Marco Camaiti is a PhD student at the Monash University in Australia. He is a herpetologist interested in the evolution of vertebrate diversity. Here, Marco shares his recent work on limb reduction and loss in skink lizards.
Marco Camaiti during fieldwork in Australia.
Personal links. Twitter
Institute. School of Biological Sciences, Monash University, Australia
Academic life stage. PhD
Major research themes. Herpetology; Vertebrate morphology; Ecomorphology; Osteology; CT-scanning
Current study system. I work with one of the largest groups of lizards: skinks. They are exceptional among tetrapods as they evolved short-limbed and elongated forms several times independently. Understanding how they underwent such dramatic shifts in body shapes, and what caused them to happen so often may provide interesting insights into the inner mechanisms of evolution. What makes them a compelling model system is also the significant variability in these forms in terms of body shapes, from fully limbed to limbless, with intermediate forms in between, plus the wide range of environments they inhabit, from the forest floor to desert sands.
Recent JBI paper. Camaiti, M., Evans, A. R., Hipsley, C. A., Hutchinson, M. N., Meiri, S., Anderson, R. O., Slavenko, A., & Chapple, D. G. (2022). A database of the morphology, ecology and literature of the world’s limb‐reduced skinks. Journal of Biogeography, 49(7), 1397-1406. https://doi.org/10.1111/jbi.14392
Motivation behind this paper. More and more, researchers rely on large datasets to analyse evolutionary trends in biodiversity. Thus, as a part of our investigation on limb reduction in skinks (our model system), we constructed a database on the body shapes of these lizards including data for all worldwide species that could be identified as limb-reduced, as well as their close relatives. Another motivation behind this work was to conduct the first comprehensive census of Scincidae – the skink family – to understand how many species and genera had undergone limb reduction, as well as how they are geographically distributed compared with fully-limbed species.
Key methodologies. We collected data on body shape and ecology for 606 species of limb-reduced skinks and their congeners (those belonging to the same genera) based on the existing literature (around 800 published works), spanning over 150 years. To process species’ body shapes, we averaged data on linear body measurements as a function of body size, as well as including digit counts and numbers of presacral vertebrae. As limb-reduced species, we considered those with forelimbs ≤15% of their body length and/or hind-limbs ≤20% of their body length; as limbless, those with lacking limbs; and fully limbed, those with higher limb proportions. To process species’ ecology, we included a broad habitat categorisation based on species’ preferred soil type. Finally, to validate the utility of the dataset, we reconstructed the number of independent events of limb reduction and loss in skinks by mapping them on a phylogeny.
Anomalopus leuckartii (left) and Lerista terdigitata (right), two limb-reduced skinks from Australia (Credits: Jules Farquhar).
Unexpected challenges. A first major challenge was sourcing and obtaining our data. This process was a massive endeavour that took more than two years to complete, given the large number and variety of sources. For example, many head length measurements were obtained directly from scaled figures in species description papers, as they were not mentioned in the text. Another obstacle we faced was compounding data from very different sources, languages, times, and units of measurement. For instance, some papers were in German, French, Italian, or even Latin. Due to how old some sources were, the units of measurement could be quite antiquated, and we had to convert quite a few measurements from the Prussian zoll or the French ligne into centimeters.
Major results. We provide a readily-available collection of data on the body shapes of skink lizards, which has a massive potential to contribute to the study of evolutionary processes based on dramatic shifts in morphology. Our data evidences an unequivocal quantification of the sheer abundance and variety of limb reduction in skinks. Based on our new definition of limb reduction (based on limb proportions instead of digit numbers), we establish that more than 20% of all skink species are limb-reduced or limbless. Our analyses demonstrate that limb reduction evolved independently an astounding number of times in skinks (53 or more), and that loss of both pairs of limbs was also recurrent (at least 24 times). These findings bring skinks into the pole-position among lizards in terms of the number of independent transitions to limb reduction.
Lerista edwardsae, a limb-reduced skink with hindlimbs only (left), and Coeranoscincus frontalis, a limbless skink (right), from Australia (Credits: Jules Farquhar).
Next steps for this research. We are investigating the patterns of limb reduction in skinks, as well as their environmental drivers, using the previously compiled dataset. As most of these skinks are fossorial (they burrow in the soil), we will examine the relationships between body shapes and their main environment, the substrate. We are also working on the relationships between the external body shapes of skinks, and their internal morphology (bones), to gather finer details of morphological transformations based on differential degrees of reduction in the bones of girdles and limbs.
If you could study any organism on Earth, what would it be? I am very partial to squamates, so I would say a lizard. They are the ultimate, and ultimately versatile study system: distributed almost everywhere on the planet, with a stunning variety of forms, and an even more stunning evolutionary history that has gone in several interesting directions through time.
Anything else to add? As I hope it has come across in my responses, limb reduction is a fascinating topic that deserves more attention. I’d like to share one last interesting and surprising fact about limb reduction: it can happen asymmetrically. That is, usually, the forelimbs end up being reduced or lost faster than the hindlimbs, but sometimes this happens to the hindlimbs instead! Interestingly, in skinks the former (forelimbs reduced first) is much more common than the latter (hind-limbs first), but in other groups, such as the South American gymnophthalmids, the general pattern is the opposite. We suspect that this difference might have to do with the type of environment the animals live in, but we are still investigating it.