or otherwise, will face a male La Palma’s wrath, as it aggressively attacks and kills other male frogs as well as predatory wasps and ants.
The International Union for the Conservation of Nature (IUCN) notes that the habitat of the La Palma glass frog is in “continuing decline” and its population is decreasing in three of the four countries where the frog still lives. Only the Costa Rica population is considered stable, though the frog is “uncommon” there and has disappeared from some areas. La Palmas face the same threats common to rainforest animals around the world, such as habitat loss due to deforestation, agricultural pollution, the spread of deadly pathogens, and the exotic animal trade.
Indeed, international trade in glass frogs has exploded in recent years, an unfortunate side effect of the tiny amphibians’ charisma. While trade of some glass frogs remains legal, it can be difficult to tell the different species and subspecies apart, and conservation groups fear that wildlife traffickers may falsify paperwork to pass off rarer species as their more abundant cousins. Just 13 glass frogs were imported to the United States in 2016, but that number rocketed to 5,744 frogs in 2021. This booming trade may be one of many reasons why the IUCN lists the majority of glass frog species (of the family Centrolenidae) as threatened. Because these elusive tree-dwelling amphibians are difficult to count and study, scientists suspect more glass frog species might be in trouble than the data reveal.
In 2022, in an effort to curb trading of all glass frogs, the Convention on International Trade in Endangered Species placed increased restrictions on exports of all glass frogs from South and Central America—including the La Palma—a big win for these tiny frogs. 
This story originally appeared in bioGraphic, an independent magazine about nature and regeneration powered by the California Academy of Sciences.
Emanuele Biggi is a scientist, photographer, and television presenter on the popular Italian series GEO, broadcast on Rai3. His photographs typically feature the world’s small creatures, helping to raise awareness about the critical roles these organisms play in their environments and the threats they face. You can find more of his work at www.anura.it.
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