Scaly-Tail Organ Enhances Static Stability during Pel’s Scaly-tailed Flying Squirrels’ Arboreal Locomotion

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Sidney Leedham, Roger Kissane and James Charles

Introduction:

An arboreal environment is a habitat found in trees, characterized by a three-dimensional structure with branches, leaves, and trunks. It is home to animals that typically require the ability to climb, jump, and move through the canopy while navigating various surfaces, such as rough bark or smooth leaves. Numerous evolutionary solutions to the problem of arboreal locomotion are observed across arboreal animals, from the prehensile tails and elongated digits of large-bodied primates, to the spines, claws, and adhesive pads seen in lizards, insects, bats and other small tree-dwellers (1).

Unique among these structures is the ‘scaly tail organ’ of the Anomaluridae, a family of African gliding squirrels (Figure 1). This structure consists of scales that originate from the caudal bones and protrude through the skin, and is thought to be an adaptation for arboreal locomotion, increasing friction against tree bark and aiding in stability when climbing and gliding by acting as a fifth point of contact alongside the limbs (2). A similar function has been demonstrated for the tails of arboreal and gliding lizards like geckos but has not been tested in scaly-tailed squirrels. Furthermore, the observation that the habitat of scaly-tailed squirrels contains many smooth-barked trees, compared to the habitats of other gliding squirrels without a scaly-tail organ, suggests that this structure may be a specific adaptation to give scaly-tailed squirrels a frictional advantage on smooth substrates (Figure 2).

In this preprint, the authors test the hypotheses that the scaly-tail organ aids the scaly-tailed squirrel in arboreal locomotion by 1) increasing friction and preventing skidding on inclined substrates and 2) increasing passive perching stability by acting as a fifth point of contact. They use an experimental setup involving a 3D-printed physical model with claws and the scaly-tail organ of Anomalurus pelii. This model was placed on a sandpaper-covered ramp and high-speed cameras were used to record the angle at which the model slipped. Additional validation and investigation of the role of the scaly-tail organ in perching stability were also provided by mathematical models.

Figure 1. Figure from Heritage et al. (3) made available under a CC-BY 4.0 International license showing the Anomalure Zenkerella insignis, with D) illustrating the scaly-tail organ.

 

Figure 2. Figure from the authors’ preprint (made available under a CC-BY-NC 4.0 International license) illustrating the hypothesis that the scaly-tail organ of Anomaluridae may be an adaptation to increase friction on smooth-barked tree

 

Key findings of the study:

Scaly tails increase friction on steep substrates

In experimental trials, the presence of a scaly tail organ significantly increased both maximum perching angle and friction coefficient (by 17% and 58%, respectively). Interestingly, this was only achieved on a ‘medium roughness’ substrate; the scaly tail performed worse than smooth-tailed models at low roughness (Figure 3). This may support the hypothesis that the scaly-tailed organ is an adaptation to maximise engagement with smooth-barked trees such as those found in the habitat of A. pelii, which the authors suggest could fall within a similar roughness range as the medium-roughness sandpaper used in this study (though this was not tested).

Figure 3. Scaly-tail organ increases maximum slip angle on medium-roughness substrate. Figure 5A of the preprint made available under a CC-BY-NC 4.0 International license.

Scaly tails enhance stability at greater perching angles

The squirrel’s limbs represent four points of contact with the substrate, enclosing a ‘support polygon’. When the animal’s centre of mass (COM) falls within this polygon, the squirrel can passively perch without any additional energetic input from the limbs, but when the COM projection falls outside this support polygon (as may occur at steep perching angles), the resulting moment must be counteracted by engagement of the claws to prevent overturning. Using mathematical models, the authors found that the scaly-tail organ acts as a fifth point of contact, providing similar frictional effects to the claws and increasing the size of the support polygon so that the COM still falls within it at steeper angles. This enables a ~4% increase in passive perch angle without the need for active engagement of the claws, representing lower energetic costs of perching for squirrels with scaly tails.

 

Why we highlight this work:

In this preprint, the authors use a neat combination of mathematical modelling and a simple experimental setup (which is nevertheless grounded in real morphology through the use of laser scanning and 3D printing) to provide the first empirical support for a stability and friction-enhancing function of the scaly-tail organ of the Anomaluridae. The study is an interesting example of how the functions of unique biological structures can be investigated using a first-principles approach and sheds light on an understudied group of mammals. We would be especially interested in further work that attempted to place the results presented here in a more robust ecological context, either by comparing substrate properties to a greater range of measured tree bark values, or by performing experiments using real bark as a substrate.

 

Questions for the authors:

1. What do you think would be the effect of adding a compliant tail and/or limbs to the model – especially as the centre of mass shifts backwards at steeper substrate angles (i.e. do you think the tail would be able to provide greater friction at these angles if it was able to bend relative to the body)?
2. Similarly, how do you think using a model with biologically informed mass properties might affect its performance?
3. Are there any other unique biological structures that are of interest as potentially applicable to robotics? (just curious as someone from a non-robotics background!)

 

References:

1. Young JW. Convergence of Arboreal Locomotor Specialization: Morphological and Behavioral Solutions for Movement on Narrow and Compliant Supports. In: Bels VL, Russell AP, editors. Convergent Evolution: Animal Form and Function [Internet]. Cham: Springer International Publishing; 2023. p. 289–322. Available from: https://doi.org/10.1007/978-3-031-11441-0_11
2. Panyutina AA, Chernova OF, Soldatova IB. Morphological peculiarities in the integument of enigmatic anomalurid gliders (Anomaluridae, Rodentia). J Anat. 2020 Sep 1;237(3):404–26.
3. Heritage S, Fernández D, Sallam HM, Cronin DT, Esara Echube JM, Seiffert ER. Ancient phylogenetic divergence of the enigmatic African rodent Zenkerella and the origin of anomalurid gliding. Hrbek T, editor. PeerJ. 2016 Aug 16;4:e2320.

 

doi: https://doi.org/10.1242/prelights.39732

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