Life-history and spatial determinants of somatic growth dynamics in Komodo dragon populations

Exceptionally rapid tooth development and ontogenetic changes in the feeding apparatus of the Komodo dragon

Dental developmental and replacement patterns in extinct amniotes have attracted a lot of attention. Notable among these are Paleozoic predatory synapsids, but also Mesozoic theropod dinosaurs, well known for having true ziphodonty, strongly serrated carinae with dentine cores within an enamel cap. The Komodo dragon, Varanus komodoensis, is the only extant terrestrial vertebrate to exhibit true ziphodonty, making it an ideal model organism for gaining new insights into the life history and feeding behaviours of theropod dinosaurs and early synapsids. We undertook a comparative dental histological analysis of this extant apex predator in combination with computed tomography of intact skulls. This study allowed us to reconstruct the dental morphology, ontogeny, and replacement patterns in the largest living lizard with known feeding behaviour, and apply our findings to extinct taxa where the behaviour is largely unknown. We discovered through computed tomography that V. komodoensis maintains up to five replacement teeth per tooth position, while histological analysis showed an exceptionally rapid formation of new teeth, every 40 days. Additionally, a dramatic ontogenetic shift in the dental morphology of V. komodoensis was also discovered, likely related to changes in feeding preferences and habitat. The juveniles have fewer dental specializations, lack true ziphodonty, are arboreal and feed mostly on insects, whereas the adults have strongly developed ziphodonty and are terrestrial apex predators with defleshing feeding behaviour. In addition, we found evidence that the ziphodont teeth of V. komodoensis have true ampullae (interdental folds for strengthening the serrations), similar to those found only in theropod dinosaurs. Comparisons with other species of Varanus and successive outgroup taxa reveal a complex pattern of dental features and adaptations, including the evolution of snake-like tongue flicking used for foraging for prey. However, only the Komodo dragon exhibits this remarkable set of dental innovations and specializations among squamates.

Functional Anatomy of the Thoracic Limb of the Komodo Dragon (Varanus komodoensis)

Since the Komodo dragon has been included on The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, it is crucial to know in detail its biology as there is a limited availability of research material on these animals-mainly those who died in zoos or whose remains were found in the wild. Anatomy is essential for understanding physiology, identification of diseases, adaptations in the environment, and behavior. In this dissection study, the relationship of individual anatomical structures was analyzed, the anatomy of the active and passive movement system of the thoracic limb was described, photographs were taken, and a radiographic examination was conducted. This species has its own differences, even within closely related lizard species. Varanus komodoensis possesses triceps muscles with three heads, and the wrist is extended with additional bones for greater flexibility of the hand. The muscles of the forelimb are analogous to the hind limb; however, they differ in the mass of individual muscles, especially those predisposed to perform the most important antigravity and locomotive functions.

Turning ghosts into dragons: improving camera monitoring outcomes for a cryptic low-density Komodo dragon population in eastern Indonesia

Abstract Context Detection probability is a key attribute influencing population-level wildlife estimates necessary for conservation inference. Increasingly, camera traps are used to monitor threatened reptile populations and communities. Komodo dragon (Varanus komodoensis) populations have been previously monitored using camera traps; however, considerations for improving detection probability estimates for very low-density populations have not been well investigated. Aims Here we compare the effects of baited versus non-baited camera monitoring protocols to influence Komodo dragon detection and occupancy estimates alongside monitoring survey design and cost considerations for ongoing population monitoring within the Wae Wuul Nature Reserve on Flores Island, Indonesia. Methods Twenty-six camera monitoring stations (CMS) were deployed throughout the study area with a minimum of 400 m among CMS to achieve independent sampling units. Each CMS was randomly assigned as a baited or non-baited camera monitoring station and deployed for 6 or 30 daily sampling events. Key results Baited camera monitoring produced higher site occupancy estimates with reduced variance. Komodo dragon detection probability estimates were 0.15 ± 0.092–0.22 (95% CI), 0.01 ± 0.001–0.03, and 0.03 ± 0.01–0.04 for baited (6 daily survey sampling events), unbaited (6 daily survey sampling events) and long-unbaited (30 daily survey sampling events) sampling durations respectively. Additionally, the provision of baited lures at cameras had additional benefits for Komodo detection, survey design and sampling effort costs. Conclusions Our study indicated that baited cameras provide the most effective monitoring method to survey low-density Komodo dragon populations in protected areas on Flores. Implications We believe our monitoring approach now lends itself to evaluating population responses to ecological and anthropogenic factors, hence informing conservation efforts in this nature reserve.

