Anole Annals

In the face of mate competition, sperm morphology can vary in a way that can enhance an individual’s chances of siring offspring of females. Studies in the past have attributed increased relative testis size as an approximate measure of an individual’s response to sperm competition. However, this does not take into account the internal architecture of the male testes that may contribute to changes in sperm morphology.

This was the focus of a poster presented by Hanna Hall titled “The evolution of sperm and testis morphology in Anolis lizards” in collaboration with Ariel Kahrl and Michelle Johnson. The authors sampled 2-20 individuals of different species of anoles in Puerto Rico and the Dominican Republic. They compared body size, sperm length ( 15 cells per individual), and the composition and size of various layers of the testis, by conducting a phylogenetic least squares regression on the average values obtained for each species.

The authors found that larger body size was associated with a larger testis size, which was in turn correlated with presence of large seminiferous tubules and a larger luminal area, where mature sperm are stored. Contrary to their expectation though, none of these aspects were associated with producing longer sperm. Further the Gonado-Somatic index (GSI), a common metric that serves as an indicator of relative testis size, was not correlated with any aspects of the internal testis architecture.

An interesting finding in this study was that species with a higher proportion of epithelial cells in the testis produced longer sperm. This result was surprising because larger number of epidermal cells may be associated with smaller spermatogonal cells, which would be predicted to form shorter sperm. The authors suggest that the correlation between lumen area and testis size may result because investment in sperm storage is more important, and that species may be producing large number of sperm which may be longer in length. Nevertheless, more data is needed to understand how changes in sperm morphology affect fertilization success and, further, under what circumstances does size and count of epithelial cells vary. The lack of correspondence of these results with that  shown in birds by Lupold et al. 2008 suggests that the mechanisms underlying sperm competition may be taxa or species-specific. We will be eyeing the Johnson Lab for more details on the same in the coming years.

An example of an aggressive display by a lizard. Photo Credits: Neil Losin Photography

Testosterone has long been though to influence male aggression behaviors. But can this same hormone influence aggressive behaviors in females too? Ellee Cook addressed this question in her talk titled “Investigating the potential for testosterone to mediate territorial aggression in female Anolis lizards.”

Ellee focused on studying a population of Anolis gundlachi in the forests of Puerto Rico. Ellee studied the response of focal females to a staged territorial intrusion by another female who was placed on a cage lid, and compared it to a scenario where she directly approached the lizard. She captured the focal females after twenty minutes of the trial and measured their size and took a blood sample to estimate the circulating levels of testosterone. Her prediction was that higher levels of aggression would be correlated with higher levels of testosterone.

Her data showed that females were indeed aggressive towards intruding females and had much higher displays of aggression in comparison to when they were presented only a lid or were directly approached. Surprisingly (or not so surprisingly), testosterone was not a significant predictor of female aggression. In fact, none of the hormonal measures corresponded to female aggression. This finding could have resulted for several reasons: A) the amount of testosterone detected in females was much lower than that found in males, making variation in testosterone  almost impossible to detect; B) High aggression may be caused by spontaneous spikes in testosterone that may be hard to detect; C) Female aggression may be governed by a completely different mechanism.

This study raises an important question about the relevance and drawbacks of existing paradigms which are male-centric and thus cloud our understanding when it comes to female behaviors. Cheers to more feminist paradigms in biology!

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Theory predicts that males should invest more in ejaculate production when the likelihood of sperm competition is high, thereby increasing the chance of fertilization. However, ejaculates can be energetically costly, and increased investment into sperm production should only occur if there are fitness benefits associated with that increased investment. Growing experimental evidence suggests that sperm traits respond plastically to social environment. However, it is not known whether fine-scale spatial variation in the local density of male competitors or potential female mates corresponds to individual variation in ejaculate production.

Island population with capture records of males (blue) and female (red) anoles.

Matt Kustra of the Cox lab examined a wild population to test the prediction that, as the risk of sperm competition increases (i.e., higher local density of male competitors), males will increase their total investment in their ejaculates (sperm count). He also tested for correlations between sperm morphology, specifically midpiece size and local density.

To do this, he and the Cox lab collected wild adults from an island population in Florida. They generated a map of each tree on the island using ArcGIS, then marked the location of males and females on this map. Using the kernel density function, they estimated the local density of individual males by taking into account all conspecific adults that were captured within a 5.8 m radius of an individual’s own capture location.

Matt found that length of the sperm midpiece increased with local density, whereas length of the sperm head and sperm count decreased with local density. Contrary to his predictions, he found that total investment in sperm count decreased with local density. This could be because males in high density environments have depleted their sperm stores because they have more opportunities to mate, or it could be because males are investing less per ejaculate if mating frequency is higher.

These findings indicate that fine-scale differences in local density within a wild population can affect sperm count and various sperm phenotypes. In the future, the Cox lab hopes to measure fitness in this populations to understand how sperm phenotypes shape individual reproductive success.

