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 -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 diVering morphology of scale surfaces at the rear of the jaws of both species. We hypothesize that capillary forces Wll 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 hingejoint channels with convoluted walls and a jaw-buccal cavity pumping-mechanism, have convergently evolved for capture, transport, and drinking of water from sporadic rainfall.

Cutaneous water collection by a moisture-harvesting lizard, the thorny devil (Moloch horridus)

“…Water is transported passively in the half-open capillary channels between the scales by capillary force (Bentley and Blumer, 1962;Gans et al, 1982;Withers, 1993;Sherbrooke et al, 2007;Comanns et al, 2011). As soon as water reaches the scales surrounding the mouth, the lizard can but does not necessarily drink.…”

“…Special surface micro-structures of the Oberhäutchen (i.e. outer layer of epidermis) and small capillary channels in between the scales allow these so-called 'moistureharvesting' lizards to collect water into their skin capillaries and transport it to their mouth for drinking (Gans et al, 1982;Withers, 1993;Sherbrooke, 2004;Sherbrooke et al, 2007;Comanns et al, 2011).…”

“…The physical ability to harvest moisture is associated with a network of capillary channels, in between the overlapping scales, that enables passive and sometimes directional transport of the collected water to the mouth for drinking (Withers, 1993;Sherbrooke et al, 2007;Comanns et al, 2011Comanns et al, , 2015. Transportation to the mouth for ingestion is necessary, because the integument is substantially waterproof to minimize evaporative water loss (Bentley and Blumer, 1962;Withers, 1993) and this precludes water absorption across the skin.…”

Directional, passive liquid transport: the Texas horned lizard as a model for a biomimetic ‘liquid diode’

“…In the case of P. cornutum, this passive transport mechanism also has a pre-determined flow direction towards the snout [5]. In more detail, the integument 1 of P. cornutum provides a network of capillary channels between the overlapping scales (figure 1b,c) [5,9,11]. These capillaries are about 100 -250 mm in width, with a smaller opening towards the surface of up to 100 mm [9,11].…”

“…In more detail, the integument 1 of P. cornutum provides a network of capillary channels between the overlapping scales (figure 1b,c) [5,9,11]. These capillaries are about 100 -250 mm in width, with a smaller opening towards the surface of up to 100 mm [9,11]. Transportation of water to the snout is necessary because the integument is almost waterproof to minimize water loss by evaporation [12].…”