Australia and New Guinea, collectively referred to as the Sahul, share many biotic elements; but could not be more different in terms of climate and geological history. Australia as a landmass is relatively geologically stable, does not have much in the way of mountains and is predominantly covered in arid deserts. In contrast, New Guinea is relatively young in geological terms, has extensive mountains ranges across the entire island and is largely covered in tropical rainforests. From a biogeographical perspective these landmasses present very different opportunities and challenges to their prospective biotic assemblages, and yet, there are some lineages that have diversified throughout both regions. Agamids, also referred to as ‘dragons’, have proliferated in arid Australia represented by over 100 species and have been relatively well studied. With 20 recognised species, Melanesian agamids are much less diverse, have received much less scientific attention, and yet have been shown to be closely related to the Australian clade. In a paper published in the Biological Journal of the Linnean Society, we examine the evolution, biogeography and ecological diversity of Melanesian agamids. We tested whether they originated on the Australian Craton or proto-Papuan islands to the north. Also, we examined to what extent mountains have played a role in Melanesian agamid diversification and tested the evolutionary trends and origins of agamid ecological diversity.
In addition to the 11 recognised Melanesian agamids sampled, we found further monophyletic divergent lineages in four taxa. This included two lineages within Hypsilurus modestus, and three within H. magnus, H. papuensis and Lophosaurus dilophus. We found little evidence for micro-endemism in Melanesia with most taxa being widespread and low genetic divergence across the Central Cordillera suggesting these mountains have only recently become a barrier to dispersal. We identified clear evidence of past over-water dispersal in Hypsilurus agamids, with H. modestus lineages dispersing between New Britain, New Ireland and northeast New Guinea. Also, the restricted range of H. schoedei and disjunct distribution of H. longii indicates insular extinction events may have occurred. We identify the Australian Craton as the ancestral area of Lophosaurus and all Australo-Papuan agamids. However, Hypsilurus were clearly able to disperse over water and show deep divergences in the Melanesia region. The distribution of extant Hypsilurus lineages suggests a history of allopatric speciation on former islands with some now accreted onto New Guinea. We infer from the biome evolution analysis that Australo-Papuan agamids were associated with rainforests which date back to the early Miocene, but also that agamids had similarly long histories in other biomes. This indicates that while the rainforest biome might well be ancestral for many Australian radiations, semi-arid or seasonal environments also have a long history in Sahul. Finally, we note the disparity in ecological diversity within Australo-Papuan agamids. Despite their long history in rainforest regions there is a complete lack of Melanesian terrestrial or semi-terrestrial agamids compared with the Australian assemblage. This reflects fundamental differences in how these lizards diversify in dry versus wet habitats and suggests adverse environmental conditions, such as reduced solar radiation, as well as biotic interactions, such as competition or predation, form a barrier to ecological diversification in rainforest biomes.
Author: Oliver Tallowin
Uncovering the evolutionary trajectories of species assemblages can provide fascinating insights into the past environmental and geological processes, as well as the biological traits, that have led to present day diversity patterns. Furthermore, time-calibrated phylogenies can shed light on the historical sequence and timing of speciation events which, in turn, can be used to complement geological models aimed at reconstructing the formation of the earth. In our paper published in Molecular Phylogenetics and Evolution, we focus on the Melanesian radiation of bent-toed geckos (Cyrtodactylus), a clade occurring throughout New Guinea and adjacent islands, and Australia’s tropical northeast. We examine the sequence and timing of diversification in Australo-Papuan Cyrtodactylus and investigated three biogeographic scenarios. Firstly, did Cyrtodactylus diversification originate on the Australian Craton or former proto-Papuan islands to the north. Secondly, does Australo-Papuan Cyrtodactylus diversity correlate with distinct geological regions and to what degree do they exhibit infra-regional clustering. Lastly, to what extent did New Guinea mountain uplift impact Cyrtodactylus diversification and if so when did this occur.
Author: Oliver Tallowin
In a recent publication in the Journal of Animal Ecology we show that the fundamental changes to the mode of life that viviparity brings to squamate females, were surprisingly not reflected in either the number of offspring produced at a single reproductive event (birth, clutch), or their size, or the total mass of offspring produced relative to the size of their mother. The distributions of all these traits in viviparous squamates are remarkably similar to those of oviparous ones. Incidentally we have found that the mass of a recently hatched squamate is (on average, despite much variation) similar to the mass of the egg its mother laid.
