Semicircular canal size constrains vestibular function in miniaturized frogs

Abstract

Miniaturization has evolved repeatedly in frogs in the moist leaf litter environments of rainforests worldwide. Miniaturized frogs are among the world’s smallest vertebrates and exhibit an array of enigmatic features. One area where miniaturization has predictable consequences is the vestibular system, which acts as a gyroscope, providing sensory information about movement and orientation. We investigated the vestibular system of pumpkin toadlets, Brachycephalus (Anura: Brachycephalidae), a clade of miniaturized frogs from Brazil. The semicircular canals of miniaturized frogs are the smallest recorded for adult vertebrates, resulting in low sensitivity to angular acceleration due to insufficient displacement of endolymph. This translates into a lack of postural control during jumping in Brachycephalus and represents a physical constraint resulting from Poiseuille’s law, which governs movement of fluids within tubes.

Diversity and function of the fused anuran radioulna

“…The one species from our dataset that lacked forelimb‐involved amplexus ( Breviceps poweri ; Figure 1 ) did not differ greatly in terms of MA calculations (Table S2 ). Regarding saltation, many frogs that jump do not brace with the forelimb upon landing (Essner et al, 2010 , 2022 ; Reilly et al, 2016 ) and yet the radius and ulna are always fused in modern frogs. In Ascaphus , the radius and ulna are fused (Figure 1a ) despite Ascaphus actively avoiding using its forelimbs during landing behaviors (Essner et al, 2010 ).…”

“…Regarding saltation, many frogs that jump do not brace with the forelimb upon landing (Essner et al, 2010(Essner et al, , 2022Reilly et al, 2016) and yet the radius and ulna are always fused in modern frogs. In Ascaphus, the radius and ulna are fused (Figure 1a) despite Ascaphus actively avoiding using its forelimbs during landing behaviors (Essner et al, 2010).…”

“…Frogs exhibit a high degree of fusion among many bones, which is thought to be an adaptation for their saltatory locomotor style. Although the ancestral condition of frogs was likely saltatory (Jenkins Jr. & Shubin, 1998 ; Přikryl et al, 2009 ; Sigurdsen et al, 2012 ; Abdala et al, 2022 ; although see Reilly & Jorgensen, 2011), many extant frogs do not typically jump (Emerson, 1978 ) or do not use the forelimbs during landing (Essner et al, 2010 , 2022 ; Reilly et al, 2016 ). All frogs have fused radioulnae (Figure 1a ) and tibiofibula, regardless of saltation ability.…”

Perceiving like a Bat: Hierarchical 3D Geometric–Semantic Scene Understanding Inspired by a Biomimetic Mechanism

“…To surmount the above challenges, we turned to nature for inspiration. Humans and animals have shown us their incredible environment perception capabilities and autonomous navigating abilities in a large-scale complex environment [ 18 , 19 , 20 , 21 , 22 ]. As humans, we understand the surrounding environments effortlessly: we receive and transmit high-level instructions and draw up long-distance travel between different cities, and even accurately predict what will happen in the future.…”

Convergent Anuran Middle Ear Loss Lacks a Universal, Adaptive Explanation

“…Beyond the potential for extrinsic factors to influence anuran ear loss, shared intrinsic factors among earless frogs, specifically sensory constraints related to smaller body size, could provide an explanation for anuran tympanic middle ear loss. Sensory system consequences of miniaturization have been linked to functional constraints in the salamander visual system [26] as well as the frog vestibular system [27]. Prior work has noted a re-lationship between smaller anuran body size and tympanic middle ear loss [28].…”