However, most of them are in the 1–2 m range and there is a group of small-bodied Australasian monitors, of which the smallest is around 20 cm long. Most of them are intelligent and active predators, and the group includes the largest terrestrial lizards to have ever existed such as the five-meter extinct Megalania and the three-meter Komodo dragon. Monitor lizards (genus Varanus) are a fascinating group of reptiles. When I arrived at Scott Keogh’s lab in the Australian National University to start my PhD, my lab-mate and friend Ian Brennan was working on the phylogeny of monitor lizards (look at the results here). Furthermore, comparative studies require a good understanding of evolutionary relationships (aka phylogeny), which are unknown in many groups of organisms. This is time-consuming and obtaining good sample sizes can be difficult for species that have rarely been collected for scientific purposes. Part of the reason is that these studies require recording morphological data for many individuals of different ages. While ontogeny has intrigued scientists for a long time, studies comparing the ontogenetic trajectories of different species are scarce and usually focus on a few species. Partially taken from Figure 1 in our paper. Heterochronic and non-heterochronic changes are shown in the left and right, respectively. The bottom panels show the different ways in which the evolution of the trajectories can proceed. The top panel illustrates isometric growth and two types of allometric scaling. Conceptualization of ontogenetic changes as trajectories. A classic example is the adult axolotl, an aquatic salamander that resembles the young of other salamander species. When it happens, some species will end up looking more “adult-like” (peramorphic) with respect to others that are “juvenile-like” (paedomorphic). Heterochrony refers to changes in the rate or timing of development. In the former, adults of different species are more similar to each other than juveniles, while in the latter it’s juveniles that are alike. Changes in direction can result in ontogenetic convergence or divergence. Changes in the intercept produce parallel trajectories. Thus, changes in body size impose limits on the variation of other traits.Įvolutionarily speaking, ontogenetic allometric trajectories can differ between species in several ways. ![]() The legs of adult t-rexes were proportionally shorter and more robust, or they would collapse under the massive weight. For example, the long and slender legs of juvenile t-rexes were built for speed. ![]() Allometry is common in nature because if body parts scaled proportionally to size they would not be as efficient biomechanically. For example, humans show allometric growth because babies have heads that are larger in relation to their bodies than those of adults. ![]() The size of traits can scale either proportionally (isometry) or not (allometry) with respect to body size. We can think of ontogenetic changes in morphology as trajectories in a two-dimensional space, where we have body size (commonly used as a representation of age) in the x-axis and another trait (e.g., head size) in the y-axis. These scientists laid out a conceptual framework that is still used today. It then comes as no surprise that ontogeny captured the attention of many influential evolutionary biologists. These examples illustrate how growth and development (ontogeny) can have a large impact on the biology of organisms. In a very dramatic example, the morphology of caterpillars and butterflies has evolved to efficiently solve their most pressing priorities: eating for one, breeding for the other. Babies and adults of the same species may also show wildly different body types that reflect how the individuals in each of these life stages interact with their environment. However, adult leatherbacks are formidable hunters of soft-bodied animals and have few natural predators. As we have learned from tearful nights watching nature documentaries, those poor souls are the favourite snack of every animal on the beach. For starters, babies are usually much smaller and this impacts how they interact with the world. Babies and adults of most animals are very different from each other.
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