In a recently published paper in the Biological Journal of the Linnean Society Shai has shown that the spread of ratios of hatchling or neonate masses to adult masses is very similar across the three classes of amniotes (mammals, birds and, of course, reptiles). This suggests that relatively large offspring are the ancestral and dominant mode of amniotes and have not evolved in response to the elaborate parental care of endotherms
The relative frequencies of the ratio of offspring size to adult size in mammals (grey), squamates (black) and birds (white). The peak at the smallest ratio is almost entirely composed of metatherians, but mammals (mostly bats) also dominate the highest ratio categories. Note that the range of values is narrower in birds than in either squamates or mammals.
We tend to think of ways to categorize animals either by shape (4 legs? 2 legs? no legs?, thin? Fat? big headed? small headed?) or phylogenetic affinities (reptiles? lesser animals? Snakes? Skinks?). But what about ecology?
The many aspects of a species ecological niche are generally quantified singly, or we refer to some abstract “multidimensional niche”, meaning we give up trying to characterize it before we even started. In a paper published recently in the Journal of Biogeography GARDians, led by Enav Vidan and Jonathan Belmaker, tried to define, and numerate, the main types of lizards out there – as reflected in their ecology.
We have selected four traits that we felt define much of the fundamental axes of ecological variation seen in lizards: 1. use of space / microhabitat preference, that defines where in the environment a lizard is active (on the ground? In trees or rocks? Under ground? In water?); 2. Activity times: species active in the same place, or even on the same branch or piece of ground, can segregate their use of the environment by dividing the temporal niche. Furthermore, being diurnal or nocturnal (or being cathemeral and enjoy both ‘worlds’) has strong implication on thermal biology and hence on metabolism and rates in which lizards take up resources and exchange them with the environment; 3. Diet: is a species insectivorous/carnivorous, as most lizards are? Or do they predominantly feed on plant matter (these guys seem to even take more leaves and plant parts with lower energy content)? Or do they in fact use both plants and animals (and then probably energy richer plant parts such as berries or sap)? This is directly related to the way a lizard affects its environment and may also influence its position across the sit and wait-active foraging continuum (no use waiting for plants); 4. Body size – while not an ecological trait per se size nonetheless strongly influences a host of ecological processes, from the degrees of metabolism and energy flow, to the types of available foods – and potential predators.
These four traits obviously interact, and some combinations may be more or less common than others: small, diurnal, terrestrial insectivore is after all the first picture to pop to mind when the term ‘lizard’ is introduced (except for the diehard gecko lovers among us, and well, there are a few of us with this infatuation). But are there tiny nocturnal herbivores? Or huge nocturnal lizards? How common is a marine iguana (large, herbivorous, swimming diurnal beast) type lizard – do we remember it simply because it is exotic?
We used Archetypal Analysis to assign lizards to types – or archetypes. Archetypal Analysis is an unsupervised machine learning technique that seeks to find the number of clusters that create the smallest convex hull in an n-dimensional trait space, by using the extreme values rather than the centroid of the clusters. AA assigns, for each species, a vector of affinities to each archetype. Most species are probabilistically assigned to several archetypes, with the partial probabilities summing to one.
We found that the most common functional trait combinations are (1) diurnal, terrestrial, carnivores (20% of the species); (2) diurnal, scansorial, carnivores (16%); and (3) nocturnal, scansorial, carnivores (13%).
Lizards could be robustly classified into seven ecological “Archetypes”:
We then partitioned the global spatial patterns of lizard richness into these seven archetypes, and found that each shows pretty distinct patterns. Turns out Australia is the main hotspot for the herbivorous, nocturnal, fossorial, and terrestrial strategies – and for lizards in general. The Amazon basin is the main hotspot for the semi-aquatic, and scansorial strategies, whereas the large strategy has pan-tropical hotspots, especially in both the Amazon Basin and Northern Australia, but also in Africa, SE Asia and Mexico.
Surprisingly, we found that functional diversity peaks in areas with medium species richness and slowly decreases toward the speciose areas. This unexpected richness-functional diversity unimodal association is also revealed within the scansorial, large, and semi-aquatic strategies. The richness patterns of terrestrial, nocturnal, herbivorous, and fossorial strategies increase with species richness, but globally functional richness peaks in areas with medium species richness.