Population structure, genomic diversity and demographic history of Komodo dragons inferred from whole-genome sequencing

Population and conservation genetics studies have greatly benefited from the development of new techniques and bioinformatic tools associated with next-generation sequencing. Analysis of extensive data sets from whole-genome sequencing of even a few individuals allows the detection of patterns of fine-scale population structure and detailed reconstruction of demographic dynamics through time. In this study, we investigated the population structure, genomic diversity and demographic history of the Komodo dragon (Varanus komodoensis), the world's largest lizard, by sequencing the whole genomes of 24 individuals from the five main Indonesian islands comprising the entire range of the species. Three main genomic groups were observed. The populations of the Island of Komodo and the northern coast of Flores, in particular, were identified as two distinct conservation units. Degrees of genomic divergence among island populations were interpreted as a result of changes in sea level affecting connectivity across islands. Demographic inference suggested that Komodo dragons probably experienced a relatively steep population decline over the last million years, reaching a relatively stable N_e during the Saalian glacial cycle (400-150 thousand years ago) followed by a rapid N_e decrease. Genomic diversity of Komodo dragons was similar to that found in endangered or already extinct reptile species. Overall, this study provides an example of how whole-genome analysis of a few individuals per population can help define population structure and intraspecific demographic dynamics. This is particularly important when applying population genomics data to conservation of rare or elusive endangered species.

Differences in growth between offspring of anadromous and freshwater brown trout Salmo trutta

In this study, individual growth of juvenile offspring of anadromous and freshwater resident brown trout Salmo trutta and crosses between the two from the River Imsa, Norway, was estimated. The juveniles were incubated until hatching at two temperatures (±S.D.), either 4.4 ± 1.5°C or 7.1 ± 0.6°C. Growth rate was estimated for 22 days in August-September when the fish on average were c. 8 g in wet mass, and the estimates were standardized to 1 g fish dry mass. Offspring of anadromous S. trutta grew better at both 15 and 18°C than offspring of freshwater resident S. trutta or offspring of crosses between the two S. trutta types. This difference appears not to result from a maternal effect because anadromous S. trutta grew better than the hybrids with anadromous mothers. Instead, this appears to be an inherited difference between the anadromous and the freshwater resident fish lending support to the hypothesis that anadromous and freshwater resident S. trutta in this river differ in genetic expression. Egg incubation temperature of S. trutta appeared not to influence the later growth as reported earlier from the studies of Atlantic salmon Salmo salar.

Identifying island safe havens to prevent the extinction of the World's largest lizard from global warming

The Komodo dragon (Varanus komodoensis) is an endangered, island‐endemic species with a naturally restricted distribution. Despite this, no previous studies have attempted to predict the effects of climate change on this iconic species. We used extensive Komodo dragon monitoring data, climate, and sea‐level change projections to build spatially explicit demographic models for the Komodo dragon. These models project the species’ future range and abundance under multiple climate change scenarios. We ran over one million model simulations with varying model parameters, enabling us to incorporate uncertainty introduced from three main sources: (a) structure of global climate models, (b) choice of greenhouse gas emission trajectories, and (c) estimates of Komodo dragon demographic parameters. Our models predict a reduction in range‐wide Komodo dragon habitat of 8%–87% by 2050, leading to a decrease in habitat patch occupancy of 25%–97% and declines of 27%–99% in abundance across the species' range. We show that the risk of extirpation on the two largest protected islands in Komodo National Park (Rinca and Komodo) was lower than other island populations, providing important safe havens for Komodo dragons under global warming. Given the severity and rate of the predicted changes to Komodo dragon habitat patch occupancy (a proxy for area of occupancy) and abundance, urgent conservation actions are required to avoid risk of extinction. These should, as a priority, be focused on managing habitat on the islands of Komodo and Rinca, reflecting these islands’ status as important refuges for the species in a warming world. Variability in our model projections highlights the importance of accounting for uncertainties in demographic and environmental parameters, structural assumptions of global climate models, and greenhouse gas emission scenarios when simulating species metapopulation dynamics under climate change.

