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In a new study hot off the press at Evolution, Stuart and colleagues experimentally test the dimorphism-richness hypothesis using several mangrove islands in south Florida inhabited by Anolis carolinensis (pictured; credit Wikimedia Commons) and A. sagrei.

New literature alert!

Male and female Anolis carolinensis maintain their dimorphism despite the presence of novel interspecific competition

In Evolution

Stuart, Sherwin, Kamath, and Veen

Abstract:

Natural selection favors sexual dimorphism that reduces resource competition between the sexes of the same species. However, niche partitioning among interspecific competitors should counter such divergence, as partitioning the niche results in smaller total niche widths for each individual species, leaving less room for the sexes to diverge. A straightforward (and long-standing) hypothesis emerges: species in competitor-rich ecological communities should show less sexual dimorphism than species in competitor-poor ecological communities. Here, we test this prediction using a well-documented natural experiment generated by the recent arrival of Anolis sagrei to a set of small islands in Mosquito Lagoon, Florida, containing Anolis carolinensis. Despite known interspecific habitat partitioning and rapid evolution in habitat-use traits by A. carolinensis in this system, sexual dimorphism between male and female A. carolinensis was not reduced as predicted on two-species islands relative to islands with only A. carolinensis. This is consistent with a small but growing body of empirical tests of the dimorphism-richness hypothesis that have been ambiguous in their support at best. A rethinking of the validity of this intuitive hypothesis is needed.

Anolis sagrei (pictured) is the star of a new study by Kahrl et al. (2021) examining selection pressures on sperm. Credit Wikimedia Commons.

New literature alert!

Selection on Sperm Count, but Not on Sperm Morphology or Velocity, in a Wild Population of Anolis Lizards

In Cells

Kahrl, Kustra, Reedy, Bhave, Seears, Warner, and Cox

Abstract:

Sperm competition is a widespread phenomenon that shapes male reproductive success. Ejaculates present many potential targets for postcopulatory selection (e.g., sperm morphology, count, and velocity), which are often highly correlated and potentially subject to complex multivariate selection. Although multivariate selection on ejaculate traits has been observed in laboratory experiments, it is unclear whether selection is similarly complex in wild populations, where individuals mate frequently over longer periods of time. We measured univariate and multivariate selection on sperm morphology, sperm count, and sperm velocity in a wild population of brown anole lizards (Anolis sagrei). We conducted a mark-recapture study with genetic parentage assignment to estimate individual reproductive success. We found significant negative directional selection and negative quadratic selection on sperm count, but we did not detect directional or quadratic selection on any other sperm traits, nor did we detect correlational selection on any trait combinations. Our results may reflect pressure on males to produce many small ejaculates and mate frequently over a six-month reproductive season. This study is the first to measure multivariate selection on sperm traits in a wild population and provides an interesting contrast to experimental studies of external fertilizers, which have found complex multivariate selection on sperm phenotypes.

A Brown Anole (Anolis sagrei)–the species of focus in the study by Cox et al. (2021)–displays a gorgeous bright-orange dewlap. Credit Wikimedia Commons.

New literature alert!

Reproductive tradeoffs and phenotypic selection change with body condition, but not with predation regime, across island lizard populations

In Journal of Evolutionary Biology

Abstract:

In a recent publication in the Journal of Tropical Ecology, we found that the herpetofauna communities of Virgin Islands National Park, on the island of St. John, USVI, were resistant to the large disturbances of hurricanes Irma and Maria, showing no significant change in habitat associations eight months post-hurricane. These findings were born from undergraduate research that grew, due to ample encouragement and funding from the National Geographic Society, into an in-depth examination of community recovery from some of the nastiest storms to hit the Caribbean region.

