Osteopilus septentrionalis shows co-ossification: dermal tissue on top of the head is fused to the underlying skull bones rather than sliding freely over them. The genus name means bony pillar. The condition occurs independently in several tree frog lineages that experience dry conditions, and it is associated with reduced evaporative water loss across the head.
Some co-ossified frogs also use the hardened head as a plug, backing into a tree hole and sealing the entrance with the skull, a behaviour called phragmosis. Osteopilus is not the classic example of that habit, but the structural basis is the same.
In Florida the species is an established invasive, arriving probably as a stowaway in cargo in the 1920s and now found throughout the peninsula. It eats native tree frogs including green and squirrel tree frogs, and it takes up residence in electrical boxes and plumbing, where it causes short circuits. Its skin secretion is irritating to eyes and mucous membranes, which is a practical reason not to handle one without washing afterwards.