Alien Earthlings

in Drake Bay, Costa Rica



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Boana rosenbergi


Gladiator Tree Frog - Hypsiboas rosenbergiGladiator Tree Frogs are common inhabitants of the humid lowlands in central and southwestern Costa Rica. These large tree frogs may measure anywhere from 70 to 90 millimeters and are easily distinguished by a pencil thin line that runs from the tip of their snout to the middle of their back. 

A close inspection of their hands will also reveal a prepollex spine that protrudes next to their thumbs, which is of vital importance during their reproductive cycle.

Gladiator Tree Frogs spend most of their lives in the tree tops. This is where they rest during the day and where they spend the driest part of dry season.

During their breeding season males descend from the canopy to marshy areas. There they excavate a nest which may measure between 100 and 300 millimeters in diameter. Once the nest is completed and fills with water, the male jumps in the nest and begins to call out to the females.

Their call is very distinctive and could be described as low hammering on a hollow log.

Gladiator Tree Frog - Hypsiboas rosenbergi

 If a female is attracted by the call, she will approach the calling male and enter his nest. As she is approaching, the male starts a courtship call which has a lower intensity than his advertising call.

While the male is engaged in his courtship call, the female inspects his nest to see if it meets her criteria. Studies have shown that females have high standards and usually reject the male and his nest about fifty percent of the time!

During the inspection, the female may frequently bump the male. If she really likes the nest, she will give the male a full body massage! The male will then mount the female in axillary amplexus, as shown in the photograph below.

Gladiator Tree Frogs Mating - Hypsiboas rosenbergi


Once in amplexus, the female may spend up to 3 hours renovating the nest! She may lay over 3000 eggs in total. Gladiator Tree Frog Eggs - Hypsiboas rosenbergiThis process takes about 10 minutes. The eggs float on the surface of the water and normally hatch within two or three days.

During this time, the parent male patrols and aggressively guards his nest site. Given the chance, rival Gladiator Tree Frog males may jump in the nest. Such an attack would have disastrous repercussions, breaking the nest's surface tension and causing the eggs to sink and drown. It is during these crucial hours, while their eggs are developing,  that Gladiator Tree Frogs really live up to their name and reputation.

Gladiator Tree Frog - Boana rosenbergiIf an intruder is detected nearby, a series of territorial and encounter calls are emitted by the parent male. These may include chuckles, hisses, mews, barks and growls. If the intruder is undeterred, the parent male will try to chase him away or tackle him in order to keep the dead beat male away from his nest.

Once the conflict gets physical, it can quickly turn brutal. Battling males use their prepollex spines as weapons and immediately attempt to gouge out their opponent's eyes and break their eardrums. These duels will often result in permanent damage to the frogs and the battle may ensue until one of them has been killed. Studies have shown that most male frogs suffer permanent damage during mating season.

If the parent male is successful in protecting his offspring, and they manage to hatch, it may take about 40 days for the tadpoles to complete their metamorphosis. When they emerge from the water, still sporting their tadpole tail, they are covered with small dark spots.

Gladiator Tree Frog - Hypsiboas rosenbergiThe individual featured here was photographed in Drake Bay.

Gladiator Tree Frogs are only known to exist in Costa Rica, Panama, and parts of Ecuador and Colombia. 



Leenders, T.  2001  A Guide to Amphibians and Reptiles of Costa Rica  Zona Tropical

Savage, J.  2002  The Amphibians and Reptiles of Costa Rica   University of Chicago Press


Mammals of the Osa Peninsula

The Frog Files

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Common Rain Frog - Craugastor fitzingeri

Gaufy Leaf Frog - Agalychnis callidryas

Gladiator Tree Frog - Hypsiboas rosenbergi

Glass Frogs Home Page

Emerald Glass Frog - Centrolenella prosobleponCascade Glass Frog - Cochranella albomaculataGranular Glass Frog - Cochranella granulosaCricket Glass Frog - Hyalinobatrachium colymbiphyllumDusty Glass Frog - Hyalinobatrachium pulveratumReticulated Glass Frog - Hyalinobatrachium valerioi

Gliding Leaf Frog - Agalychnis spurrelli

Hourglass Tree Frog - Dendropsophus ebraccatus

Giant Marine Toad - Bufo marinus

Masked Tree Frog - Smilisca phaeota

Smoky Jungle Frog - Leptodactylus petadactylus

Tink Frog - Diasporus diastema

Salamanders - Order: Caudata



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