In a recent publication in Global Ecology and Biogeography GARDians explored the global diversity and distribution of lizard clutch sizes.
We tested the geographic factors that affect clutch sizes in across nearly 4000 lizard species. We found similar patterns to those that have long been known in birds but were never seriously studied in other groups of organisms: lizards lay large clutches at high latitudes and at highly seasonal regions. We postulate that high latitudes with their short, pronounced productivity peals both allow the production of large clutches and promote putting all the eggs in one basket – because the window of opportunity is short in highly seasonal regions. We hypothesize that this may further be a factor preventing taxa with fixed clutch sizes from colonizing high latitudes.
Median log‐transformed clutch size in 96 km × 96 km grid cells globally. Top: all lizards; bottom: only lizards with variable clutch sizes. Note that the colour scale differs between the maps. To the right of each map is a curve showing a generalized additive model of the mapped variable (in black), the 95% confidence intervals of the mapped variable per 96‐km latitudinal band (shaded dark grey), and the range of values of the mapped variable per 96‐km latitudinal band (shaded light grey).
Authors: Shai Meiri, Uri Roll
It has long been thought that animals that ‘live slowly’, having a slow rate of metabolism, live longer than those that live their lives at a fast rate: having high metabolic rates. The notion is based on the assumption that animals with fast metabolic rates are more active, more exposed to predators, have higher rates of potentially harmful somatic mutations and produce more harmful metabolic by products such as free radicals. This tradeoff between metabolism and lifespan is commonly referred to as the ‘rate-of-living’ theory.
In a recent publication in the Journal Global Ecology and biogeography, we (Gavin Stark; Daniel Pincheira donoso and Shai Meiri) showed that the assumption under the “rate of living” theory which have been around for almost a century is unsupported by the results of our largest scale study (4,100 land vertebrate species: 2,214 endotherms & 1,886 ectotherms) to date of this theory. We could not find any connection between animal metabolic rate and longevity, either when we tested all land vertebrates (i.e. Mammals, Birds, Reptiles and Amphibians) or when we tested each group separately. In contrast, we did find other factor that did affect the lifespan of ectotherms (Reptiles and Amphibians), and it is ambient temperature. In colder regions around the world we expect species of reptiles and amphibians to live longer than other ectotherms living in warmer environments. The link between ectothermic (amphibians and reptiles) lifespan and ambient temperatures could mean that they are especially vulnerable to the unprecedented global warming that the planet is currently experiencing. Indeed, if increasing ambient temperatures reduces longevity, it may make ectothermic species more prone to go extinct as the climate warms. Our findings add a previously overlooked layer to the range of factors that are commonly thought to imperil species in the Anthropocene.
The relationship between longevity (y axis of all panels, log10 transformed) and (a–d) body mass (in g, log10 transformed) of amphibians (red circles), reptiles (black circles), birds (green triangles) and mammals (inverted blue triangles). (e–h) Mean annual temperature (regression lines only shown for amphibians and reptiles for which the relationship is significant), (i) basal metabolic rate (in ml O2/hr, log10 transformed), according to the color codes depicted in the top plots and (j) field metabolic rate (kJ/day, log10 transformed)
In a recent publication in the Israel journal of ecology and evolution, we (Gavin Stark, Rachel Schwarz and Shai Meiri) showed that nocturnality does not prolong lifespan among the within gekkotan species. Species from the infraorder Gekkota are known to be predominately nocturnal as opposed to other lizard clades. Diurnal lizards demonstrate higher metabolic rates than nocturnal ones. Moreover, exposure to solar radiation is thought to reduce ectothermic longevity by increasing both metabolic costs and the rate of accumulating harmful mutations through UV radiation. Thus, we assume that by being nocturnal, ectothermic species may reduce their intrinsic mortality rates and thus live longer. We compared groups of nocturnal and diurnal species across all gekkotan families, and also compared all non-gekkotan species to geckos (740 lizard species, of which 185 are geckos) to test whether nocturnality select for longer lifespans. We found that geckos live relatively long for lizards of their size, however their activity time was found to be unrelated to longevity, contradicting our predictions. We suggest that mortality through extrinsic causes (e.g., predation) may impose much stronger selective pressures than intrinsic causes.
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.
Mainly maintained by Shai Meiri and Uri Roll