Anolis stratulus dewlapping. Photo by Sam DiGiulio

I started this work in 2016 as a third-year undergraduate at Northern Michigan University (NMU). My research partner, Sam DiGiulio, and I jumped at the chance to enroll in Dr. Jill Leonard’s class, Field Marine Biology, which took students on a spring-break field excursion to St. John, USVI. During our stay at the Virgin Islands Environmental Resource Station (VIERS), we experienced the biodiversity of Caribbean coral reefs, observed marine sampling techniques, and learned how terrestrial processes affect marine environments. As an additional component to the class, Sam and I designed an independent research project to carry out during our time on St. John. Due our shared passion for herpetofauna, we decided to implement a herpetological inventory of Virgin Islands National Park (VINP), following up on work done by the US Geological Survey (USGS) in 2001 (Rice et al. 2001). The initial goal of our survey was to provide the National Park Service (NPS) with a report that would act as a snapshot of the herp communities 15 years after the USGS survey, but it ended up being a lot more than that.

There is not much better field work than that in VINP. We hiked the trails and conducted visual encounter surveys; documenting the anoles, dwarf geckos (Sphaerodactylus macrolepis), and the occasional green iguana (Iguana iguana). Each transect we walked was classified by one of five landcover types (moist forest, dry forest, scrub, estuarine, and gut – small areas that hold freshwater for extended periods), per the USGS inventory. We also set out to the trails at night to conduct vocalization surveys for the island’s frog species. VINP is home to 19 species of reptile and amphibian, though this number fluctuates as populations of introduced species wink in and out. St. John is the smallest of the US Virgin Islands and does not host some of the rarer endemic species like St. Croix, such as Anolis acutus, or Ameiva polops, or hold the diversity found on Puerto Rico. However, that doesn’t make its residents any less charming, and with over 56% of the island of St. John included within VINP, there are ample places to observe them.

Anolis cristatellus. Photo by Sam DiGiulio

The most common anole on the island is the Puerto Rican crested anole (Anolis cristatellus). We observed all age classes, sizes, and crest shapes of these lizards and in every habitat type on the island. It was a treat to see two huge males duke it out on a tree trunk, head bobbing, dewlapping, and charging at one another, turning almost black, as opposed to their usual brown hues. The fight ended with one tossing the other off the trunk with its mouth.

We were also fortunate to find some of the rarer island residents, flipping just the right logs and rocks to reveal blind snakes (Antillotyphlops richardii) and a Virgin Islands worm lizard (Amphisbaena fenestrata). We conducted our surveys anywhere that we could hitch a ride with the class, or with staff from VIERS as they traveled to and from town for camp supplies. Even without our own vehicle, we were able to cover much of the park on foot in the course of the week and after returning to NMU for the end of the semester, we wrote up the report for the NPS, and presented the data at several scientific conferences.

Come September of 2017, category 5 Hurricanes Irma and Maria struck St. John within the course of two weeks. The forests and infrastructure of the islands sustained serious damage. Countless trees had fallen, and those that were still standing were completely defoliated. I anxiously stayed informed about the damage, but it wasn’t until November of 2017 that we talked about the potential of returning to resurvey the herps of VINP. Jill pointed us in the direction of the program. With encouragement from our advisor, we applied for a National Geographic Society’s Early Careers grant and in March of 2018 we received the funds necessary to return to St. John to re-conduct our herp survey.

Early that summer, Sam and I arrived back on St. John. This time, we were able to secure a vehicle and cover even more of the park, in addition to resurveying the areas that we had visited in 2016. We hit the ground running, first by revisiting our previous survey areas to ensure we repeated the same procedures as 2016. As we had arrived nine months after the storms hit, there were already plenty of signs of recovery. Canopy gaps created by the storms were being filled by early successional growth. Vines covered fallen trees and standing trees were quickly replacing lost leaves. The red mangroves were very slowly sprouting new shoots and leaves from the standing skeletons of prop roots. Clean-up operations and construction projects were coming along in the developed areas of the island. Unfortunately, VINP infrastructure had yet to be repaired, and VIERS had been destroyed. As of writing this post in 2021, the incredible education center is still not functional. However, as we traversed the trails and conducted our visual encounter surveys, the herps of the island appeared to be rather unperturbed.

