Grip It and Ribbit: Mechanisms Contributing to Grasping in Australian Green Treefrogs (Ranoidea Caerulea)
Arboreal habitats provide expanded niches for species by offering access to new food sources and providing safety from predators. To securely navigate these challenging 3-D environments, treefrogs generate gripping forces using both active muscle recruitment and passive mucus adhesion mechanisms to maintain contact with the substrate and prevent falling. This study focused on Australian Green Treefrogs (Ranoidea caerulea) as a model for exploring grasping abilities in an arboreal context. Using a portable grip strength tester, we collected pulling forces from five animals (mean body mass: 34.1 ± 7.6 g) across five different substrate diameters (2.5, 5.0, 7.5, 10.0, and 12.5 mm). Our findings revealed that mucus adhesion accounts for approximately 75% of pulling forces, as indicated by a significant reduction in force when using an oiled substrate. Additionally, hindlimbs exhibit greater pulling forces than forelimbs, which is consistent with the larger size and primary propulsive function of hindlimbs in anurans. We also observed a decrease in force generation on larger diameter substrates, which was likely due to length-tension properties of the flexor musculature. Comparisons with other arboreal species demonstrated that treefrogs exhibit similar relative pulling strengths. Mucus adhesion represents a crucial mechanism facilitating movement on arboreal supports, allowing amphibians to navigate arboreal substrates without extensive anatomical adaptations. Further comparative studies across diverse anuran taxa are needed to explore the role of hindlimb morphology in grasping adaptations. Understanding the interplay between active muscle recruitment and mucus adhesion mechanisms will provide valuable insights on evolution of grasping abilities in arboreal tetrapods.ABSTRACT
Experimental setup for evaluation of pulling strength.
White arrow: Custom 3-D printed gripping substrate measuring at 2.5, 5, 7.5, 10, and 12.5 mm in diameter. Two experimental conditions were measured: an unaltered grip and an adhesion-knockout grip, where adhesion grip was prevented by coating the substrate with a nonstick oil. Black arrow: Harvard Apparatus grip strength tester.
Pulling forces in all trials of unaltered grip (unshaded, top) and adhesion-knockout grip (shaded, bottom) across varying substrate diameter sizes (2.5, 5, 7.5, 10, and 12.5 mm) stratified by forelimb (left) and hindlimb (right) represented as jittered boxplots.
Box plots show median (horizontal line) values, upper and lower quartiles (box), 1.5 × interquartile range (whiskers), and outliers (dots).
Mean-of-maximum force, calculated by taking the average of each frog's highest trial within each experimental condition, to control for the influence of low-motivation cycles.
Pulling forces in newtons (N) of unaltered grip (unshaded, top) and adhesion-knockout grip (shaded, bottom) across varying substrate diameter sizes (2.5, 5, 7.5, 10, and 12.5 mm) stratified by forelimb (left) and hindlimb (right) represented as jittered boxplots. Box plots show median (horizontal line) values, upper and lower quartiles (box), 1.5 × interquartile range (whiskers), and outliers (dots).
Relative pulling force (adjusted for body size) in other tetrapods represented as a percentage of body weight (%BW).
Dwarf chameleon data from da Silva et al. (2014); mouse lemur data from Thomas et al. (2016); mouse data from Nevins et al. (1993); and rosy-faced lovebird data from Dickinson et al. (2022). All literature data points represent dual-sided, forelimb (i.e., bimanual) pulling barring lovebird data, which represent unipedal pulls.
Schematic of differences of frog hand posture wrapped around largest (A) and smallest (B) substrates. On our largest substrate (12.5 mm), frogs wrap the entirety of their digits over the dowel without circumferential overlapping of their digits, while on the smallest substrate (2.5 mm), the digits overlap, and the substrate is positioned securely at the distal portions of the phalanges.
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