Thus increases in richness do not necessarily stem from increased functional diversity. Species diversification within specific strategies often dominates richness patterns. Our findings support the contention that it is important to consider different functional and ecological subgroups when studying richness patterns.
Author: Shai Meiri
Throughout the years, scientists have formulated various ecological "rules" describing how body size evolves as an adaptation to various climatic factors – the first and most famous of these being Bergmann's Rule which posits animals increase in size in cold habitats as an adaptation to minimize heat loss.
In our recent paper published in Global Ecology and Biogeography, we examined trends in body size of squamates, utilizing GARD's massive dataset of distributions and body sizes. We examined these trends both at the assemblage level (how median size of squamate assemblages changes from one area to the next, and how it's correlated with climatic conditions in those areas) and at the species level (how body size changes from one species to the next, and how it's correlated with the climatic conditions experienced by each species).
Our most basic prediction was that if the proposed mechanisms behind these rules work, we'd see the expected correlations in most cases. What do we mean by that? If, for instance, Bergmann's Rule works, in most cases (squamates on different continents, or in different families, etc.) we'd see a negative relationship between size and temperature.
What we found is, for lack of a better term, a huge mess - the spatial patterns for squamates differ from the spatial patterns for lizards and snakes separately, and from continent to continent, and between different families. For each of the climatic variables we examined, we found positive relationships with size in roughly a third of the cases, negative relationships in roughly a third of the cases, and no relationships in about a third of the cases.
When we examined patterns at the species-level we found an extremely strong phylogenetic signal, which makes sense (geckos and skinks are typically all small, varanids and pythons are typically all large, etc.), and we found that climatic variables explain about 1-2% of the interspecific variation in body size, a fraction so small as to be almost negligible.
To sum it all up, our conclusion was that the effect of climate on size evolution in squamates is negligible at best, at least at the interspecific level. Of course, climate can be very important – it can serve as an ecological filter for dispersal and colonization of different groups, which can create spatial patterns in body size when these groups differ in size, as we indeed find (for instance – most squamates in Australia are skinks, and most skinks are very small). In any case there doesn't seem to be some general "rule" we can formulate on how climate affects body size evolution, and we think such evolutionary relationships, if they exist, are highly species-specific and should be examined on a case-by-case basis.
Author & photographer: Alex Slavenko
In a recent publication in Biological Journal of the Linnean Society, we present a comparative analysis exploring patterns and drivers of longevity of 1320 reptile species, spanning all orders.
In recent years, there have been many studies focusing on the effect of different ecological variables and life-history traits on the variation in longevity of specific taxonomic groups, focusing mostly on birds and mammals (with the only large- scale ectothermic study done on squamates by Scharf et al. 2015). In order to expand our knowledge on the effect of environmental and life-history variables on the variation in longevity of animals, we tested the effect of ecological variables (through various hypotheses) related to extrinsic mortality (e.g. predation) on the variation in longevity among and within lizards, snakes, turtles and crocodiles. We found that species living on islands, and in colder and more seasonal environments, live longer. Moreover, sampling more individuals increases the chances of finding older specimens, and should be corrected for when studying maximum longevity.
We hope these analyses will enable us to better understand the drivers of longevity in reptiles (and other taxa). We hope this paper will facilitate more large-scale comparative studies on the causes of the variation in longevity of tetrapods in general.
Author: Gavin Stark
In a recent publication in Global Ecology and Biogeograpy, I present a vast dataset of over 20 body size, ecological, thermal biology, geographic, phylogenetic and life history traits for global lizards.
I hope these data will facilitate more study into the biology of these most fascinating of creatures, and that the database publication will encourage others to add yet more data and to correct errors I surely have made when assembling them.
Author: Shai Meiri
In a recently published paper in Diversity and Distributions we try to illuminate aspects regarding the biology, and conservation of all narrow ranged lizard species, across the globe.
When exploring different attributes of small ranged species we found that most of them inhabit relatively inaccessible places in tropical climates worldwide. Furthermore, they are mostly small bodied species; many of them are active at night; and live in rocky habitats. Among the different lizard groups geckos and skinks dominate with many rare species.
This work could help better focus conservation efforts by pointing at the species, and places, that are in the greatest need of protection. Many of the species, especially those which have not been observed for decades, may well be already extinct. However, to-date only six of the species studied have been officially recognized as such. In order to examine the true extent of such extinctions, and try to prevent future ones, the study provides invaluable information for directing future research and conservation efforts.
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