How we perceive and interact with the world is strongly shaped by natural light. How much light there is at a given time determines whether we’re sleepy or awake, and whether we’re bracing for winter or excited for long summer days. The annual switch to daylights savings time shows us how even a small shift in our perceived light cycle can disrupt our internal clocks!
The amount of illumination an animal experiences – its photoperiod – is also vitally important in nature, from determining sleep cycles to the seasonal timing of reproduction. However, the increasing urbanization of natural habitats has led to huge shifts in the exposure of wildlife to light due to the use of electric lighting at night in urban areas. This artificial light at night has the potential to seriously affect the life cycle of many organisms through, for example, changes in endocrine function, melatonin production, and reproductive timing. On the other hand, artificial lighting at night may also have positive effects in some cases: organisms that forage or hunt at night, for instance, may have increased perception of prey and greater success in obtaining nutrients.
In a recent paper, Thawley and Kolbe investigate the effect of artificial light at night on one of our favorite species of lizards, Anolis sagrei. Using captured anoles from a wild population in a forested area with low levels of artificial lighting at night, they use laboratory experiments to see the effects of this artificial lighting on the anole’s body condition, glucocorticoid levels, and reproductive cycles.
The authors found that artificial lighting at night actually increased the growth rate of anoles in captivity! Females under ‘artificial light at night’ condition grew 1.8 times faster than their counterparts experiencing the ‘dark at night’ condition, and males grew 1.2 times more! This suggests that artificial lighting at night could have a huge biological consequences for this species. As the authors hypothesized, the female anoles exposed to artificial lighting at night also started laying eggs much earlier than their control counterparts. In particular, smaller females under artificial lighting at night managed to produce similar total egg output to that of larger females, potentially representing a tremendous gain in reproductive fitness for these smaller individuals.
All this would seem to suggest that artificial lighting might be a boon for Anolis sagrei! However, the authors suggest some caution in this interpretation, as their experimental set-up did not include foraging costs or predation – both of which could also interact with the effects of artificial lighting to create downsides for Anolis sagrei in these new lit-up urban environments.
Overall, this paper is an illuminating look at how the artificial lighting at night associated with increased urbanization can impact the lives of Anolis sagrei – I encourage you to check it out!
Citation: Thawley, Christopher J., and Jason J. Kolbe. 2020. Artificial light at night increases growth and reproductive output in Anolis lizards. Proceedings of the Royal Society B 287.1919: 20191682. [https://doi.org/10.1098/rspb.2019.1682]
The origin and maintenance of reproductive isolation between species is a central question to evolutionary biologists. Divergent sex chromosomes can play an important role in this process, and are generally assumed to have outsized importance in the establishment of reproductive barriers. Studying the origin and evolution of sex chromosomes – and their respective fusions and fissions – may therefore provide key insights into their role in these processes.
Anole are known to vary in sex chromosome size and content, although all anoles are male heterogametic. In a poster at Evolution, de Mello et al. investigate the neo-sex chromosomes of Anolis distichus, one of the “model anoles” of speciation research. Starting from a newly assembled genome, these researchers used differences in coverage, k-mer comparisons, and synteny mapping to the Anolis carolinensis genome, to identify the sex-linked genomic regions of A. distichus.
From these results, de Mello et al. were able to identify deep conservation of the X chromosome between A. distichus and A. carolinensis – implying an ancient origin of a shared anole X chromosome. They also identified explicitly Y-linked scaffolds for the first time in any Anolis species, which will prove useful for future work on the evolution of these sex chromosomes. However, perhaps most excitingly, de Mello et al. identified a chromosome fusion of the Anolis carolinensis microchromosomes 11 and 12 to the A. distichus X chromosome. In other words, the A. distichus X chromosome has expanded through the fusion of these two microchromosomes.
de Mello et al.’s result that the A. distichus sex chromosomes are simultaneously ancient and newly expanding provides a fascinating look at the dynamic lives of these sex chromosomes. Future investigations into the evolution of Anolis sex chromosomes will surely prove fruitful to understanding their role in the diversification of the Anolis lineages.