An effect size statistical framework for investigating sexual dimorphism in non-avian dinosaurs and other extinct taxa

Despite reports of sexual dimorphism in extinct taxa, such claims in non-avian dinosaurs have been rare over the last decade and have often been criticized. Since dimorphism is widespread in sexually reproducing organisms today, under-reporting in the literature might suggest either methodological shortcomings or that this diverse group exhibited highly unusual reproductive biology. Univariate significance testing, especially for bimodality, is ineffective and prone to false negatives. Species recognition and mutual sexual selection hypotheses, therefore, may not be required to explain supposed absence of sexual dimorphism across the grade (a type II error). Instead, multiple lines of evidence support sexual selection and variation of structures consistent with secondary sexual characteristics, strongly suggesting sexual dimorphism in non-avian dinosaurs. We propose a framework for studying sexual dimorphism in fossils, focusing on likely secondary sexual traits and testing against all alternate hypotheses for variation in them using multiple lines of evidence. We use effect size statistics appropriate for low sample sizes, rather than significance testing, to analyse potential divergence of growth curves in traits and constrain estimates for dimorphism magnitude. In many cases, estimates of sexual variation can be reasonably accurate, and further developments in methods to improve sex assignments and account for intrasexual variation (e.g. mixture modelling) will improve accuracy. It is better to compare estimates for the magnitude of and support for dimorphism between datasets than to dichotomously reject or fail to reject monomorphism in a single species, enabling the study of sexual selection across phylogenies and time. We defend our approach with simulated and empirical data, including dinosaur data, showing that even simple approaches can yield fairly accurate estimates of sexual variation in many cases, allowing for comparison of species with high and low support for sexual variation.

Knee deep in trouble: rusa deer use an aquatic escape behaviour to delay attack by Komodo dragons

We document six observations of an aquatic behaviour used by rusa deer (Rusa timorensis) to delay an imminent attack from Komodo dragons (Varanus komodoensis). This unusual behaviour arose after rusa deer fled into the nearby seawater following an attack from a solitary Komodo dragon. Once in the sea, rusa deer remained relatively stationary by standing in shallow water (<1 m deep) for up to 4 h. This behaviour generally allowed rusa deer to avoid an in-water attack from Komodo dragons. However, if rusa did not die from injuries, they moved back onto land and were subsequently killed by Komodo dragons. The aquatic behaviour delays subsequent attacks on rusa deer by Komodo dragons, but this appears only to postpone, rather than prevent, the deer’s death.

Tail regeneration and other phenomena of wound healing and tissue restoration in lizards

Wound healing is a fundamental evolutionary adaptation with two possible outcomes: scar formation or reparative regeneration. Scars participate in re-forming the barrier with the external environment and restoring homeostasis to injured tissues, but are well understood to represent dysfunctional replacements. In contrast, reparative regeneration is a tissue-specific program that near-perfectly replicates that which was lost or damaged. Although regeneration is best known from salamanders (including newts and axolotls) and zebrafish, it is unexpectedly widespread among vertebrates. For example, mice and humans can replace their digit tips, while many lizards can spontaneously regenerate almost their entire tail. Whereas the phenomenon of lizard tail regeneration has long been recognized, many details of this process remain poorly understood. All of this is beginning to change. This Review provides a comparative perspective on mechanisms of wound healing and regeneration, with a focus on lizards as an emerging model. Not only are lizards able to regrow cartilage and the spinal cord following tail loss, some species can also regenerate tissues after full-thickness skin wounds to the body, transections of the optic nerve and even lesions to parts of the brain. Current investigations are advancing our understanding of the biological requirements for successful tissue and organ repair, with obvious implications for biomedical sciences and regenerative medicine.

The effect of growth rate and ageing on colour variation of European pond turtles

Many chelonians have colourful dots, patches and stripes throughout their body that are made up, at least in part, of carotenoids. Therefore, turtles are very suitable models to study the evolution and functionality of carotenoid-based colouration. Recent studies suggested a close link between colouration and immune system in these taxa. However, more research is needed to understand the role of these colourful stripes and patches in turtle visual signalling. The purpose of this study was to explore the relationship between growth rate and colouration in European pond turtles. In particular, we wanted to answer the question of whether there is a trade-off between growth and colour expression. We also aimed to explore the effect of body size and age on colour variation. Turtles from a reintroduction-breeding program were recaptured, weighed and measured over an 8-year period to estimate their growth rates and age. We also measured with a spectrometer the reflectance of colour patches in two different body parts: shell and forelimb. We found that turtles with a faster growth rate had brighter limb stripes independently of their age. On the other hand, shell colouration was related to body size with larger turtles having brighter shell stripes and higher values of carotenoid chroma. Our results suggest that fast-growers may afford to express intense colourful limb stripes likely due to their higher intake of carotenoids that would modulate both growth and colour expression. However, shell colouration was related to body size probably due to ontogenetic differences in the diet, as juveniles are strictly carnivorous while adults are omnivorous. Alternatively, shell colouration might be involved in crypsis as the shell is visually exposed to predators.

