Marco is a PhD student at the Monash University, Australia. He is a evolutionary biologist with special focus on biogeography and ecomorphological evolution of lizards. Here, Marco shares his recent work on biogeographic patterns of limb reduction in skinks.
The evolutionary biologist Marco Camaiti
Personal links. Twitter
Institute. Monash University
Academic life stage. PhD student
Major research themes. Evolution, ecomorphology, biogeography, lizards, osteology
Current study system. Currently, I am studying skinks (a group of lizards), specifically forms with reduced or absent legs. These animals are fascinating because of their incredible variety of body shapes, which include a series of intermediates between lizard-like and long-limbed, and snake-like and limbless. Limb reduction also tends to be coupled with the elongation of the trunk and an increase in the number of presacral vertebrae. Skinks also make a great model system to study phenotypic evolution and adaptations because they independently evolved these body shapes multiple times (more than fifty, in fact) and are widespread on almost all continents and through a variety of environments.
Recent JBI paper. Camaiti, M., Evans, A. R., Hipsley, C. A., Hutchinson, M. N., Meiri, S., Anderson, R. O., Slavenk, A., & Chapple, D. G. (2023). Macroecological and biogeographical patterns of limb reduction in the world’s skinks. Journal of Biogeography, 50(2), 428-440. https://doi.org/10.1111/jbi.14547
Motivation behind this paper. The evolution of limb-reduced forms in squamates has been variously investigated; however, the environmental drivers that gave rise to the existing variability of limb-reduced shapes have so far remained unexplored beyond the simple distinction between fossorial and above-ground ecomorphs. For example, different groups of skinks have reduced or lost their legs in different ways, some reducing their forelimbs faster, others their hindlimbs, and in some groups, both are reduced at the same rate. Moreover, skinks have evolved limb reduction and loss in different continents and environmental contexts, ranging from arid desert sands to humid forest floors. In our recent paper, we aimed to investigate the link between these morphologies and their environments and whether similar, if independently evolved morphologies correspond to similar environmental conditions.
Fitzroy Sandslider (Lerista simillima), a limb-reduced skink without forelimbs.
Key methodologies. We first quantified morphological variation in skinks by regressing measurements of body proportions to identify divergent evolutionary trajectories of limb reduction across subfamilies and focal clades. One of the innovative aspects of this approach was to use limb disparity as a new metric to quantify variations in limb proportions, which tends to 1 for species with longer hindlimbs than forelimbs and to -1 for species with longer forelimbs than hindlimbs. Secondly, we investigated whether aspects of the skink’s morphologies (i.e., limb proportions and elongation of the body) were associated with specific substrate categories for our global dataset of limb-reduced skinks. To investigate ecomorphological associations beyond simple categories, we conducted a parallel analysis using ecological data extracted from the distributions of a large subclade of limb-reduced skinks, the Australian sphenomorphines.
North-western Sandslider (Lerista bipes), a limb-reduced skink without forelimbs. Detail of the foot.
Unexpected challenges. It was certainly challenging to conduct comparative analyses of the relations between all possible combinations of body measurements across clades for almost 300 species. One unexpected result from these analyses was that some clades really have unique ways of becoming limb-reduced and elongating their bodies. In particular, we were surprised to find that one clade, the skink genus Glaphyromorphus, had evolved elongated bodies by increasing the length of its vertebrae instead of their number. Another significant hurdle we encountered was obtaining precise microhabitat data across the distribution of Australian species for our downscaled analyses, given that each had to be extracted and averaged across thousands of data points. Surprisingly, ecomorphological analyses conducted on these data found that more dramatically limb-reduced forms associate with humid, cool, and carbon-rich microhabitats as opposed to dry, arid environments, challenging the assumption that limb reduction is necessarily an adaptation to desert conditions.
Major results. Different clades of skinks evolved limb reduction following different trajectories. Contrary to what previously thought, we found that the shortening of limbs does not correspond to the elongation of the trunk in all clades, and that trunk elongation is not always achieved via an increase in the number of presacral vertebrae. Beyond that, our global ecomorphological investigation found that skinks with longer hindlimbs than forelimbs (positive limb disparity) associate with poor, sandy substrates, as opposed to skinks with equal or shorter hindlimbs, which associate with richer substrates. In other words, more penetrable substrates like humus and sand are generally associated with smaller body sizes. The smaller-scale investigation of the Australian sphenomorphines found a similar result, showing that positive limb disparity is associated with drier, poorer substrates with higher sand percentages. Contrary to previous assumptions, both limb reduction and body elongation are associated with rich, more humid substrates instead.
Dampier Land Limbless Slider (Lerista apoda), a limbless skink.
Next steps for this research. We are interested in investigating other aspects of the morphology of skinks with limb-reduced forms, specifically the internal changes that accompany the evolutionary transitions to limb reduction. We also would like to investigate aspects sensorial adaptations that make fossorial forms so efficient at navigating subterranean environments. Currently, we are working on examining the morphology of the internal bone structures of these animals using computerized tomography (CT) scanning techniques.
If you could study any organism on Earth, what would it be? A squamate, no doubt. They are critically understudied, but they deserve way more attention than they’re currently getting! The extent to which they can change according to different environments is almost unparalleled among tetrapods, making them a unique model system to study evolution.
Anything else to add? I find that the most impressive forms are the ones where a pair of legs is completely gone or vestigial, while the other pair still remains and can actually be quite well-developed. Interestingly, hindlimb-only forms seem to have independently evolved across continents and islands, being found in different lineages across Australia, Madagascar, Africa and Asia, always in association with sand and sandy soil. While forelimb-only forms are rarer, they do exist, and appear to associate with rich substrates like humus.
Jackson’s Three-toed Slider (Lerista jacksoni), a limb-reduced skink with three fingers and toes.