Hypsiboas rosenbergi

Gladiator 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. They have extensive webbing on their hands and feet and enlarged disks on their fingers and toes.

A close inspection of their hands will also reveal another interesting characteristic of Gladiator Tree Frogs. They have a prepollex spine that protrudes next to their thumbs, which is of vital importance during their reproductive cycle. These spines grow from the base of the frog's wrist and are covered by a skin sheath; their sharp point left uncovered.

Gladiator Tree Frogs spend most of their lives in the tree tops. This is where they rest during the day and where they spend most of the dry season. Their breeding season usually spans from March to September, and it's at this time that this frog's life is truly remarkable. Males descend from the canopy to a marshy area or to the bank of a slow moving stream. There they will excavate a nest which may measure between 100 and 300 millimeters in diameter. Once the nest is completed and fills with water, the male will begin his advertising call. Their call is very distinctive and could be described as low hammering on a hollow log.

A gravid female will approach the calling male and enter his nest. When she approaches, the male will start a courtship call which has a lower intensity than his advertising call. While the male is engaged in his courtship call, the female will inspect his nest to see if it meets her criteria. Studies have shown that females have high standards and will reject the male and his nest about fifty percent of the time!

While the inspection is taking place, the female will frequently bump the male. As her inspection  continues, and she begins to succumb to his charms, she will continue touching him. Her affections will reach their zenith with a full head and body massage which she will perform with her hands, chest, and chin. The male will then mount the female in axillary amplexus, as shown in the photograph below.

Once in amplexus, the female will usually spend several hours renovating the nest. She will lay her eggs in small groups every few seconds and may lay over 3000 eggs in total. This process takes about 10 minutes. The eggs float on the surface of the water and will normally hatch within two to three days. During this time, the father will patrol and aggressively guard his nest site. Given the chance, other Gladiator Tree Frog males will 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 his eggs are developing,  that the Gladiator Tree Frog really lives up to its name and reputation.

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

Once the conflict gets physical, it will very quickly turn brutal. Males will use their unsheathed prepollex spines as weapons and will 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.

We have often witnessed Gladiator Tree Frogs on The Night Tour moving through the vegetation with parts of their face missing, presumably the result of a confrontation.

If the father is successful in protecting his offspring, and they manage to hatch, it will normally 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. The individual featured here was photographed in Drake Bay near a breeding site located by a marshy area.

Interestingly, it seems that not all Gladiator Tree Frogs in Costa Rica exhibit these heroic traits. Studies conducted in La Gamba, also located on the Osa Peninsula, have shown that male frogs in that location exhibit "wimpy" behavior! Males in La Gamba don't tend to build nests, but instead use cattle footprints and small puddles to breed in. They are not aggressive and do not guard their nests to protect their offspring. Scientists theorize that it may be another species altogether, one that is yet undescribed. Gladiator Tree Frogs are only known to exist in Costa Rica, Panama, and parts of Ecuador and Colombia. 

References:

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