Anolis pulchellus. Photo by Sam DiGiulio

While the vegetation structure appeared different from our initial surveys in 2016, community composition and the landcover types in which they were found  remained unchanged. Puerto Rican crested anoles remained the most abundant species across the landcover types, taking full advantage of the coarse woody debris as display platforms to show off their dewlaps. The male anoles retained their cantankerous attitude towards conspecifics; this time around, we observed two dueling sharp-mouth anoles (Anolis pulchellus). These anoles are much lighter than the bulky crested anoles, favoring grasses and shrubbery to perch, allowing their background to more easily match their greenish hues. As diligently as we searched, we were unable to find more blind snakes and worm lizards, although this was no surprise due to the rarity of specimens recorded in previous surveys. Sufficient planning allowed the 2018 surveys to unfold even more smoothly than in 2016, and after canvasing the national park, we headed home to work through our results.

As a conclusion to our expedition, we determined that the reptile and amphibian communities of VINP had no observable changes in community composition, or landcover associations after hurricanes Irma and Maria. Interestingly, the communities appear resistant to large disturbances, indicating that they are either able to survive and adapt to the habitat changes, or underwent rapid population growth. Because the hurricanes struck the island over the course of just two weeks, they may have been perceived as a single disturbance event for the island fauna.

While the herpetofauna communities appeared to survive the hurricanes intact, they remain under pressure from many other threats, including invasive species like rats (Rattus rattus) and Indian mongoose (Herpestes auropunctatus), increasing human impact in VINP, and climate change. We recommend a regular monitoring program that would allow managers to observe abundance and population changes of both the herpetofauna species and invasive predators on the island, and give further insight to how these animals deal with disturbances, especially in a time of global biodiversity loss.

The findings from this study were recently published in the Journal of Tropical Ecology (Richter et al. 2021). What started as a class project was able to grow into grant writing, presentation, and publication opportunities that have helped me gain critical skills I’ll be using while pursuing my master’s degree at the University of Georgia’s Warnell School of Forestry and beyond. This work would not have been possible without the encouragement and guidance of our advisors and coauthors, and serves as an example of the advances that can be made by investing in undergraduate research.

Rice KG, Waddle JH, Crockett ME, Carthy RR and Percival HF (2005) Herpetofaunal Inventories of the National Parks of South Florida and the Caribbean. U.S. Geological Survey Professional Paper 1631. Open-File Report 2005-1301: 2, 1-45.

Richter, C. J., S. M. DiGiulio, C. D. Marshall, and J. B. K. Leonard. 2021. Herpetofaunal community response to hurricanes Irma and Maria in Virgin Islands National Park. Journal of Tropical Ecology 37(4): 185-192.

Using a fine-toothed genetic comb, Johnson et al. (2021) investigated the mating system of Anolis cristatellus (pictured above; photo credit Aryeh H. Miller) to better understand both female mate choice and male competition in an explicit spatiotemporal context.

New literature alert!

What Determines Paternity in Wild Lizards? A Spatiotemporal Analysis of Behavior and Morphology

In Integrative and Comparative Biology

Johnson, Kamath, Kirby, Fresquez, Wang, Stehle, Templeton, and Losos

Abstract:

Mating behavior in animals can be understood as a sequence of events that begins with individuals encountering one another and ends with the production of offspring. Behavioral descriptions of animal interactions characterize early elements of this sequence, and genetic descriptions use offspring parentage to characterize the final outcome, with behavioral and physiological assessments of mates and mechanisms of copulation and fertilization comprising intermediate steps. However, behavioral and genetic descriptions of mating systems are often inconsistent with one another, complicating expectations for crucial aspects of mating biology, such as the presence of multiple mating. Here, we use behavioral and genetic data from a wild population of the lizard Anolis cristatellus to characterize female multiple mating and the potential for sexual selection through female mate choice in this species. We find that 48% of sampled females bore offspring sired by multiple males. Moreover, spatiotemporal proximity between males and females was associated with whether a male sired a female’s offspring, and if yes, how many offspring he sired. Additionally, male body size, but not display behavior, was associated with reproductive outcomes for male–female pairs. While much remains to be learned about the mechanisms of mating and targets of sexual selection in A. cristatellus, it is clear that female multiple mating is a substantial component of this species’ mating system in nature.

Javier Lobon Rovira, a graduate student working on geckos, decided to up his game and pay attention to anoles. Here’s what resulted! The lizards were found on 24th of August in the surroundings of Gainesville, Florida, displaying from a tree branch around one meter high. He found a second specimen close by sleeping at night on a small bush close to a water pond.