The Brazilian tree-frog Corythomantis greeningi inhabits the Caatinga, a semi-arid habitat. This species exhibits putative adaptations to a dry environment, including a flat head, co-ossification of the cranial skin, and phragmotic behaviour (retreat to crevices using the head to seal the entrance). Real frogs were compared with agar models to study water balance under simulated Caatinga conditions and to investigate the ability of C. greeningi to find small water sources. The benefits of skin co-ossification were also studied by comparing agar models with and without nail polish on the head. Tree-frogs repeatedly preferred a humid test tube out of 10 possible choices, and adopted phragmotic behaviour inside it. In both agar models and tree-frogs, total water evaporation increased with body size, but relative water loss was higher in smaller individuals. The presence of nail polish on agar models barely reduced water loss, suggesting that cranial skin co-ossification has a small role in water economy. Although both models and tree-frogs evaporated about 90% more water when exposed than when phragmotic, tree-frogs evaporated about 50% less water than agar models at any test condition. We conclude that reduced skin permeability, phragmotic behaviour and a keen capacity to identify humid crevices, explain the success of this species in a semi-arid habitat.