Red-Eyed Treefrog Eggs Hatch a Plan to Escape Warming Temperatures

A new study from the Smithsonian Tropical Research Institute (STRI) shows that red-eyed treefrog embryos hatch early when exposed to high ammonia levels — an environmental cue that it’s too hot and dry for the eggs to survive

Olivia Milloway

July 15, 2024

With their bright green coats, orange feet, and blue stripes, red-eyed treefrogs have become cover models for Central American rainforest conservation. Along with flashy accessories, they’ve evolved to hatch in response to environmental cues. This phenomenon helps them survive myriad jungle threats like hungry predators, dangerous pathogens, suffocating floods, and maybe even climate change.

Three decades ago, Dr. Karen Warkentin, current Boston University professor and STRI Research Associate, realized that red-eyed treefrog embryos can hatch up to three days shy of their normal seven-day development cycle. If they deem life in their gelatinous nursery to be too risky, they can slip out of their eggs and into the relative safety of the water below. But how do embryonic treefrogs make this call?

Red-eyed treefrogs lay their eggs on leaves that hang over ponds so that as the tadpoleshatch, they fall into the water below. Surprisingly, about half of all anurans (frogs and toads) lay their eggs on land. Astrid Katerina Lisondro-Arosemena

Red-eyed treefrogs can be found throughout Central America with isolated populations in Colombia in low to mid-elevation rainforests. Males, which are usually smaller than females, make a “chack” call to defend their territories during the rainy season. Astrid Katerina Lisondro-Arosemena

The Experimental Pond at the Smithsonian Tropical Research Institute in Gamboa, Panama, is a herpetologist’s (a scientist who studies amphibians and reptiles) dream. Decades of behavioral and developmental research on iconic species like red-eyed treefrogsand túngara frogs has been conducted here. Karen Warkentin

Warkentin discovered that red-eyed treefrog embryos are discerning observers of the sensory world around them, able to differentiate between the vibrations caused by a munching snake or hungry wasp and a harmless tropical storm. More recent studies showed that red-eyed treefrogs also hatch in response to hot, dry conditions that could be fatal.

“We wondered if ammonia might not just be a toxin, but information.” Unable to detect any obvious clues with their human senses, Warkentin’s team didn’t know how these embryos could tell their eggs were dangerously dehydrated. Javier Méndez Narváez, a former graduate student in the Warkentin lab, showed that ammonia concentrations — a waste product in developing eggs — increase as terrestrial eggs dry out. “Whether that’s in a natural setting or under a lab microscope, we always start with observations,” said Warkentin. “So, we wondered if ammonia might not just be a toxin, but information.”

Red-eyed treefrog egg clutches (a group of eggs laid together) stay on their leaves as long as they can, as the water below is fullof aquatic predators like giant water bugs and fishes. After at least a month—though developmental timing is quite variable—the tadpoles metamorphose into frogs and leave the water for good. Karen Warkentin

One way that climate change affects red-eyed treefrogs is during their hatching process. Well-hydrated embryos easily slide out of their eggs and into the water below, while dry embryos can get stuck after hatching—like the embryo on the left side of this photo—or even get trapped inside their eggs during the hatching process. Karen Warkentin

One of Méndez’s volunteers, Astrid Katerina Lisondro-Arosemena, received an internship funded by STRI and the Panamanian National Secretariat of Sciences, Technology, and Innovation (SENACYT) to test whether elevated ammonia levels could trigger early hatching in red-eyed treefrogs in an otherwise safe, nurturing environment. Over the course of two years, Lisondro-Arosemena, a graduate from the Universidad Autonoma de Chiriquí and lead author on the newest paper from the Warkentin Lab, conducted experiments at STRI’s laboratory in Gamboa, Panama.

The study’s results linked ammonia exposure to near-immediate hatching in 95% of trials, uncovering one of the ways that embryonic treefrogs can sense oncoming climate danger. “I love it when the embryos give us such clear answers to our questions,” said Warkentin. The findings weren’t the only exciting part of the project; “we needed to figure out how to make [an experiment] no one had ever thought of before happen in the field, cheaply,” explained paper co-author and doctoral student at Boston University, María José Salazar-Nicholls. Salazar-Nicholls mentored Lisondro-Arosemena throughout the project, teaching her methods in developmental biology and troubleshooting experimental design in an ambient laboratory space in Gamboa, Panama. “Sometimes, the discovery comes in figuring out how to ask the embryos, ‘Why are you doing what you’re doing?’” said Warkentin.

Because there were no previous studies on how quickly ammonia is absorbed by frog eggs, Lisondro-Arosemena conducted many pilot experiments to find the “sweet spot” of ammonia concentration for exposure experiments in the ambient laboratory in Gamboa, Panama. María José Salazar-Nicholls

To expose the embryos to ammonia without reducing the oxygen concentration, which is known to induce hatching, Lisondro-Arosemena flooded the eggs only partially. Astrid Katerina Lisondro-Arosemena

Within the 30-minute experiment, 95% of the embryos exposed to high ammonia levels hatched (right). Their siblings who were given the “control” treatment (flooded partially but without added ammonia) remained inside their egg capsules (left). Astrid Katerina Lisondro-Arosemena

Since she first worked with Warkentin as a STRI intern in 2016, Salazar-Nicholls has learned how to tell when embryos are happy, annoyed, or just about to hatch. “That internship introduced me to this field of frog embryo behavior that I didn’t know even existed ... and I fell in love."

Originally from Ecuador, Salazar-Nicholls didn’t speak English when she started working with Warkentin but learned through the lab’s bilingual environment, including Warkentin, who themself speaks Spanish. “If your leader invests the time and the effort into learning the language, if you receive training in your own language, you’ll immediately feel more welcomed,” said Salazar-Nicholls.