Small but mighty: How pantless tree frogs are withstanding the sixth mass extinction

Courtesy of Justin Touchon.

As ecosystems across the globe transform due to climate change, the resilience of organisms is being put to the test. Fluctuations in atmospheric composition, global temperature and other climatic measures are just a few of the changes to which species must adapt. As a result of this phenomenon, many animals have been eradicated over the last few centuries at alarming rates and continue to be threatened by extinction. Some biologists and paleontologists have even referred to the current time period as the sixth mass extinction

While evolution occurs much too slowly to save many of the species at risk today, there is still hope for some flora and fauna. Phenotypic plasticity, evolution’s less popular associate, may be the reason certain species are more suited to survive widespread climate change. 

Justin Touchon, an Assistant Professor of Biology at Vassar, described phenotypic plasticity as “The idea that an organism’s single set of genes…can be expressed in different ways in response to the environment.” What makes this particularly interesting is that some environmental cues may prompt a different phenotypic expression during life history events, such as embryonic development, than other cues. In other words, the same genotype might produce different morphological characteristics in an organism based on external signals. 

The complex and sometimes misleading nature of plasticity meant that it was regarded as unimportant and a potential nuisance in the 20th century. Only in recent years has it become a less disputed area of study. Touchon is one of the biologists committed to elucidating some of its mysteries.

Touchon combines his love of herpetology (the study of amphibians and reptiles) with his interest in predator and prey interactions in his research. As a self-proclaimed “herp-dork,” his species of interest is the pantless tree frog, or Dendropsophus ebraccatus. “Since coming to Vassar I have primarily focused on the tadpoles of [D. ebraccatus],” he said. “Before coming to Vassar…I studied the plasticity of all three life stages of this frog.” 

Plasticity applies to both behavior and morphology, making it highly important for different elements of an organism’s survival and reproduction. According to Touchon, “[A species] can be flexible in aspects of their life history—[such as] when [they] hatch, or when [they] metamorphose.” 

“The eggs of [D. ebraccatus] have plasticity when they hatch,” Touchon explained. “They can detect the vibrations of a predator that is trying to eat them.” The frog eggs will hatch earlier if they detect an ant or other predator on the leaf where they were laid. Once hatched, they are able to fall into the water below and escape the predator. 

In the case of the developing tadpoles, they detect chemical cues from the predators which affect the nature of their tails. Touchon’s research showed that in the presence of fish, the tadpoles grow a long slender tail. The long tail allows them to out-swim the fish pursuing them. In the presence of invertebrate predators like dragonflies, however, the tadpoles grow a large paddle-like tail with a large red spot at the end. The paddle allows for quick turns to evade the invertebrates, while the spot tricks the predators to target the tail rather than the head. This increases the probability of the tadpoles to survive an attack, since they are able to survive if some of their tail is ripped off.

The specificities of phenotypic plasticity have yet to be determined, and Touchon hopes to make headway in future studies of his tadpoles. One potential study might address the ability of tadpoles to adapt to the presence of invertebrate predators after only being in the presence of fish predators for multiple generations. 

After multiple generations of isolated exposure to a particular predator, the gene expression of the tadpole will be adjusted to living alongside one type of threat. This may impede its ability to shift in response to another predator, or revert to a more neutral phenotype. Touchon described this as two distinct layers of a species’ flexibility: “So you have plasticity, [which] is changes in gene expression, and then you have evolution of that by changing the baseline that an organism is expressing.” 

Phenotypic plasticity offers a potential solution to conservation efforts in the face of climate change, with climates becoming more and more extreme. Species that are able to physically adapt to certain predators, such as pantless tree frogs, may be more fit to survive in a volatile world. 

Another example of the pantless tree frog’s grit is their ability to lay eggs in multiple locations. As explained by Touchon, they can lay their eggs both in a pond and outside of the pond depending on the level of shade available. This is significant because the tree frogs are able to fully develop in both locations. While ponds offer greater risk of predation, they eliminate the risk of drying out. On the other hand, shaded terrestrial locations offer the opportunity for safer eggs. Touchon explained: “While those eggs are developing they are going to be shaded, they are going to be protected from the sun and be more likely to survive.” 

Without shade, however, laying eggs on land can be catastrophic. “If there’s no shade, they would die,” Touchon explained. “They would dry out from the sun.” Fortunately, female pantless tree frogs are somehow able to predict the appropriate place to lay eggs. 

On the other hand, red-eyed tree frogs are not as versatile. Instead, they evolved to lay entirely terrestrial eggs. The intensely humid climate of the tropics allowed for egg development on land, avoiding the predation of ponds. Red-eyed tree frogs would be unable to adapt to a dry climate because of this.

Thus, an intermediate stage characterized by phenotypic plasticity may be the key to witnessing the complexity behind evolution. Animals such as pantless tree frogs may stand the test of time more readily than their red-eyed counterparts. From their propensity for survival against the deadly chytrid fungus ravaging amphibian populations around the world, to their knack for growing predator-dependent tails specialized for evasion, pantless frogs may be an indicator of the resilience required to survive the sixth mass extinction.

Professor Touchon’s humble Vassar lab may hold the key to discovering the intricacies of plasticity and what it takes to survive—all by studying our (very) distantly related scantily clothed cousins, the pantless tree frogs.

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