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“In animal life there are certain moments where what happens at that moment could be more important than you would think…hatching is one of those moments.”

- Dr. Karen Warkentin

While wading through a densely forested pond in some Neotropical realm, one might come face-to-face with a translucent and gelatinous globule of treefrog eggs. Peering into the individual egg sacs fastened to a leaf and hanging above the water may reveal the distinct characteristics of life: a head, a tail, perhaps a movement. The most discerning eye might observe—through the transparent skin of this yet to be born animal—a beating, red heart. Leaves tremble from even the slightest maneuvers through the habitat. The tiny beings in their pods start to quake as well, and plop! Those who were unborn just a moment ago are now swimming in the water below. Was it a coincidence that these embryos hatched so quickly? Certainly, some have interpreted similar observations as such. Karen Warkentin, on the other hand, thought differently – leading her to form the hypothesis of predator-induced escape-hatching and embark on a decades-long journey studying embryo behavior.

In Warkentin’s very first paper on the subject she described how these pre-natal beings have the ability to make evaluative decisions in response to environmental conditions in order to stay alive. Warkentin explains that, while eggs are extremely vulnerable to predation, the embryos inside them can receive information about the world around, and – during some developmental period – make the decision to hatch.

Phenomena subtle as these can go unnoticed in the jungle. The competing chorus of tropical birds and insects often drowns out the nearly noiseless drop of a tadpole into its aquatic home. Even more so, how perceptive one would have to be in order to capture the stirrings of these minuscule creatures hesitant in their egg capsule waiting for the right moment to hatch – or not. Warkentin’s research embarks on the journey of red-eyed treefrogs from conception to hatching and into their adolescent and adult lives. Understanding how the life patterns of these organisms are shaped is informed by a “critical transition point,” as Warkentin describes, when embryos make the decision to hatch.

Taking a broader view of the many elements of Warkentin’s research, we can begin to gauge the further implications behind the abilities of these organisms. The Narrow Escapes that treefrog embryos make from their predators happen too often to be called a coincidence, but how are they distinguishing external threats— like a parrot snake attacking a clutch— from something as harmless as rainfall. Despite being faced with these terrestrial dangers, somewhere along the organisms’ evolutionary history, the species was able to leave an aquatic habitat and colonize land. At the River Frijolito we can gain insight into the challenges of making such a transition, by observing the habits of another lineage of terrestrial egg layers, the glassfrog. Perhaps both the aforementioned phenomena can be clarified by observing these embryos under a microscope. When layer upon layer of body tissue is examined it is revealed that certain physiological mechanisms are triggered in the embryo in response to external cues. These Microscopic Revelations suggest that the interaction between embryos and the outside world may be happening at a neurological level, even more complex than previously believed. Perhaps it is with this possibility that we can begin to question what it means for an organism to inherit the will to survive. Consider the concept of adaptive plasticity in Agalychnis callidryas, the red-eyed treefrog. 

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Metamorphosis in the animal world is the transformation from a juvenile form into an adult one. Scientist and laymen alike might consider metamorphosis as simply a developmental process wherein the transformation is determined by a physiological mechanism that at some point leads to a new stage. Warkentin, however, describes both metamorphosis and hatching as being “critical transition points” during a “developmental window of possibility” within which the developing animal can make a decision that has a major impact on their life. When frog embryos hatch – and move from one life stage (in the egg) to the next (in the pond) – depends on what the embryo decides to do and when it decides to do it. Similarly – within the developmental window of possibility – when the animal leaves the pond for life on land depends on when it decides to climb out of the water.

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Like the red-eyed treefrog, glassfrogs lay their eggs terrestrially, on land. While studying the behavioral similarities between these species of frog can give scientists key insights into their evolutionary histories, finding a tiny frog that is nearly transparent amidst the dense vegetation along a rainforest stream can be quite the challenge. Join Field Biologist Javier Mendez as he leads us into the realm of the elusive glassfrog.

A. Aquatic Life vs. Terrestrial Colonization 

Lineages of frogs that evolved the ability to lay eggs on land, as opposed to in water, also had to evolve mechanisms to deal with the differences in biotic and abiotic components found in these environments. How can comparing glassfrogs and their behaviors give us further knowledge of plastic hatching? 

B. Glassfrog Dads

In the world of glassfrogs, it is common for the male to provide most of the parental care to their eggs. How they do so while fending off the various threats posed by the environment is an example of an evolved behavior that went on to alter the fate of a species. 

C. Parental Care and Offspring Success

“Waste disposal is an issue that life faces,” explains Dr. Warkentin, and in many animals waste disposal is aided  by water, through its ability to flush out toxic substances. This makes waste disposal much easier for aquatic animals. So how do terrestrial animals who have far less access to water cope with the issue? One answer is to let parents take care of it, but how can frog embryos be sure that they will be given this life-saving tender love and care?

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When observed under a microscope and encourage to hatch, treefrog embryos reveal the ability to release enzymes that can free them from their egg.

The Warkentin Lab's "Development of Adaptive Embryo Behavior" project is supported by the National Science FoundationBoston University, and the Smithsonian Tropical Research Institute. Arjun Collins Research Experience for Teachers was supported by the National Science Foundation. For more information on research in the Warkentin Lab, see the Warkentin Lab website. Below, read more about Warkentin’s work, and peek behind the scenes at STRI’s Experimental Pond in Gamboa, Panama:

Observe the nocturnal behaviors of frogs with the help of infrared technology.