Sticking under Wet Conditions: The Remarkable Attachment Abilities of the Torrent Frog, Staurois guttatus

Abstract

Tree frogs climb smooth surfaces utilising capillary forces arising from an air-fluid interface around their toe pads, whereas torrent frogs are able to climb in wet environments near waterfalls where the integrity of the meniscus is at risk. This study compares the adhesive capabilities of a torrent frog to a tree frog, investigating possible adaptations for adhesion under wet conditions. We challenged both frog species to cling to a platform which could be tilted from the horizontal to an upside-down orientation, testing the frogs on different levels of roughness and water flow. On dry, smooth surfaces, both frog species stayed attached to overhanging slopes equally well. In contrast, under both low and high flow rate conditions, the torrent frogs performed significantly better, even adhering under conditions where their toe pads were submerged in water, abolishing the meniscus that underlies capillarity. Using a transparent platform where areas of contact are illuminated, we measured the contact area of frogs during platform rotation under dry conditions. Both frog species not only used the contact area of their pads to adhere, but also large parts of their belly and thigh skin. In the tree frogs, the belly and thighs often detached on steeper slopes, whereas the torrent frogs increased the use of these areas as the slope angle increased. Probing small areas of the different skin parts with a force transducer revealed that forces declined significantly in wet conditions, with only minor differences between the frog species. The superior abilities of the torrent frogs were thus due to the large contact area they used on steep, overhanging surfaces. SEM images revealed slightly elongated cells in the periphery of the toe pads in the torrent frogs, with straightened channels in between them which could facilitate drainage of excess fluid underneath the pad.

Morphological studies of the toe pads of the rock frog,Staurois parvus(family: Ranidae) and their relevance to the development of new biomimetically inspired reversible adhesives

“…Elongated epithelial cells do not appear to be a common feature of rock frog pads in spite of their habitat in the vicinity of waterfalls. However, as shown by Endlein et al [34] in S. guttatus and here in S. parvus, there are channels crossing the pad ( particularly in a distal/proximal direction as would be appropriate for water drainage) that, while not straight, are significantly shorter than would be found in an array of regular hexagons ( figure 3a). A role in drainage of excess fluid thus seems probable.…”

“…In ranid frogs, this feature has been observed in torrent frogs such as Amolops spp. [33], in rock frogs of the genus Staurois ( [34] and this study) and also, somewhat surprisingly, in the unexpanded toe pads of the amphibious leopard frog, Rana pipiens [45]. In S. parvus, the channels are restricted to the ventral epithelium of the toes; i.e.…”

“…Experimental evidence is lacking, but an obvious function for these shorter pathways from centre to edge of pad would be to promote rapid drainage of excess fluid from underneath the pad. Such elongated channels were also found in the torrent frog, Odorana hosii, but only around the edges of the toe pads in the rock frog, Staurois guttatus [34]. A biomechanical study has been carried out on the Trinidadian stream frog, Mannophryne trinitatis, by Barnes et al [35], while Endlein et al [34] made a detailed comparison between the adhesive abilities of the rock frog S. guttatus and a rhacophorid tree frog of comparable size.…”

“…Such elongated channels were also found in the torrent frog, Odorana hosii, but only around the edges of the toe pads in the rock frog, Staurois guttatus [34]. A biomechanical study has been carried out on the Trinidadian stream frog, Mannophryne trinitatis, by Barnes et al [35], while Endlein et al [34] made a detailed comparison between the adhesive abilities of the rock frog S. guttatus and a rhacophorid tree frog of comparable size. In both studies, the stream/rock frog was good at adhering to rough wet surfaces, but very poor at sticking to dry rough ones.…”

Nanoscale friction and adhesion of tree frog toe pads

“…Although the surface of each cell is macroscopically flat, microscopically it is covered by a dense array of nanopillars, called pegs by some early authors, though the name is inappropriate as the nanopillars, indeed the whole pad epithelium, has a low elastic modulus [15]. Although adhesion and friction forces measured from individual toe pads with a custom-built force transducer are in the region of 1-5mN/mm 2 [10,16], a recent study of the forces acting on the toes of tree frogs landing on a wooden stick could be as high as fourteen times the body weight of the frogs [17]. This equates to a force of ca 26 mN/mm 2 , based on a mean force of 0.55 N and a pad area (2 toes) of 21 mm 2 (or as high as 57 mN/mm 2 if the frog lands using only a single toe).…”