Ecological and evolutionary influences on body size and shape in the Galápagos marine iguana (Amblyrhynchus cristatus)

Lessons from evolution by natural selection: An unprecedented opportunity to use biomimetics to improve planetary health

Planetary health embraces the concept that long-term human welfare depends on the well-being of its ecological systems. Current practices, however, have often ignored this concept and have led to an anthropocentric world, with the consequence of increased greenhouse gas emissions, heat stress, lack of clean water and pollution, that are threatening the environment as well as the health and life of Homo sapiens and many other species. One consequence of environmental stressors has been the stimulation of inflammatory and oxidative stress that may not only promote common lifestyle diseases, but the ageing process. Despite the harshness of the current reality, treatment opportunities may exist 'in our backyard'. Biomimicry is an emerging field of research that explores how nature is structured and aims to mimic ingenious solutions that have evolved in nature for different applications that benefit human life. As nature always counteracts excesses from within, biodiversity could be a source of solutions that have evolved through the natural selection of animal species that have survived polluted, warm, and arid environments - i.e. the same presumptive changes that now threaten human health. One example from the emerging science suggests that animals use the cytoprotective Nrf2 antioxidant pathway to combat environmental stress and this may be a case example that we can apply to better human health. Learning from nature may provide opportunities for environmental management and solutions to the most challenging issue that face the future of the planet.

Phylogeography and Prevalence of Hemoparasites (Apicomplexa: Eucoccidiorida) in Galápagos Marine Iguanas, Amblyrhynchus cristatus (Reptilia: Iguanidae)

Parasitism is among the most common forms of coexistence of organisms of different species. Hemoparasites live in the bloodstream of the host where they complete different life-cycle stages. Members of the phylum Apicomplexa constitute a large portion of all hemoparasites infecting reptiles and their parasite transmitting vectors, including arthropods. In this study, we carried out a survey and molecular identification of hemoparasites in blood samples of the iconic Galápagos marine iguana (Amblyrhynchus cristatus). Major island populations of marine iguanas were sampled to examine large-scale biogeographic patterns of parasite diversity and prevalence. Nested PCRs were used to amplify segments of the 18S rRNA-gene of hemoparasites. Furthermore, ticks attached to marine iguanas were collected and analyzed in the same way to assess their potential use as a non-invasive method for the detection of hemoparasites in vertebrate host species. PCR products were sequenced and a phylogenetic analysis was carried out showing the presence of two genetically distinct clusters of hemoparasites, one more commonly distributed than the other one, belonging to the genera Hepatozoon and/or Hemolivia (Apicomplexa: Eucoccidiorida). Overall, 25% of marine iguanas were infected by hemoparasites. However, infection rates varied strongly among particular island populations (from 3.45% to 50%). Although marine iguanas are an extremely mobile species that has colonized all islands in the Galápagos archipelago, parasite occurrence was not related to geographical distance, suggesting that dispersal behavior has a minor role in parasite transmission. On most islands, females tended to have higher infection rates than males, but this relationship was only significant on one island. Overall, ticks and marine iguanas had similar prevalence and diversity of parasites. However, the infection profiles of ticks and their corresponding hosts (marine iguanas) did not mirror one another, indicating that this method cannot be used reliably to assess marine iguana infection status. Interestingly, we found that hemoparasite prevalence in marine iguanas and ticks tended to be positively correlated across islands. Our results indicate that certain populations of marine iguanas may have special mechanisms and adaptations to cope with parasite infection. In addition, other factors such as vector density, anthropogenic-related activities or the immunological state of marine iguanas could potentially affect the striking variation in hemoparasite prevalence across island populations.

Morphological and performance modifications in the world’s only marine lizard, the Galápagos marine iguana, Amblyrhynchus cristatus

The Galápagos marine iguana (Amblyrhynchus cristatus), the world’s only extant marine lizard, may have had one of the most unique and challenging transitions to aquatic life. Curiously, previous studies have identified relatively few physiological adaptations in the marine iguana; however, little is known about the extent of morphological specialization and performance trade-offs associated with the marine environment. By examining the morphology and locomotory performance of the marine iguana in comparison to their closely related mainland ancestors, the black spiny-tailed iguana (Ctenosaura similis) and green iguana (Iguana iguana), we found variation reflected specialization to ecological niches. However, variation was more pronounced among subspecies of marine iguana, suggesting that little morphological or performance modification is required for iguanids to successfully invade aquatic environments, thus raising the question why there are so few extant aquatic reptilian lineages. Our findings indicate that specialization for the marine environment likely resulted in a trade-off in sprint speed in a terrestrial environment, which may explain why other lizards have not undergone transitions to the marine environment. Additionally, we found that the magnitude of morphological and performance variation was more pronounced between subspecies of marine iguana than between iguanid species. This illustrates that the form-function relationship is more complex than previously thought and sheds light on the ecomorphological mysteries of the marine iguana.

