Functional morphology of scale hinges used to transport water: convergent drinking adaptations in desert lizards (Moloch horridus and Phrynosoma cornutum)

Abstract

The Australian thorny devil, Moloch horridus Gray, 1841, and the Texas horned lizard, Phrynosoma cornutum Harlan, 1825, have the remarkable ability to rapidly move water through interscalar spaces on their skin’s surface to their mouth for drinking. The morphology of these scale hinges has not been studied. We used histological and SEM techniques to examine and compare the scale hinges of both species. Additional taxa in their respective lineages were examined in order to evaluate the potential that convergent evolution has occurred. In the two species that transport water, each scale hinge has a basally expanded and semi-enclosed channel formed by the hinge joint that is interconnected with all scale hinges on the body. We hypothesize that it is within this semi-tubular channel system of hinge joints, where the β-layer keratin of the integument is very thin, that water is transported. Hinge joint walls are covered by a complex topography of fractured surfaces that greatly expand the channel's surface area and probably enhance capillary transport of water. In addition, we note differing morphology of scale surfaces at the rear of the jaws of both species. We hypothesize that capillary forces fill the scale-hinge system and additional forces, generated within the mouth by observed motions during drinking, depress local water-pressure to pull water through the channels of the hinge-joint system. We conclude that the combined features in the two species, semi-tubular hinge-joint channels with convoluted walls and a jaw-buccal cavity pumping-mechanism, have convergently evolved for capture, transport, and drinking of water from sporadic rainfall.