Get a Grip on It! Cling Force and Perch Diameter

An actual perch used by Anolis cristatellus – neither smooth nor flat! (photo by K. Winchell)

In the real world, lizards cling to everything from smooth, flat concrete walls to rough,  rounded tree trunks. So why is it that most studies on cling force in anoles focus on clinging to smooth flat substrates? Does cling force differ if the substrate is rounded or rough? Jason Kolbe sought to answer this question in his recent publication, “Effects of Hind-Limb Length and Perch Diameter on Clinging Performance in Anolis Lizards from the British Virgin Islands” (Kolbe 2015).

We know that morphology impacts performance in anoles and that performance varies with environment. For example, sprint speed is correlated with limb length, but this relationship depends on the diameter of the substrate (e.g. Losos and Sinervo, 1989). We also know a little about clinging performance in anoles. Greater cling force is correlated with larger toepads and more lamellae on smooth flat surfaces (Irschick et al., 1996; Zani 2000; Elstrott and Irschick, 2004), but adhesion on rougher surfaces may be influenced by claw and toe morphology (Zani 2000).

There appears to be an unexplored interaction between substrate properties and clinging ability that involves more than just toepad characteristics. Specifically, Kolbe points out that claws can increase clinging ability by digging into the perch or simply by increasing friction on the surface. Limbs can also increase friction via the application of compression forces to the substrate. In other words, cling force can be increased, particularly on rough surfaces, by using muscular force to grasp rather than relying on van der Waals forces from the toepads, which are more effective on smooth flat surfaces.

Anolis cristatellus (photo by KMW)

Anolis stratus (photo by KMW)

Anole species used in this study: Anolis cristatellus (left) and Anolis stratulus (right). Photos by K. Winchell.

Kolbe investigated this further by looking at the interaction between limb length and clinging ability on perches of different diameters with Anolis cristatellus and Anolis stratulus from the British Virgin Islands. Specifically, he hypothesized that cling force should increase as the ability of a lizard to obtain a firm grasp on a substrate improves (i.e. when it can wrap its limbs around the substrate). This ability to form a secure grasp is dependent on both the diameter of the perch and on lizard limb length.

To test this, Kolbe captured 44 A. cristatellus (22 males, 22 females) and 38 A. stratulus (19 males, 19 females), two common species in the British Virgin Islands and Puerto Rico. Anolis cristatellus is larger and tends to perch on lower and broader perches compared to A. stratulus. Male A. cristatellus are larger than females (by about 10mm SVL on average) while males and females of A. stratulus are of similar size. Lizards were positioned on a vertical smooth wooden dowel so that their limbs wrapped around to the greatest extent possible. Using a force gauge, Kolbe slowly pulled the lizards away from the dowel and measured the force required to separate the lizard from its perch.

Cling posture depends on limb length and perch diameter (D). Hindlimb lengths (HL) depicted here are typical of adult male A. cristatellus (50mm), female A. cristatellus and both sexes of A. stratulus (30mm). Black arrows indicate the direction of force applied (away from the perch) and white arrows indicate the direction of the compression force applied by limbs. (Figure 1 from Kolbe 2015)

Kolbe found that cling force varied with species, sex, and perch diameter. Male A. cristatellus generated the greatest cling force and A. stratulus the least, and all lizards generated greater cling force on the smaller diameter perch compared to the larger diameter perch. Cling force was significantly related to limb length, but not body size (SVL). This correlation is likely explained by two factors: longer limbed lizards are able to encircle more of the perch (which results in a more effective grip), and longer limbed lizards are able to apply stronger compressive forces to the perch (since limb length is likely correlated with muscle size).

Mean and standard deviation of the log-transformed cling forces measured. All lizards had stronger cling force on the smaller perch. (Figure 3 from Kolbe 2015)

The interaction between cling force and perch diameter is intriguing because it implies a tradeoff in performance. On thinner perches long-limbed lizards will run more slowly, but their grip will be more secure. Even more interesting, Kolbe points out, is that despite being able to cling best to smaller diameter perches, these species tend to use broad substrates. This could be because of an interaction between surface roughness and claw use (if broader surfaces are rougher, which would enable more effective gripping with claws). Although this hypothesis has yet to be tested, we do know that claws are an important part of this story. For example, Crandell et al. (2014) (discussed previously on Anole Annals) found that toepad area and lamellae number were positively correlated with larger claws (height and length) – suggesting that these traits are evolving together, perhaps to improve clinging performance. Kolbe also proposes that lizards may prefer broader substrates because sprinting and jumping performance are more important than clinging ecologically, or perhaps that weak cling force is sufficient for the majority of activities.

Hindlimb length is correlated with cling force and this varies with species and sex. (Figure 4 from Kolbe 2015).

In summary, Kolbe has provided us with another example of how morphology and habitat interact to determine performance: cling force depends on both limb length and perch diameter. This research opens the door for many other interesting questions. How important is cling strength for daily activities? What are the fitness benefits of stronger clinging? How does claw morphology influence cling force? What other environmental variables influence clinging ability? Cling force is certainly more complex than just toepad area and much work remains to be done on this interesting habitat-morphology-performance relationship.

Citation:  Kolbe, J.J. (2015) Effects of Hind-Limb Length and Perch Diameter on Clinging Performance in Anolis Lizards from the British Virgin Islands. Journal of Herpetology 49(2): 284-290. doi:10.1670/13-104

Works Referenced:

Crandell, K.E., A. Herrel, M. Sasa, J.B. Losos, and K. Autumn. (2014) Stick or grip? Co-evolution of adhesive toepads and claws in Anolis lizards. Zoology 117: 363-369.

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Kristin Winchell

I am an Assistant Professor of Biology at New York University. My first love is herpetology, but my lab works on urban evolutionary ecology in diverse organisms (but primarily anoles!).

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