No impact of a short-term climatic "El Niño" fluctuation on gut microbial diversity in populations of the Galápagos marine iguana (Amblyrhynchus cristatus)

Gut microorganisms are crucial for many biological functions playing a pivotal role in the host's well-being. We studied gut bacterial community structure of marine iguana populations across the Galápagos archipelago. Marine iguanas depend heavily on their specialized gut microbiome for the digestion of dietary algae, a resource whose growth was strongly reduced by severe "El Niño"-related climatic fluctuations in 2015/2016. As a consequence, marine iguana populations showed signs of starvation as expressed by a poor body condition. Body condition indices (BCI) varied between island populations indicating that food resources (i.e., algae) are affected differently across the archipelago during 'El Niño' events. Though this event impacted food availability for marine iguanas, we found that reductions in body condition due to "El Niño"-related starvation did not result in differences in bacterial gut community structure. Species richness of gut microorganisms was instead correlated with levels of neutral genetic diversity in the distinct host populations. Our data suggest that marine iguana populations with a higher level of gene diversity and allelic richness may harbor a more diverse gut microbiome than those populations with lower genetic diversity. Since low values of these diversity parameters usually correlate with small census and effective population sizes, we use our results to propose a novel hypothesis according to which small and genetically less diverse host populations might be characterized by less diverse microbiomes. Whether such genetically depauperate populations may experience additional threats from reduced dietary flexibility due to a limited intestinal microbiome is currently unclear and calls for further investigation.

Galapagos Islands Endemic Vertebrates: A Population Genetics Perspective

The organisms of the Galapagos Islands played a central role in the development of the theory of evolution by Charles Darwin. Examination of the population genetics factors of many of these organisms with modern molecular methods has expanded our understanding of their evolution. Here, I provide a perspective on how selection, gene flow, genetic drift, mutation, and inbreeding have contributed to the evolution of 6 iconic Galapagos species: flightless cormorant, pink iguana, marine iguana, Galapagos hawk, giant tortoises, and Darwin's finches. Because of the inherent biological differences among these species that have colonized the Galapagos, different population genetic factors appear to be more or less important in these different species. For example, the Galapagos provided novel environments in which strong selection took place and the Darwin's finches diversified to produce new species and the cormorant adapted to the nutrient-rich western shores of the Galapagos by losing its ability to fly and genomic data have now identified candidate genes. In both the pink iguana, which exists in one small population, and the Galapagos hawk, which has small population sizes, genetic drift has been potentially quite important. There appears to be very limited interisland gene flow in the flightless cormorant and the Galapagos hawk. On the other hand, both the marine iguana and some of the Darwin's finches appear to have significant interisland gene flow. Hybridization between species and subspecies has also introduced new adaptive variation, and in some cases, hybridization might have resulted in despeciation. Overall, new population genetics and genomics research has provided additional insight into the evolution of vertebrate species in the Galapagos.

Marine subsidies likely cause gigantism of iguanas in the Bahamas

We utilized natural experiment opportunities presented by differential conditions (presence/absence of seabirds and invasive species) on cays in the Bahamas to study whether interisland variations in food resources contributed to gigantism in Allen Cays Rock Iguanas (Cyclura cychlura inornata). We analyzed the stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values from iguana tissues and resources from each island food web to test the predictions that (1) food webs on islands with seabirds exhibit the influence of marine subsidies from seabird guano, whereas those from non-seabird islands do not, and (2) size differences in iguanas among cays were due to either (a) supplemental food availability from mice and/or seabird carcasses killed by barn owls (Tyto alba) and/or (b) access to more nutrient-rich vegetation fertilized by seabird guano. Food web components from the seabird island (Allen Cay) had 5-9‰ higher δ¹⁵N values than those on the other cays and Allen Cay plants contained nearly two times more nitrogen. Bayesian stable isotope mixing models indicated that C₃ plants dominated iguana diets on all islands and showed no evidence for consumption of mice or shearwaters. The iguanas on Allen Cay were ~ 2 times longer (48.3 ± 11.6 cm) and ~ 6 times heavier (5499 ± 2847 g) than iguanas on other cays and this was likely from marine-derived subsidies from seabird guano which caused an increase in nitrogen concentration in the plants and a resultant increase in the δ¹⁵N values across the entire food web relative to non-seabird islands.

Self-righting potential and the evolution of shell shape in Galápagos tortoises

Self-righting, the capacity of an animal to self-turn after falling on its back, is a fitness-related trait. Delayed self-righting can result in loss of mating opportunities or death. Traits involved in self-righting may therefore be under selection. Galápagos giant tortoises have two main shell morphologies - saddleback and domed – that have been proposed to be adaptive. The more sloped shape on the sides of the shell and the longer extension of neck and legs of the saddlebacks could have evolved to optimize self-righting. The drier environments with more uneven surfaces where the saddleback tortoises occur increases their risk to fall on their back while walking. The ability to fast overturn could reduce the danger of dying. To test this hypothesis, we used 3D shell reconstructions of 89 Galápagos giant tortoises from three domed and two saddleback species to compare self-righting potential of the two shell morphotypes. Our results indicate that saddleback shells require higher energy input to self-right than domed ones. This suggests that several traits associated with the saddleback shell morphology could have evolved to facilitate self-righting. Studying the functional performances of fitness-related traits, as in this work, could provide important insight into the adaptive value of traits.