Cofilin Activation Is Temporally Associated with the Cessation of Growth in the Developing Hippocampus

Dendritic extension and synaptogenesis proceed at high rates in rat hippocampus during early postnatal life but markedly slow during the third week of development. The reasons for the latter, fundamental event are poorly understood. Here, we report that levels of phosphorylated (inactive) cofilin, an actin depolymerizing factor, decrease by 90% from postnatal days (pnds) 10 to 21. During the same period, levels of total and phosphorylated Arp2, which nucleates actin branches, increase. A search for elements that could explain the switch from inactive to active cofilin identified reductions in β1 integrin, TrkB, and LIM domain kinase 2b, upstream proteins that promote cofilin phosphorylation. Moreover, levels of slingshot 3, which dephosphorylates cofilin, increase during the period in which growth slows. Consistent with the cofilin results, in situ phalloidin labeling of F-actin demonstrated that spines and dendrites contained high levels of dynamic actin filaments during Week 2, but these fell dramatically by pnd 21. The results suggest that the change from inactive to constitutively active cofilin leads to a loss of dynamic actin filaments needed for process extension and thus the termination of spine formation and synaptogenesis. The relevance of these events to the emergence of memory-related synaptic plasticity is described.

Embryonic metabolism of the ornithischian dinosaurs Protoceratops andrewsi and Hypacrosaurus stebingeri and implications for calculations of dinosaur egg incubation times

The embryonic metabolisms of the ornithischian dinosaurs Protoceratops andrewsi and Hypacrosaurus stebingeri have been determined and are in the range observed in extant reptiles. The average value of the measured embryonic metabolic rates for P. andrewsi and H. stebingeri are then used to calculate the incubation times for 21 dinosaurs from both Sauischia and Ornithischia using a mass growth model based on conservation of energy. The calculated incubation times vary from about 70 days for Archaeopteryx lithographica to about 180 days for Alamosaurus sanjuanensis. Such long incubation times seem unlikely, particularly for the sauropods and large theropods. Incubation times are also predicted with the assumption that the saurischian dinosaurs had embryonic metabolisms in the range observed in extant birds.

Relative role of population density and climatic factors in shaping the body growth rate of Lilford’s Wall Lizard (Podarcis lilfordi)

The body growth rate in small reptiles is modulated by per-capita food resources and recent evidences suggested that this constraint is the mechanism underlying differences between cohorts. Per-capita food resources depend on population size and climatic factors, but their relative role in explaining the variations in growth rate is unclear. We used morphological data collected over 6 years to model the body growth of an insular lizard (Lilford’s Wall Lizard, Podarcis lilfordi (Günther, 1874)). We used nonlinear equations to describe the appropriate length-at-age relationship. For each sex, seasonal growth was observed and the oscillatory von Bertalanffy curve was fitted to the data. Three age classes were recognized, and we investigated the relative role of density, spring rainfall, and temperature in explaining the variation of the growth rate in each sex-by-age class. Results showed that the relative role of density and climatic factors varied according to the age considered. While population size and temperature had a negative effect on the growth rate of juveniles, rainfall had a positive influence on the growth of subadults. Adult growth was near zero and constant over time. The different role of density-dependent and climatic factors in explaining age-dependent growth rate provides an important insight in understanding lizard population dynamics and life-history tactics.

Energetic constraint of non-monotonic mass change during offspring growth: a general hypothesis and application of a new tool

