The Spanish woman who spent a year on a Philippine island and discovered another way frogs reproduce

The 18th and 19th centuries marked the golden age of natural history. In the midst of the Enlightenment, naturalists and explorers set out to traverse the globe with one ambition: to discover, describe, and classify life in all its diversity. Aboard maritime expeditions or venturing into remote territories, they collected plants, animals, and minerals that were then transported to Europe, where they were studied and organized, expanding our understanding of the world.

Two centuries later, that task is far from over. Even today, scientists continue to explore the least-known corners of the planet. Ignacio de la Riva, a herpetologist (a scientist dedicated to the study of reptiles and amphibians) at the Spanish National Museum of Natural Sciences (MNCN), has dedicated a significant part of his career to studying a group of Andean frogs distributed from Ecuador to Argentina and Chile. But chytridiomycosis, a global disease caused by a fungus, proved devastating for them. “Several of the species I had discovered and described are possibly already extinct. I dedicated many years to studying them, and it was terribly frustrating to return there and find they were gone,” he explains. “I realized it was time for a change of scenery.”

He found this new environment in the Philippines. There lives a primitive frog (Barbourula busuangensis) surprisingly similar to Andean frogs, adapted to the same type of habitat: fast-flowing, rocky rivers. They are relatively large animals — adults reach 10 centimeters — completely aquatic, with robust bodies, flattened heads, prominent eyes located on the top of their heads, and broad webbing.

However, what was most intriguing wasn’t their appearance, but their possible method of reproduction. The few available observations indicated that the females carried large, unpigmented eggs, an unusual trait in amphibians. In contrast to the more common pattern — many small eggs left to their fate — these characteristics suggested alternative reproductive strategies. “When an egg is large and completely unpigmented, it suggests direct development — ovoviviparity — or that it will remain hidden, without exposure to light. And the larger the egg, the more evidence there is of parental care or oviposition outside of water… something unusual,” De la Riva points out.

Because if there’s one thing that defines frogs, it’s precisely the extraordinary diversity of their reproductive methods. Although the most widespread image is that of eggs laid in the water from which tadpoles emerge and, over time, complete their metamorphosis, the reality is much richer and more surprising.

For example, Oophaga pumilio is a poison dart frog from the Neotropics that climbs a tree and lays an egg in each leaf axil of a bromeliad, where a larva grows. The frog then visits from time to time to feed it with its own unfertilized eggs. Even stranger is the case of the Australian Rheobatrachus silus, which was declared extinct in 2006. The female swallowed the fertilized eggs, the young developed inside her stomach, and then emerged from her mouth as fully formed froglets.

The Philippine frog that De la Riva wanted to study is a very difficult animal to observe and capture. It is rarely heard calling and, at the slightest sign of danger, hides under large, immovable rocks. Studying it was quite a challenge, and De la Riva and his colleague Patricia Burrowes found in their doctoral student, Marta Miñarro, the necessary enthusiasm to attempt it. After overcoming a complex bureaucratic process and obtaining all the necessary permits, Miñarro traveled to the Philippines.

She settled in the village of Mabalto, on Busuanga Island, and, accompanied by a local guide, ventured into the jungle each night through virtually unexplored areas. After about two hours of walking, they reached the river where they worked. There, they established three transects — sections of the river marked every few meters — and systematically traversed them, moving upstream in search of specimens.

The frogs took refuge in cavities formed under large rocks. With the help of headlamps, they searched for the reflection of their eyes in the darkness. If they found one exposed, they had to act quickly to capture it by hand; sometimes they also managed to catch them in shallower areas, where they came to feed. “Catching that frog was like trying to pick up a wet bar of soap, but one that was also strong,” Miñarro recalls. “In one night I could catch about 20; it seems like a lot, but in similar studies with other species, you can catch up to five hundred.”

At the end of each day, they tagged the frogs with a microchip — a kind of biological ID — recorded their measurements, checked for eggs or malformations, and noted the exact location of their capture. This information allowed them to build a precise picture of the population. Then, they retraced their steps and released each frog precisely where it had been found.

Miñarro never stopped lifting stones and exploring every nook and cranny in search of any clue about the species’ reproduction. The complete absence of tadpoles led them to believe that it might have direct development: they found very small specimens, but no one — neither she nor other researchers before her — had ever observed a larval stage. Faced with this enigma, they decided to use an endoscopic camera: a tablet connected to a long cable with a small, illuminated lens at the end. Miñarro inserted it into cracks and cavities in the river for weeks without results. “I was in the Philippines for 12 months, and I didn’t see anything until the eighth month,” she recalls.

That’s when the stroke of luck came. “Some photographer friends who were visiting came across a small dam in the river and called me over because there were a lot of adults. We started inserting the endoscope and, suddenly, I saw some tiny frogs that had a bit of a tail. I said: if it has a tail, it must be a tadpole. We looked more closely and there I found the first one.”

But this was no ordinary tadpole. It was almost transparent — a typical trait of organisms that live in darkness, without exposure to sunlight — lacked teeth for feeding, and had a large oral sucker. Months later, Miñarro located a complete nest. “I saw an adult that started to become very aggressive. I followed it, and the entire cave wall was covered with tadpoles stuck to it. The adult was protecting them. In other words, they were exhibiting parental care,” she explains.

The next day, they improvised a system to extract one of the specimens: they bought hoses from a car repair shop and, using a water bottle, built a negative pressure device. Thanks to this ingenious method, they managed to extract a tadpole from the cave gallery and take it to the cabin, where they kept it in a container with rocks and river sediment. Minarro recalls: “What struck us was seeing that this tadpole wasn’t feeding on anything; it completed its entire development using the yolk from the egg it had already ingested and which was in its intestine. It also reabsorbs nutrients from its tail. This is what is known as an endotrophic tadpole.” Two weeks later, it had transformed into a small frog and was returned to the river. The particular combination of developmental and behavioral characteristics observed allowed them to define a new mode of reproduction.

“I have the tadpole tattooed on me,” Miñarro says enthusiastically. “Imagine, this species was described 102 years ago and since then no one had seen the tadpole,” she recounts, adding: “For me, with my imposter syndrome and being a woman in a field mainly led by men… I won’t lie to you, finding the tadpole was a real source of pride.”

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