Corticosterone and immune responses to dehydration in squamate reptiles.

Many environments present some degree of seasonal water limitations; organisms that live in such environments must be adapted to survive periods without permanent water access. Often this involves the ability to tolerate dehydration, which can have adverse physiological effects and is typically considered a physiological stressor. While having many functions, the hormone corticosterone (CORT) is often released in response to stressors, yet increasing plasma CORT while dehydrated could be considered maladaptive, especially for species that experience predictable bouts of dehydration and have related coping mechanisms. Elevating CORT could reduce immunocompetence and have other negative physiological effects. Thus, such species likely have CORT and immune responses adapted to experiencing seasonal droughts. We evaluated how dehydration affects CORT and immune function in eight squamate species that naturally experience varied water limitation. We tested whether hydric state affected plasma CORT concentrations and aspects of immunocompetence (lysis, agglutination, bacterial killing ability and white blood cell counts) differently among species based on how seasonally water limited they are and whether this is constrained by phylogeny. The species represented four familial pairs, with one species of each pair inhabiting environments with frequent access to water and one naturally experiencing extended periods (>30 days) with no access to standing water. The effects of dehydration on CORT and immunity varied among species. Increases in CORT were generally not associated with reduced immunocompetence, indicating CORT and immunity might be decoupled in some species. Interspecies variations in responses to dehydration were more clearly grouped by phylogeny than by habitat type.

Savanna vegetation increase triggers freshwater community shifts.

Tropical savannas are globally extensive and ecologically invaluable ecosystems. As most ecosystems however, they are subject to serious anthropogenic stress. Defaunation, and especially the loss of large mammals, is pervasive in tropical savannas and known to trigger wide-ranging ecological effects, from vegetation changes to the loss of ecosystem function. Despite what is currently known about the terrestrial consequences of defaunation, and the potential cross-ecosystem influence of large mammals, virtually no research has investigated associated effects on small adjacent water bodies. This research gap persists because (1) tropical savannas have been historically neglected, (2) the ecological value of small water bodies (e.g. ponds) is only recently being recognized, and (3) empirical baseline data are often lacking. In this paper, we compared a rare pre-change dataset with newly collected data on 213 freshwater assemblages, to investigate community structure and composition before and after a major defaunation event. Our research focused on a diverse species assemblage of amphibian larvae (i.e. tadpoles) in temporary savanna ponds. We found that pond vegetation cover increased from 16.0% to 45.6% post-defaunation, that is, a near three-fold increase. Such habitat changes seemed to have benefitted those species that use vegetation during reproduction (e.g. the leaf-folding Afrixalus spp.), while others have declined. Interestingly, we found a strong correlation between tadpole community shifts and other freshwater organisms, which indicates that habitat changes have affected a wide variety of aquatic organisms. Given that organisms inhabiting temporary aquatic habitats often have complex life histories with terrestrial adult life stages, we propose that the terrestrial effects of defaunation have indirectly led to distinct aquatic communities, in addition to direct habitat effects. These results shed new light on the potential role of large-bodied mammals in shaping adjacent ecosystems, and raise important questions concerning the functioning of temporary aquatic systems in the Anthropocene.

Les savanes tropicales sont des écosystèmes étendus à l'échelle mondiale et d'une valeur écologique inestimable, mais qui sont soumis à une pression anthropique croissante. La défaunation, en particulier la perte de grands mammifères, est omniprésente dans les savanes tropicales et pouvant déclencher des effets écologiques de grande envergure allant des changements de végétation à la perte des fonctions écosystémiques. Malgré ce qui est connu des conséquences terrestres de la défaunation, presque aucune recherche n'a étudié les effets de la défaunation sur les plans d'eau temporaires adjacents qui sont utilisés par les grands mammifères. Cette lacune persiste parce que (1) les savanes tropicales ont été historiquement négligées, (2) la valeur écologique des plans d'eau temporaires a souvent été sous-estimée et (3) les données empiriques de référence sont souvent absentes. Dans l'étude présente, nous avons utilisé des données pré/post-défaunation sur 213 assemblages aquatiques de savane, dans le but d'étudier la structure et la composition de ces communités avant et après qu'un événement majeur de défaunation ait eu lieu. Notre recherche se focalise sur des plans d'eau temporaire comptant un nombre important d'espèces de larves d'amphibiens (têtards). Nous avons détecté une multiplication moyenne par près de trois de la couverture végétale des plans d'eau après la défaunation (16,0% à 45,6%). Ces changements d'habitat semblent avoir profité aux espèces qui utilisent la végétation pour leur reproduction (par exemple, Afrixalus spp.), tandis que d'autres espèces avec d'autres préférences d'habitat ont décliné. Nous avons calculé une forte corrélation entre la composition des têtards et celle de leurs prédateurs, ce qui indique que les changements d'habitat ont affecté la plupart des membres de ces communautés aquatiques. Étant donné que les organismes d'habitats aquatiques temporaires ont pour la plupart un cycle biologique complexe figurant à la fois un stade larvaire aquatique et un stade adulte terrestre, nous proposons qu'en plus des effets directs sur l'habitat aquatique (augmentation de la végétation), les effets terrestres de la défaunation ont indirectement altéré les communautés. Ces résultats suggèrent un rôle important des grands mammifères par leur influence sur les écosystèmes aquatique adjacents et soulèvent des questions urgentes concernant la fonctionnalité des systèmes aquatiques temporaires dans l'Anthropocène.

Dehydration enhances multiple physiological defense mechanisms in a desert lizard, Heloderma suspectum.

The physiological challenges associated with dehydration can induce an increase in plasma glucocorticoid concentrations, a response thought to provide the mechanism for dehydration suppressing immune function. However, a comprehensive examination of the inter-relationship of dehydration, stress and immune function has not been conducted within a single species. We previously demonstrated that Gila monsters (Heloderma suspectum), which inhabit a xeric environment with a predictable seasonal drought, have enhanced measures of innate immunity when dehydrated. These results suggest that, in this species, dehydration may not induce a glucocorticoid response, but, instead, enhances physiological defense mechanisms. To explore this possibility, we examined multiple measures of innate immunity as well as initial and reactive plasma concentrations of glucocorticoids in captive and free-ranging Gila monsters at various hydration states. Our results show that, in this species, dehydration alone does not cause a substantial increase in plasma glucocorticoids, and we provide broader evidence that dehydration enhances defensive mechanisms including stress reactivity and various measures of innate immune function. These findings suggest that physiological responses to dehydration may depend heavily on an organism's ecology. More research on the effects of dehydration on the glucocorticoid response and immunity will help clarify the interactive roles they play in response to hydration challenges and whether adaptations to water-limited environments influence these interactions.