“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.
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.
Escape Ability Changes from Three to Five Days Old In the wild, red-eyed treefrog embryos are a natural source of food for snakes. In some ponds, over fifty percent of treefrog embryos are eaten by predators. The low success rate for hatching of the embryos is due mostly to predation on clutches that are not yet able to hatch. In the lab, Warkentin was able to manipulate the age of the clutches that snakes came into contact with, giving us a detailed look at how embryo response to predation changes as they develop.
The Makings of a Snake Snake charmers are those said to have the ability to control a snake; however, doing so in a scientific lab would call for a far greater level of discretion, abstraction and sometimes creativity. Here Warkentin and her student-scientists discuss the many approaches used to simulate predatory vibration patterns on frog clutches and the creation of a Snake Puppet.
Snake Puppet Master To create a puppet that resembles a snake is a feat alone. However, making that puppet slither, hiss, and command the presence of a serpent requires the discrete knowledge of a scientist and the craft of a Puppet Master. Adeline Almanzar, a student in the Warkentin lab, IS the Snake Puppet Master. See what her experimentation and analysis with three different snake species can tell us about how red-eyed treefrogs respond to predation in the wild.
Return to the Wild After experimentation, the animals in the Warkentin Lab are returned to the wild. The surviving frog tadpoles, once embryos have hatched, are returned to the same pond where their clutches were found. Snakes, like the one seen in this video, usually spend just a few weeks in the lab, giving the scientists valuable data.
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.
View fullsize
Aquatic Life v.s. Terrestrial Colonization
View fullsize
Glassfrog Dads
View fullsize
Parental Care and Offspring Success
View fullsize
Hatching 101
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?
When observed under a microscope and encourage to hatch, treefrog embryos reveal the ability to release enzymes that can free them from their egg.
Enzyme Mediated Hatching and Confocal Microscopy Watching treefrog embryos hatch using the naked eye has revealed a lot about the conditions under which these organisms might choose to escape. Using the latest technology in confocal microscopy can tell us even more about how the embryos do so and the physiological implications behind this ability.
Complications at Birth - Induced Hatching Similar to human birth, the development and birth of a frog embryo is not without its complications. Without a parent to aide in the hatching process, how will individual embryos deal with the complications that often mean life or death? Perhaps there is a Plan B.
Brain Power? The Possibility of Innervation While physiological processes like the release of hormones or the digestion of nutrients in the body can be widely mediated and lengthy in time, interactions of nerves within the brain and their control of the rest of the body can be faster and more discretely operated. Here we examine how escape hatching, given its speed and specificity, might be a result of innervation, the connection of nerves cells to the hatching glands of treefrog embryos.
The Warkentin Lab's "Development of Adaptive Embryo Behavior" project is supported by the National Science Foundation, Boston 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:
Science News: Behaviors Unhatched: Embryo's Employ Survival Strategies
Discover Magazine - Escape Hatch
Journal of Experimental Biology - Embryos Sense Seismic Events
Observe the nocturnal behaviors of frogs with the help of infrared technology.
Permalink