Two of the authors (Colin Donihue and Raphaël Scherrer) with their poster at Evolution 2018
How do new species form? At ESEB 2018, Colin Donihue uses Anolis lizards to answer this fundamental question in evolutionary biology.
Anoles are known for their adaptive radiation in the Carribean and the corresponding diversification into distinct “ecomorph” categories. Each ecomorph is associated with distinct morphologies and behaviors that allow it to live easily in a different habitat. This pattern is repeated across the Greater Antillean islands, but what we see is the end result of an adaptive radiation – each ecomorph corresponds to a separate species.
Donihue and his co-authors embarked on an ambitious project to capture the beginning of an adaptive radiation. To do so, they turned to the ubiquitous brown anole, Anolis sagrei. As Anolis sagrei is found across the Bahamas in a variety of different habitats, you might expect to see them adapting to those different habitats through changes in morphology; in other words, looking at the early adaptation of Anolis sagrei populations in different habitats is a natural experiment reflecting the early stages of ecomorph development. And since Anolis sagrei is on islands across the Bahamas, there isn’t just one experiment, but several replicated ones. Donihue et al. could therefore also question the role of contingency vs deterministic evolution though their study.
The authors captured 20 individuals from coastal scrub, mangrove, and primary coppice forest habitat across 11 islands in the Bahamas, and measured a suite of morphological traits for all individuals; these traits include the “usual culprits” of ecomorph differentiation, such as forelimb length, hindlimb length, and lamella count. This effort resulted in an enormous data set that the authors could use to test whether brown anoles had adapted to the different habitats across all the islands.
So are the Bahamian brown anoles adapting along early ecomorph lines? Well…sort of. On any given island, lizards living in different habitats have different morphological characteristics. But, looking across islands, Donihue et al. observe different patterns of morphological specialization on each island. This suggests that contingency, in this case represented by the island of origin, is playing a large role in how the lizards adapt to the three different habitats.
In an interesting twist to the project, Donihue et al. used supervised machine learning to test whether lizards could be assigned to the correct habitat categories based on morphology. They found that this algorithm could assign lizards to their habitat correctly based on the input of their morphological measurements across islands. This result implies that determinism is playing a role in the specialization of these brown anoles, but may only be detectable when looking at a lizard’s holistic phenotype rather than any individual trait measurement. Looking forward to seeing the paper on these results!
Sexual dimorphism, or phenotypic differences between the sexes, is characteristic of nearly all animal species. Males and females often differ in size, shape, color, and many other morphological and behavioral phenotypes. This dimorphism can often make it difficult to study selection on various phenotypic traits – how do you measure selection on a trait accurately when that trait may be expressed differently in each sex?
Anolis sagrei exhibits sexual dimorphism. (Photo by Bob Reed)
In a talk at the annual Evolution meeting, Robert Cox and Joel McGlothlin help us answer this question. Using dewlap and skeletal measurements – which differ widely between males and females – and data from breeding experiments on Anolis sagrei, they examine the quantitative genetic architecture of these sexually dimorphic traits. Using a matrix-based model, which accounts for genetic correlations between and within sexes, Cox and McGlothlin are also able to see how these sexually dimorphic traits react to a variety of selection regimes, including selection that acts in opposite directions in males and females. In addition, using these simulations, they are able to estimate how different traits can be evolutionarily constrained: genetic correlations between the sexes appear to constrain selection on skeletal phenotypes, but not dewlap-related phenotypes.
These methods are likely to be extremely useful to anyone hoping to measure selection in natural population of anoles, or any other sexually dimorphic species. Sex differences often play an important role in how an organism can evolve in the wild, and introducing them into the way we quantify selection and its response is a key contribution to understand this process. I encourage anyone interested in the details of this method to check out the recent paper by the authors below for more details!
The adult sex ratio is an important characteristic of a population, influencing the number of available mates in an area, the strength of sexual selection, and the evolution of mating systems. In our new paper in the Journal of Zoology, Michele Johnson and I use anoles to look at variation in sex ratios within and across species within a clade.
