Very small animals reside their entire lives encompassed by physical and sensory challenges disparate from our own. A parasitoid wasp struggles to break the water’s surface tension, and a tiger beetle moves so rapidly that it becomes functionally blind. By simply changing the scale of the surrounding environment, body size can have profound effects on an organism’s ecology, and as a consequence, the cognitive processes needed to interpret and respond to stimuli in the periphery.
There is a predominance of work examining the evolutionary changes in body size and its effects, but less often a focus specifically on the small extremes — a phenomenon often called ‘miniaturization’. In my recent paper with Manuel Leal, we draw from studies of vertebrates and invertebrates to provide a theoretical framework for miniaturization (download). Central to miniaturization is a decrease in body size beyond a certain threshold at which marked anatomical or ecological changes occur to maintain organismal function. Perhaps wings become so small that aerodynamics requires distinct feathery structures, or brain structures become deformed or reduced (yet still functional!) to fit inside tiny skulls.
In our review, we advocate that squamates (and lizards, specifically!) are a good group to examine how biological processes are affected by miniaturization. First, we provide a conservative body size threshold for miniaturized lizards, informed by previous work on unusual skull arrangements in very tiny species. Next, we use phylogenetic tools to illustrate high variability in body size and the repeated evolution of miniaturization in the evolutionary history of lizards— spanning across 11 families. In case you’re wondering, a chameleon currently holds the title of the most diminutive of the bunch, and a dwarf gecko is a close second!
We then conducted a literature review to gather ecological data for miniaturized species, revealing a tendency for miniaturized lizards to occupy terrestrial microhabitats, as opposed to arboreal ones. Small body size puts miniaturized lizards at a high risk of overheating and dehydration. In some cases, even minutes of exposure can result in death. Ground-dwelling and burrowing habits allow miniaturized lizards to make use of small pockets within fallen leaves or the sediment to adopt the temperature and humidity levels of their chosen refuges. Although living and hiding on the ground’s surface has its advantages, that isn’t to say miniaturized species cannot thrive high up in trees. In Kenya, I remember at times I would sit and watch dwarf geckos (Lygodactylus keniensis) enter and exit the domatia on acacia trees to relieve themselves from the savanna heat. Many species are also crepuscular which allows them to avoid exposure to the hottest parts of the day, and in deserts there may be a strong pattern towards nocturnal activity. Indeed, actively moving towards optimal sites in the environment (and at the right times), on the whole, appears to be crucial to the survival of miniaturized species.
In addition to exploring the ecological correlates with miniaturization, we touch upon neuroanatomy using micro-CT scans of two gecko specimens that have a large size disparity. Through quantifying the relative volumes of various brain regions, we observe a disproportionately large forebrain (telencephalon) in the miniaturized species. The forebrain in reptiles is primarily responsible for sensory integration, which allows animals to use distinct sensory information (i.e., visual & olfactory) synergistically to achieve a better representation of their surroundings. This is very relevant to miniaturized taxa because small nervous systems experience greater noise, making them less able to rapidly and accurately interpret sensory information. A relative increase in size of the telencephalon may then confer advantages in computational power for multimodal integration in miniaturized species. Ideally, I would have brain data various lineages to test whether miniaturization has caused convergent changes in brain morphology— much like I have done in my analysis using ecological traits. However, at the moment I cannot confidently generalize the effect of miniaturization on the lizard brain. In my future work, I plan to incorporate more brain data to explore this question.
Throughout this post I’ve shared several examples of miniaturized geckos from the lowland tropical rainforests of Costa Rica— all photographed after capture . Hopefully in the coming years I will continue gaining insight into the behavioral ecology of various miniaturized lizards by encountering and observing them in nature. In particular, I will work with the Puerto Rican radiation of dwarf geckos in the genus Sphaerodactylus to more rigorously examine the ideas and predictions I have proposed.