Post-natal growth is an important life-history trait and can be a sensitive indicator of ecological stress. For over 50 years, monotonic (never-decreasing) growth has been viewed as the predominant trajectory of post-natal mass change in most animal species, notably among birds. However, prevailing analytical approaches and energetic constraints may limit detection of non-monotonic (or multiphasic), determinate growth patterns, such as mass recession in birds (weight loss prior to fledging, preceded by overshooting adult mass), which is currently believed to be restricted to few taxa. Energetic surplus and shortfall are widespread conditions that can directly influence the degree of mass overshooting and recession. Thus, we hypothesize that in many species, prevailing energetic constraints force mass change away from a fundamental non-monotonic trajectory to instead follow a monotonic curve. We observed highly non-monotonic, mass change trajectories (overshooting adult mass by up to almost 20%) among common tern Sterna hirundo chicks, a well-studied species long-established as growing monotonically. We quantified the prevalence and magnitude of non-monotonic mass change prior to fledging for 313 common tern chicks that successfully fledged from two discrete populations in multiple years. We used a new approach for analysing non-monotonic curves to examine differences in mass change trajectories between populations under contrasting abiotic (freshwater vs. saltwater) and biotic stresses (low rates of food provisioning). Some degree of mass recession occurred in 73% of all study chicks. Overshooting adult mass followed by extensive mass recession was most prevalent at our freshwater colony, being detected among 34-38% of chicks annually. Non-monotonic trajectories were less marked in populations experiencing ecological stress and among lower quality individuals. Chicks that were provisioned at higher rates were more likely to both overshoot adult mass and experience subsequent mass recession. Our results in common terns provide strong support for the hypothesis that non-monotonic trajectories are the fundamental pattern of mass change but are constrained to be monotonic under energetic shortfall. This justifies future tests of the generality of this hypothesis across a broad range of taxa. We also demonstrate a recent analytical tool that prevents routine fitting of monotonic curves without prior investigation of non-monotonic trends.

Metabolism of dinosaurs as determined from their growth

A model based on cellular properties is used to analyze the mass growth curves of 20 dinosaurs. This analysis yields the first measurement of the average cellular metabolism of dinosaurs. The organismal metabolism is also determined. The cellular metabolism of dinosaurs is found to decrease with mass at a slower rate than is observed in extant animals. The organismal metabolism increases with the mass of the dinosaur. These results come from both the Saurischia and Ornithischia branches of Dinosauria, suggesting that the observed metabolic features were common to all dinosaurs. The results from dinosaurs are compared to data from extant placental and marsupial mammals, a monotreme, and altricial and precocial birds, reptiles, and fish. Dinosaurs had cellular and organismal metabolisms in the range observed in extant mesotherms.

Dinosaur physiology. Evidence for mesothermy in dinosaurs

Were dinosaurs ectotherms or fast-metabolizing endotherms whose activities were unconstrained by temperature? To date, some of the strongest evidence for endothermy comes from the rapid growth rates derived from the analysis of fossil bones. However, these studies are constrained by a lack of comparative data and an appropriate energetic framework. Here we compile data on ontogenetic growth for extant and fossil vertebrates, including all major dinosaur clades. Using a metabolic scaling approach, we find that growth and metabolic rates follow theoretical predictions across clades, although some groups deviate. Moreover, when the effects of size and temperature are considered, dinosaur metabolic rates were intermediate to those of endotherms and ectotherms and closest to those of extant mesotherms. Our results suggest that the modern dichotomy of endothermic versus ectothermic is overly simplistic.

Can camera traps monitor Komodo dragons a large ectothermic predator?

Camera trapping has greatly enhanced population monitoring of often cryptic and low abundance apex carnivores. Effectiveness of passive infrared camera trapping, and ultimately population monitoring, relies on temperature mediated differences between the animal and its ambient environment to ensure good camera detection. In ectothermic predators such as large varanid lizards, this criterion is presumed less certain. Here we evaluated the effectiveness of camera trapping to potentially monitor the population status of the Komodo dragon (Varanus komodoensis), an apex predator, using site occupancy approaches. We compared site-specific estimates of site occupancy and detection derived using camera traps and cage traps at 181 trapping locations established across six sites on four islands within Komodo National Park, Eastern Indonesia. Detection and site occupancy at each site were estimated using eight competing models that considered site-specific variation in occupancy (ψ)and varied detection probabilities (p) according to detection method, site and survey number using a single season site occupancy modelling approach. The most parsimonious model [ψ (site), p (site*survey); ω = 0.74] suggested that site occupancy estimates differed among sites. Detection probability varied as an interaction between site and survey number. Our results indicate that overall camera traps produced similar estimates of detection and site occupancy to cage traps, irrespective of being paired, or unpaired, with cage traps. Whilst one site showed some evidence detection was affected by trapping method detection was too low to produce an accurate occupancy estimate. Overall, as camera trapping is logistically more feasible it may provide, with further validation, an alternative method for evaluating long-term site occupancy patterns in Komodo dragons, and potentially other large reptiles, aiding conservation of this species.