How Frankfurt biologist Diana Abondano Almeida discovered unknown communication pathways in neotropical frogs.

Most people have probably seen them: wriggling tadpoles darting about in a pond or even a puddle. Rarely, however, does one encounter an adult frog. The situation is different with neotropical poison frogs (Dendrobatidae). These small amphibians, only around 2 to 3 cm in size, care for their offspring with great dedication. Even the eggs – much smaller than those of European species – are guarded around the clock, usually while perched on a leaf.

But where does the clutch come from? During the mating season, which can vary depending on the climate zone, females lay their eggs in water, where they are fertilized by the males. Physical contact is not necessary for this. However, frogs and toads worldwide exhibit more or less affectionate amplexus, where the male clasps the female with its front limbs. Depending on the species, this grip can be quite firm. In the case of neotropical poison frogs, though, the females could easily shake off the smaller male.

The small tree frogs engaging in typical cephalic amplexus during mating. (Photo: Diana Abondano Almeida)

This behavior has puzzled biologists: why do the frogs cling if physical contact isn’t needed for fertilization? Why do they expose themselves to predators in this way? And why do the females tolerate it when they could easily escape? A clue might lie in the chemical communication of these small amphibians. While their acoustic signals, commonly known as croaking, have been well studied, little is known about the use of other sensory channels during reproduction.

Swollen Fingers

For this reason, the research group Wildlife-/Zoo-Animal-Biology and Systematics, led by Prof. Dr. Lisa Schulte at Goethe University’s Institute for Ecology, Evolution, and Diversity, focuses on these underexplored modes of communication. While the group leader has specialized in the parental care behavior of certain amphibians, PhD student Diana Abondano Almeida noticed something else: unlike their European relatives, neotropical tree frogs do not embrace their partner’s abdomen or chest during amplexus, but rather their head – a behavior known as cephalic amplexus. In this position, the male places its fingers close to the female’s nostrils. It is also noticeable that during the mating season, one of the male’s front fingers swells to several times its normal size in some subspecies. The team quickly recognized that this swelling might be linked to an overlooked mechanism. This observation became the starting point for Almeida’s dissertation.

Biologist Diana Abondano Almeida on one of her field trips. (Photo: Luca Livraghi)

During an expedition to her home country Colombia in 2018, Almeida observed the species Leucostethus brachistriatus in its natural rainforest habitat and collected tissue samples from different fingers and toes. This was no easy task given their small size – a pinhead is larger than the tip of the tiny amphibian’s finger. She returned to Frankfurt with the prepared samples, where she also collected similar samples from a related species, Epipedobates anthonyi, which is commonly sold in the pet trade. Almeida’s goal was to sequence the RNA (ribonucleic acid) and identify which genes were active in the finger glands (see info box). Despite the widespread presence of these inconspicuous brown frogs in Colombia, little was known about their molecular biology.

It’s not just singing that counts

When the biologist finally had the results after months of intense analysis, she was amazed: in both species – whether or not the finger was swollen – a particular gene region in the front limbs showed increased activity. These were the so-called SPF genes (Sodefrin Precursor-like Factors), known for producing pheromones, i.e. chemical messengers that influence the behavior or physiological functions of other animals, especially in mate selection. While they had been primarily identified in salamanders and newts, their discovery in frogs – and in the fingers no less – was a surprising revelation. Up to now, it had been believed that frogs primarily relied on vocalizations. The possibility that they might also communicate via scent offers an entirely new perspective on their mating behavior.

Now, Almeida’s dissertation is almost finished. Her results show that neotropical poison frogs may rely more on chemical signals during mate selection than previously thought. The discovery of pheromone-related genes in the finger glands provides the first hints of a sensory world in these animals that has been little explored. Whether cephalic amplexus in neotropical frogs is indeed a form of targeted signal transmission remains to be studied in more detail. But perhaps it was no coincidence that the fingers play a key role – after all, not everything that starts special needs a golden ball.

How is RNA sequenced? RNA sequencing is a method used by researchers to decode the genetic information from a cell’s RNA. First, RNA is isolated and converted into complementary DNA (cDNA). This cDNA is then fragmented and sequenced using special devices. The resulting data shows which genes are active in the cell and how much of each gene is being produced. In the case of the frogs, this method was used to identify the genes responsible for pheromone production in the finger glands.

Source – Goethe-Universität Frankfurt am Main