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  • Writer's pictureCésar L. Barrio-Amorós CRWild

CRWILD /Agalychnis spurrelli, Gliding tree frog reproductive explosion

Updated: Dec 11, 2020

Anurans (frogs and toads) are amphibians that usually need water to reproduce. The vast majority of them go in the rainy season to ponds, lagoons, rivers and streams to mate (males calling the females by their particular sound), the females full of eggs, choose their favorite male, who will do the embracement known as amplexus, which means that the male is on the female for hours or days, depends on the species. In almost all species of anurans, females are larger than males. When a male makes the amplexus with a female, he never leave her, and often he must defend his catch against other males, sometimes with kung-fu kicks. With the male on top, the pair arrive to the place where the female decides to lay eggs. In that moment, the male fertilizes them, spraying his sperm on the soft-jelly eggs (external fertilization as in fish). Fertilized eggs develop, either in water, in humid areas or in leaves over water, until tadpoles hatch, end their development and metamorphose.

Some frogs are solitary, others are gregarious; some always live near water, others live deep in the forest far from the water, and only approach it during the breeding season. It is the case of the paraglide frog (Agalychnis spurrelli) a beautiful species, green with yellow or orange flanks and disproportionally huge hands and feet, their fingers webbed with a patagium-like membrane that, when deployed, makes them glide tree to tree (hence its common name). This species can be one of the most abundant in a specific place, but not seen more than in just a few times every year. What is an extraordinary fact is that several thousands of frogs gather at the same time in a small area to mate and reproduce. There is a harsh competition, as the sex-ratio is about 20 males per female, and males fight to each other to mate. Also, females compete for the best places for oviposition, etc. This event is not the norm in that species, as does not always occur. It seems that this event is associated with certain elements, both natural and climatic. In most of the distribution,from Costa Rica to Ecuador, this frog is not abundant, and just a few couples meet in a single pond.

In some places of the Costa Rican Caribbean lowlands, and in the Colombian Chocó, reproductive aggregations of hundreds of specimens have been registered. But only in the Costa Rican Pacific lowlands, and specifically in the Osa Peninsula and surroundings, there are such agglomerations of thousands and thousands of specimens. The first study that was conducted in 1970 yielded a number (very conservative, I would say) of 13,000 specimens in an artificial lagoon. The truth is that, a priori, without doing an exhaustive calculation, counting the surface of palms and trees occupied, for a square meter in which you see 128 frogs, you can make an approximate calculation of about 300 m2 where the frogs are, there would be over 35,000 frogs ...

Taking this in count, I started to figure out when it could be the first reproductive explosion of the year, and in that way, I went to the Osa Peninsula, the only place in the world where this phenomenon occurs. These explosions cannot be predicted or calculated, since they occur in a crescent moon, but during any appropriate night after heavy rains. Some of these aggregations last a few days, others only one night. After waiting three days with almost no rain ... and with nothing special to do, more than wait for the event, on the third day a persistent rain lasted all evening and night. So I decided to stay another day. I went early to the lagoon, and what I observed was something so sensational that I get excited just to remember it. I had never seen so many frogs together, and less by day, with such activity. The frogs jumped everywhere, mating, fighting, kicking, some males cleaning eggs from the leaf where he wanted its female to lay (the truth is that almost all the appropriate leaves were already covered with eggs). I thought you would see more predators, snakes, birds, etc. but we only saw a tiger heron and some wasp stealing some eggs. However, at night I am sure that many opportunists will come to feast on eggs, such as snakes, turtles, caimans, nocturnal mammals ... etc.

Anyway ... another of my goals was to record everything with my camera as with the drone. So, I was able to produce the above video.

I deeply thank the people of Osa Conservation who welcomed me, Brandon, who is doing his doctoral thesis on these frogs, Katherine, and Marina, for their help and company.


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