Herpetologist Paul Freed agreed with my guess that these are young Hyla rosenbergi. Thanks Paul!

This is a largish treefrog with large webbed feet and powerful jumping abilities. The one in the top picture is a full-size adult; the one in the bottom picture is a youngster.

Simon Lailvaux of the Department of Ecology and Evolutionary Biology at Tulane University explained to me why they are called Gladiator frogs:

"the reason is that male H. rosenbergi frogs have a sharp bony spine on each hand that is used as a dagger in fights between males over females and nests. Many males are wounded in these fights, and some are even killed. This is probably due to the small size of the breeding ponds, and unusually strong predation pressure - these frogs seldom live longer than a year, so males that fail to defend a decent nest probably won't get another chance in the next season."

Biologist Gerlinde Hoebel wrote me to add more information about this behavior. The fighting behavior among male H. rosenbergi was recorded from a study in Panama. Hoebel's studies in Costa Rica showed no evidence of such behavior, possibly due to the abundance of desirable basins for egg-laying. So the common name of this frog seems to be an overgeneralization based on a behavior that does not occur everywhere in the frog's range.

These frogs were photographed on a herping trip with GreenTracks. Here is a complete list of the species we found on this GreenTracks trip.

This large adult was the only treefrog we encountered on a night hike on Pipeline Road in the midst of a long dry spell for this part of Panama.