Whereas in the Greater Antilles islands anoles evolved ecomorphs and live in communities with up to 11 species in sympatry, islands in the Lesser Antilles support only one or two species each. However, islands such as Dominica have populations of anoles that experience selective pressures resulting in different ecotypes.

Figure from Thorpe et al. 2004

While Dominica is relatively small, the mountainous topology results in highly variable environmental conditions across the island with cool mountainous regions and warm coastal regions and thermal vents. The single endemic anole species present on the island, Anolis oculatus, exhibits four morphologically distinct ecotypes (Montane, Atlantic, North Caribbean and South Caribbean) and despite levels of gene flow between these ecotypes are high, adaptive differentiation in this system is maintained.

Photo by Aurélien Miralles

Tricia Neptune, a graduate student in the Watson lab, at Midwestern State University, explored whether these ecotypes also show any differences in metabolic rate (by measuring oxygen consumption) and its sensitivity to temperature (Q10) at ecologically-relevant temperatures.

Results show that size differences between ecotypes are reflected in their physiology with the south Caribbean ecotype exhibiting higher oxygen consumption and Q10 compared to the other three ecotypes. Tricia hypothesize that these differences in metabolism and temperature sensitivity are in part responsible for maintaining relaxed geographic segregation among ecotypes.

Tricia plans to incorporate data on sprint speed, bite force as well as investigate thermoregulation strategies in this species. It will also be interesting to see a comparative study between the A. oculatus ecotypes and the introduced Puerto Rican crested anole, A. cristatellus.

Figure from Thorpe et al. 2004

The cover slide of Michael Yuan’s talk at SICB 2018.

Convergent forms of anoles can be found across the Greater Antilles, with similar phenotypic and ecological morphs filling similar microhabitats from island to island.  Anole ecomorphs are in part defined by the extent of arboreality, as most species in the Greater Antilles spend a lot of time in trees.  Crandell et al. 2014 found arboreality to be associated with significant differences in claw characteristics in Costa Rica and Panama.  In Greater Antillean anoles, similar research into claw morphology has yet to investigate if this relationship holds across ecomorphs. Michael Yaun, a PhD student in the Wang lab at UC Berkeley, set out to investigate the patterns of variation of claw morphology in the Greater Antillean anoles.

Anolis barbouri is shown as an outlier in a PCA. The flattened claws of this ground-dwelling anole are illustrated in black to the right.

Michael sampled 566 individuals, which included 55 species of anoles, all 6 ecomorphs, and another 8 species without any ecomorph designations.  His results suggest that perch height and diameter produced differential effects on claw characteristics.  Performance traits like toepad lamellae number and area were not correlated with claw height and length.  Michael’s study uncovered only one anole that conformed to previous research: Anolis barbouri, the only truly terrestrial species in the data set, possessing flattened claws.  Intriguingly, twig anoles have the most divergent claws, an inspiring result for future directions!

Photo by Andrea Westmoreland

Human-mediated range expansion is rapidly forming novel populations of anoles. The ancestry of these new populations typically traces back to a handful of individuals, and with repeated invasions the genetic history can be complex. These scenarios may be common in non-native populations of Anolis carolinensis, but what does the genetic history look like in such a system? In Sozos Michaelides’ talk at SICB 2018, he discussed his recently published findings.

Michaelides et al., 2017 tackled the question by inferring colonization history using mitochondrial haplotypes from Hawaiian Islands (Oahu, Hawaii, Maui, and Lanai) and some western Pacific islands (Guam, Palau, Saipan, Yap, and Rota). After genotyping 576 anoles, population genetic diversity and differentiation was assessed between native and non-native ranges. Results indicated geographically disparate haplotypes were identical (Hawaii to Brownsville, Texas), demonstrating that source populations may be from Texas or Louisiana. And a minimum of two introductions to Hawaii and Guam were uncovered, with subsequent within-population stepping-stone model colonization.

Overall, lower genetic diversity was found in non-native island populations as distance increased from the southeastern United States source population, and between the two archipelagos, genetic differentiation was high. Persistence of these non-native populations is not guaranteed because they are isolated, small in population size, and low in genetic diversity. It will be interesting to study the adaptive response of these introduced populations to stochastic climatic events!

The insulin signaling network has an essential role in growth, reproduction, and aging. Insulin-like growth factors, or IGFs, are important protein hormones within this network and are typically conserved across vertebrates. However, some proteins in the insulin signaling network have experienced selection in reptiles. Also, not a whole lot is known about the specific functions of components of this network within reptiles.

Amanda Clark, a PhD student in Dr. Tonia Schwartz‘s lab at Auburn University,  investigated the the function of purified IGFs on cell function for brown anoles (Anolis sagrei) and crested anoles (Anolis cristatellus). She had five different treatments for cell plates from both species: brown anole (BA) IGF-1, BA IGF-2, green anole IGF-1, a positive control, and a negative control. Cell proliferation was not different among all of the treatments, possibly due to incorrect protein folding or low concentrations of IGF. As expected, cell viability was also not affect by the IGFs. In the future, this experiment will be conducted again with increased sample size and an improved positive control.