Photo by Michele A. Johnson
This paper had its roots when Jonathan Losos put me in touch with Michele in my first semester of grad school. Michele had compiled a massive database of detailed behavioral observations for Anolis populations and species across the Greater Antilles during her PhD on territoriality and habitat use (see Johnson et al. 2010 for more details!). While still trying to familiarize myself with the data set, I came across papers by Bob Trivers on sexual selection in anoles and his publication on the name-sake Trivers-Willard hypothesis; the combination of these topics made me curious about sex ratios and their role in sexual selection. I decided to quickly calculate the sex ratios of our localities, and given their distribution, realized that we should definitely look into this more.
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Sex ratios are generally very hard to measure in the field. You need to be certain that you haven’t had any biased sampling, or in other words, that you’ve made a fair attempt at censusing the population. This is quite difficult during short sampling periods! However, Michele conducted extended behavioral observations, and carefully tagged and monitored every individual in large habitat areas for ~3 weeks in each locality. This meant that we could be fairly confident that she had captured every individual in the population during her sampling periods, and her total counts of male and females in the population would be accurate. Even more, she had these adult sex ratios for 14 species, with some of those species being sampled at multiple localities. Given these data, we could actually both look at sex ratios across the Anolis clade, and within multiple anole species, for the first time.
We had two main questions: 1) were the sex ratios of these anole populations significantly skewed (i.e., were they very far off from a 50:50 male-to-female ratio?) and 2) did the adult sex ratio of a population correlate with the strength of sexual selection in that population? For question 2, we used two measurements of sexual size dimorphism as a proxy for the strength of sexual selection. Sexual selection generally drives an increase in sexual size dimorphism (i.e., the difference between males and females in body size), but is also thought to be related to sex ratio skew (as the more skewed a population sex ratio, the more competition for mates or mating opportunities). We predicted that species with more skewed sex ratios would show an increase in sexual size dimorphism. Given that ecomorphs are an important component of evolution in anoles, and are commonly associated with varying levels of sexual size dimorphism, we also decided to test for a correlation between sex ratio skew and ecomorph type.
We found that sex ratios varied widely across and within anoles, ranging from a very female biased 0.32 in Anolis krugi to a male biased 0.61 in Anolis smaragdinus (sex ratios are expressed as the total number of adult males divided by the total number of both adult males and females in the population). Adult sex ratios also varied between different localities within a species (we had six species with multiple localities). We found two populations with significantly skewed sex ratios (Anolis krugi and Anolis valencienni) but based on Fisher’s test of combined probabilities, the sex ratios of anoles overall are not skewed away from 50:50.
I should note, however, that it is intrinsically extremely difficult to detect a skewed sex ratio in a natural population. We’re trying to measure deviations from a 50:50 sex ratio, and this requires surprisingly high population sizes since the binomial distribution has a broad center. For instance, to detect a true underlying sex ratio of 0.4 or 0.6 (away from our null of 0.5), we would need population sizes of >780 lizards to detect a significant skew 80% of the time. This is just an illustration, but the main point is that these population sizes might not exist for a given species – and so detecting significantly skewed sex ratios might not be possible at all. This is especially difficult when looking at small or endangered populations – there sex ratio skew might be a big problem, but impossible to demonstrate statistically. The general takeaway here is that sex ratio skew in a population can be biologically important, but not statistically significant.
We then used both the categorization of the anole species by sexual size dimorphism (low or high SSD) and the measured sexual size dimorphism of each population (calculated by average male SVL divided by average female SVL, minus 1). We used both of these estimates of SSD to test whether the sex ratio of a population correlated with the sexual size dimorphism of that population, as predicted by sexual selection theory. Turns out we were completely off – there was really no correlation between sex ratio skew and measured SSD, categorical SSD, or ecomorph (see figure 1, posted below, for a visual of this lack of correlation!).
