Can Anyone Climb? The Skills of a Non-specialized Toad and its Bearing on the Evolution of New Niches

Abstract

Several studies of arboreal anuran species show morphological specializations for clinging onto narrow substrates. However, little is known about these capacities in non-specialized anurans, which is crucial to understand the initial phases of adaptation to a new niche. To assess the functional requirements related to the evolution of arboreality in anurans we analyzed climbing performance, and correlated anatomical traits, in the terrestrial toad Rhinella arenarum, a species choose as a proxy for the ancestral condition regarding the evolution of this specialized niche. We studied the impact of a substrate of wooden rods with different diameters, arrangements, and slopes on locomotion, grasping, and climbing with a comparative framework. Animals were confronted with climbing tests, video recording their behaviors. Preserved specimens were dissected to assess limb myology, osteology, and tendons' characteristics. Our results show that how terrestrial toad R. arenarum climbs is different from those displayed by specialized tree frogs. Animals flexed their fingers and toes, grasping the substrate displaying hookings and partial graspings. The palm was scarcely involved in the grip, as in specialized anurans. These actions were performed although flexor and extensor muscles of the digits are highly conserved and generalized. Further, we formally assess the evolutionary history of ecological and anatomical traits related to climbing among Rhinella species to improving the comprehension of the relation between morphofunctional patterns and behavioral climbing skills. Our experiments revealed that this terrestrial toad possesses unexpected climbing capacities, suggesting a way in which evolution of new niches could have developed in the evolution of anurans.

Why link diverse citizen science surveys? Widespread arboreal habits of a terrestrial amphibian revealed by mammalian tree surveys in Britain

“…However, why apparently substantial numbers of adult toads climb trees, how long they remain there, and how they select trees with cavities or arboreal nests remains unknown. An arboreal niche might allow toads opportunities to survive either as a resting site where predators can be avoided, or as novel foraging areas (8; 39) compared to the ground level where they risk being hunted or parasitized. The toadfly Lucillia bufonivora is the obligate agent of myiasis in amphibians and an important specific parasite of common toads, found in both open habitats and shaded woodland in different European studies (40).…”

“…Most animal species use a characteristic primary mode of locomotion for the majority of their daily activities, but several species were shown to be capable of expanding their locomotion mode in order to access atypical habitats or substrates, such as some European terrestrial rodents when climbing vegetation (27). Even if rarely used, this ability to adjust the movement type to access otherwise inaccessible areas may confer those individuals important or even critical advantages in particular situations such as during dispersal, when facing stressful environmental situations such as drought, fires or flooding, or during the generation of new ecological niches (8). The collated data from arboreal mammal surveys in Britain demonstrates that some amphibian species regularly climb trees in Britain and do so across their active period in the year, although with an apparent increase in summer and autumn months.…”

“…Anurans can use a range of movements on the ground, including leaping, walking, crawling or hopping. However, while arboreal species have no difficulty to switch to terrestrial locomotion (as most anurans are capable of hopping), the reverse is far more problematic for terrestrial anurans with short limbs and a heavy body, and thus cases of terrestrial frogs or toads climbing trees or cliffs remain rare (8). What is particularly remarkable in our dataset is the height of several observations, with a record of a toad in a tree feature with the entrance at 2.8 m height from the ground.…”

“…Pedostibes hosii ) (7). However, even some typical terrestrial toads which use hopping for locomotion, such as Rhinella arenarum from South America, were recently shown during climbing tests and morphological analyses of the limbs to be able to climb wooden structures of up to 90% inclination but were using different strategies compared with tree frogs, including flexing their fingers and toes to grasp at the substrate and displaying hooking and partial grasping (8). Climbing ability was also recently noted for other Rhinella toad species but based on few chance observations in the field (9; 10).…”

Why link diverse citizen science surveys? Widespread arboreal habits of a terrestrial amphibian revealed by mammalian tree surveys in Britain

“…Pedostibes hosii ) [ 8 ]. However, even some typical terrestrial toads which use hopping for locomotion, such as Rhinella arenarum from South America, were recently shown during climbing tests and morphological analyses of the limbs to be able to climb wooden structures of up to 90% inclination but were using different strategies compared with tree frogs, including flexing their fingers and toes to grasp at the substrate and displaying hooking and partial grasping [ 9 ]. Climbing ability was also recently noted for other Rhinella toad species but based on few chance observations in the field [ 10 , 11 ].…”