The first Anole Annals post, as Anthony Geneva reminded us a few days ago on AA’s fifth anniversary, consisted of a few anole-related haikus. One of them was:
Dewlap and Toepad.
Key innovations are traits that are thought to enable lineages to diversify greatly, as these traits are adaptations that remove constraints and allow lineages access to new niches and adaptive zones. In anoles, the dewlap is considered a key innovation as it provides greater potential for diversity in signaling traits important for mate and species recognition, thereby increasing the rate of speciation across Anolis.*
The toepad is also thought to be a key innovation. Toepads allow anoles to climb on smooth surfaces. By climbing, anole species can partition the habitat not just in horizontal space, but vertical space as well. Thus, toepads have opened the arboreal niche to anoles, thereby playing a likely role in Anolis community assembly during adaptive radiation.
Several lines of evidence support the toepad’s role as an adaptation to arboreality. (1) Large toepads do tend to be found on those anole species that live higher in the canopy; (2) larger toepads impart better clinging ability; and (3) a recent study showed that populations of A. carolinensis that shifted to higher perches rapidly evolved larger toepads.
However, anoles also have claws.
As anyone who has ever had a cat jump onto their head knows, claws are also useful for climbing. Claws can interact with surfaces by interlocking with surface irregularities, or through friction, and the morphology of the claw determines in part how useful it is for climbing. Very few studies have been conducted on the Anolis claw, however, so we don’t have a good sense for whether and how anoles use the claw during climbing. We also don’t know how the claw might co-evolve with the toepad, if it does. Thus, the role of the claw in anole evolution, and its relation to arboreality, remains unknown.
To that end, Crandell et al. investigate the claw (and toepad) in a new paper just out in Zoology. They find that the toepad doesn’t tell the whole story of Anolis climbing. Perhaps, the claw also determines which Anolis have to stay on the ground and which can go into the trees. Read on…
Crandell & co. chose 12 mainland Anolis species and measured claw morphology, toepad morphology, toepad clinging performance (by dragging the toepad across a force plate), and perch height in those species. The authors also binned the species into arboreal versus non-arboreal, depending on their average perch heights. The authors then removed the effect of body-size and corrected for phylogeny in their analyses, which included correlations of traits, performance, and habitat characteristics.
Similar to other studies, the authors found that toepad area and the number of scales (lamellae) on the toepad were both positively correlated with toepad clinging force. Moreover, toepad clinging force was positively correlated with perch height.
Toepad area and lamella number were also positively correlated with claw height and claw length, suggesting that toepads and claws are undergoing correlated evolution under shared selection pressures in the arboreal habitat.
Interestingly, claw traits also were positively correlated with toepad clinging force.It is unclear to me, however, why claw traits would be expected to be related to clinging force. In the narrow sense, clinging force was measured on a smooth glass surface where claws were unlikely to provide any additional grip. In the broad sense, the authors themselves argue that claws and toepads likely serve different functions (clinging to rough versus smooth surfaces, respectively). Thus, the above correlations need not be taken as evidence that claws are useful for climbing in anoles. Indeed, claw traits did not correlate with perch height. Instead, the correlations in Figure 4 may be an indirect result of correlated evolution between toepads and claws for other reasons.**
Yet, there were claw differences among the species that were explained when species were categorized into arboreal versus non-arboreal. Habitat dependent claws: important for climbing after all.
In resolving their contradictory findings, the authors suggest that the relationship between claw morphology and climbing function may not be continuous (as it is for toepads), relying instead on a threshold effect. In other words, it may be that any curvature in the claw is sufficient for climbing, thereby eliminating an expected correlation between claw traits and perch height, yet still explaining the claw differences between arboreal and non-arboreal species. This seems a reasonable proposition to me. Field experiments in which toepads are ablated and climbing performance is measured based on claws alone will provide key evidence. Moreover, such a test would shed light on whether toepads are necessary and sufficient for arboreal living or whether claws are also necessary.***
In all, this was an exciting study and one of the first to investigate anole (and, more generally, vertebrate) claw morphology as it relates to habitat use. The authors found that claw morphology and toepad morphology both appear to be involved in adaptation to arboreal living, the former for rough surfaces and the latter for smooth.
* At one point, about five years ago, there was an effort to correlate dewlap size with speciation rate in subclades across the Anolis phylogeny, thereby testing the dewlap as a key innovation. However, the analysis suffered from a lack of power. Has anyone in anole-land picked this up?
** It’d be great if the authors would discuss this in the comments. Likeley, I misunderstood this section or fell subject to poor reasoning. If not, however, it’d be good to know the authors’ rationale for the clinging force vs. claw morphology comparison.
*** This is a interesting question to answer, I think, for Anolis evolution. Has anyone looked at this, even in the lab?