Figure 1 (from the paper) : Sex ratio versus sexual size dimorphism. Sex ratio is represented as the proportion of males among adults in the population, while sexual size dimorphism was calculated dividing the average SVL of the larger sex by the average SVL of the smaller sex, and subtracting 1 for each population. Each circle represents 1 of the 21 localities sampled in this study. The dashed line represents an equal sex ratio of 0.5. We found no relationship between sexual size dimorphism and sex ratio across the 21 localities (PGLS: adjusted R2 = −0.08, P = 0.86).
So what’s the general message here? Sexual size dimorphism does not correlate with adult sex ratios across anole species, and so the relationship between strength of sexual selection, sex ratio bias, and sexual size dimorphism may be more complicated than we initially assumed. However, anole sex ratios can range widely between species, and within populations. Given the variance within anole species, the adult sex ratio is probably a better description of a locality, or population, than an intrinsic quality of an entire species. We also think that the influence of various localized environmental factors may impact sex-specific mortality or dispersal, which in turn which cause differences between localities in adult sex ratio skew.
This is my first anole paper, and it’s really nice to see all the brainstorming and discussions put into print. It was also great to get to know and work with Michele, and learn more about her research and behavioral work in anoles (we even got to meet in person at the Evolution conference last year!). This paper was also my first small step into the world of sex ratio and sex determination theory which now forms a large part of my PhD work, so I’m very grateful for the introduction to the subject. Anyway, feel free to email us with any questions and we hope you enjoy the paper!
Parallel evolution and convergent evolution are big themes within anole biology, so our lab was excited to discuss a new paper by Thorpe et al looking at these concepts in Lesser Antillean anoles. The paper focused on evidence for parallel evolution across seven small islands that contained both xeric and montane habitats with at least one species of anole split between the two habitats. Xeric habitats tend to occur along island coasts and are hotter, drier, and have less canopy cover, while montane habitats occur in the interior of islands and are cooler and wetter. There are many physical differences consistently found between the anoles associated with each type of habitat, even within a species; perhaps the most obvious examples are the repeated differences in skin color and pattern between habitats, beautifully illustrated in the first figure of the paper.
Figure 1 from Thorpe et al, showing the repeated evolution of characteristic xeric and montane color patterns in the Lesser Antilles
Thorpe et al. set out to conduct tests of parallel evolution among seven anole species using 18 phenotypic traits that vary between habitats, including both morphological and pattern measurements. In addition, they used mitochondrial DNA sequencing to produce a new phylogeny of these species and control for phylogenetic interference in their comparisons. The authors first used a principal components analysis to confirm that the major source of climatic variation is found within each island and between different habitats, rather than across different islands. The authors found convincing evidence for parallel morphological evolution in multiple phenotypic traits, especially those associated with skin pattern and hue: anole populations in xeric habitats consistently converge on a grey skin color and those in montane habitats converge on green. Thorpe et al. also go on to suggest that divergence in coloration may be the result of signal optimization in environments with different chromatic backgrounds (characterized by variance in background vegetation or sun exposure). The authors describe a possible evolutionary scenario in which an anole population first colonizes the coastal areas of each island after a dispersal event, and then rapidly expands into the interior montane areas of the island and adapts to new conditions there. Given the constant concern of climate change, repeated evolution in response to different climatic conditions may offer hope that anole populations can respond to rapid environmental change.
The most famous story of parallel evolution in anoles is the convergent evolution of ecomorphs across the islands of the Greater Antilles. This paper offers the tantalizing possibility of another type of convergent evolutionary pattern, this time within species but across habitat types. The smaller islands of the Lesser Antilles may be too constrained to allow for speciation driven by ecomorph specialization, but could still promote significant population divergence across habitats. More information on the adaptive differences between these xeric and montane populations, along with characterization of their genetic structure, could shed light on these possibilities. Based on these results in the Lesser Antillean populations, there is also the possibility that this type of xeric and montane divergence exists within species in the Greater Antilles, and fine-scale studies of population structure could reveal another level of convergent evolution in those species.
Thorpe, R. S., Barlow, A., Malhotra, A. and Surget-Groba, Y. (2015), Widespread parallel population adaptation to climate variation across a radiation: implications for adaptation to climate change. Molecular Ecology, 24: 1019–1030. doi: 10.1111/mec.13093
