My research incorporates both strong laboratory and field components. I mostly study tropical reptiles and amphibians, since their highest species diversity resides in these areas. I have worked in 14 different countries spanning every continent except Antarctica. However, I have spent the most time in Mexico and Costa Rica. I feel that science often proceeds best as a collaborative effort, and over many years I have formed a strong network of international collaborators from the US, Mexico, Costa Rica, Ecuador, Colombia, Honduras, Guatemala, Panama, Venezuela, Belize, Brazil, Bahamas, France, Australia, and Taiwan. My current research interests span a wide range of topics:
Phylogenetic Systematics, Phylogeography, and Patterns of Diversification:
The Neotropical snake subfamily Dipsadinae is the dominant snake lineage throughout much of Central America, yet relationships among most of its species remain largely unresolved. For my dissertation research, I used mitochondrial and nuclear genes to infer relationships among this subfamily and its member tribe Dipsadini (Sheehy, 2012). These data were analyzed using statistical model-based approaches (i.e., Bayesian Inference and Likelihood methods) as well as parsimony and distance methods to estimate phylogenetic relationships. This work uncovered a large adaptive radiation that appears to have been driven by a major dietary shift (vertebrates to invertebrates) and subsequent dietary (snail) specialization. I will be making many taxonomic changes as a result of this work that include the description of at least three new species and up to five new genera. This research is important because it helps understand patterns of diversification in one of the largest groups of colubrid snakes (ca. 750 spp.) and helps understand how dietary specialization has evolved in its snail-eating members.
Identifying population structure and morphological variation helps to understand how populations diverge over time both genetically and phenotypically. I use phylogeography to investigate population structure and species delineation of several amphibians and reptiles. I investigated the phylogeography of the yellow-bellied sea snake (Pelamis platurus) to assess gene flow across the Pacific Ocean, as well as to quantify genetic divergence between two chromatically different and potentially allopatric populations in Costa Rica. In collaboration with Christian Cox (University of Virginia), I quantified levels of landscape genetic diversity in a Mexican treefrog (Smilisca fodiens). In addition, I have also collaborated with Randy McCranie on a study identifying morphological and molecular variation in a pitviper (Atropoides indomitus) population from Honduras and with Jeff Streicher (University of Texas at Arlington) on an anuran (Lithobates chiricahuensis) population from Mexico. I have described a new frog species from Bolivia (Harvey and Sheehy, 2005), and I currently have in review a manuscript describing a new snake species from Ecuador (Sheehy et al., in review).
Freshwater Requirements in Marine Snakes – a Paradigm Shift:
Current dogma states that sea snakes are able to acquire freshwater by drinking sea water and using extrarenal salt glands to remove the excess salt. I have been testing this for several years in collaboration with Harvey Lillywhite (University of Florida) using a number of marine snake lineages and the results suggest that the current dogma may need to change. We studied three sea krait (Laticauda) species in Taiwan and found that they dehydrate in sea water, that they require freshwater to drink, that their distribution is strongly correlated with the availability of freshwater, and that their amount of cutaneous water loss is negatively correlated with their terrestrial tendencies (Lillywhite et al., 2008, 2009). We recently conducted drinking experiments in Australia with “true” hydrophiid sea snakes and with file snakes (Acrochordus), and the results suggest that these species also require freshwater to survive. We are conducting an ongoing field study in Costa Rica where we have been studying water requirements of the yellow-bellied sea snake (Pelamis platurus) during the wet and dry seasons for four years. These and related studies all suggest that sea snakes require freshwater to survive, which is changing our understanding of how these secondarily marine reptiles are persisting through time and space. Furthermore, many sea snakes are important and specialized predators in many coral reef habitats, and their distributions will likely be affected by changes in rainfall patterns associated with global climate change. The sea snake work is funded by a NSF grant to Harvey Lillywhite, and this grant is currently funding my postdoctoral research appointment at the University of Florida. I was recently invited to present my research at the 2012 SICB meeting as part of an NSF-funded symposium entitled “New Frontiers from Marine Snakes to Marine Ecosystems“. I was also recently invited to become a member of the IUCN Species Survival Commission Sea Snake Specialist Group.
Adaptation to Arboreality in Snakes:
Arboreal snakes possess a suite of morphological and behavioral adaptations for living in complex three-dimensional habitats. In collaboration with Harvey Lillywhite (University of Florida), I used various univariate, multivariate and comparative methods (e.g., Phylogenetic Independent Contrasts analyses) to identify that climbing or scansorial snakes worldwide have longer relative tail-lengths than their non-climbing counterparts (Sheehy, 2006). This pattern is not correlated with phylogenetic lineage but rather with gravitational stress imposed by gravity on blood circulation. We subsequently discovered that the length of the vascular lung and anterior heart position are negatively correlated in arboreal snakes (Lillywhite et al., 2012). These cardio-pulmonary characteristics may also be adaptations for countering gravitational stress on blood circulation while climbing. This research helps in understanding how macrohabitat use influences the evolution of vertebrate morphology.
Feeding Behavior:
I conducted chemosensory prey preference and feeding behavior studies on three genera of Neotropical gastropod specialists from Mexico in collaboration with Ruben Tovar (Texas State University). These studies were conducted on live snakes that we captured in Mexico and brought back to the lab at UTA. This research has identified a new feeding behavior in snakes, which is of interest in terms of functional morphology and the origins of evolutionary novelty in these gape-limited predators (Sheehy, 2012; Sheehy and Tovar, in review).