Urea and glucose modulation during freezing exposure in three temperate frogs reveals specific targets in relation to climate.

Amphibian diversity is most prominent in the warm and humid tropical and subtropical regions across the globe. Nonetheless, amphibians also inhabit high-altitude tropical mountains and regions at medium and high latitudes, exposing them to subzero temperatures and requiring behavioural or physiological adaptations to endure freezing events. While freeze tolerance has been predominantly reported in high-latitude zones where species endure prolonged freezing (several weeks or months), less is known about mid-latitudes amphibians exposed to occasional subzero temperatures. In this study, we employed a controlled ecological protocol, subjecting three frog species from the Iberian Peninsula (Rana parvipalmata, Epidalea calamita, and Pelobates cultripes) to a 2-h exposure to temperatures of -2 °C to investigate the accumulation of urea and glucose as physiological mechanisms associated with survival at freezing temperatures. Our results revealed a moderate response in the production of cryoprotectant metabolites under experimental freezing conditions, particularly urea, with notable findings in R. parvipalmata and E. calamita and no response in P. cultripes. However, no significant alterations in glucose concentrations were observed in any of the studied frog species. This relatively weak freezing tolerance response differs from the strong response exhibited by amphibians inhabiting high latitudes and enduring prolonged freezing conditions, suggesting potential reliance on behavioural adaptations to cope with occasional freezing episodes.

Commentary: Ultraviolet radiation triggers "preparation for oxidative stress" antioxidant response in animals: Similarities and interplay with other stressors.

Preparation for oxidative stress (POS), i.e., the upregulation of endogenous antioxidants, is a widespread response of animals exposed to extreme conditions. This response has been described for more than 80 animal species belonging to eight phyla during hypometabolism or situations that limit oxygen availability. The pattern of the typical POS-response, in which a mild redox imbalance triggers antioxidant adjustments that results in increased tolerance to subsequent oxidative insults, roughly follows the curve of hormetic phenomena. A similar pattern has been reported for various animal species exposed to ultraviolet radiation (UVR) - these studies, on animals from six phyla, are discussed herein. In the light of the similarities in the redox-response of animals exposed to either oxygen restriction or UVR, we argue in this essay that UVR elicits a type of response that fits the POS theory. Exposure to UVR induces both reactive species formation and antioxidant adaptation, which is the essence of typical POS-responses. Thus, antioxidant response to UVR in animals can be categorized as a POS-type mechanism. Moreover, considering that animals are exposed to multiple stressors simultaneously in nature, this would represent an ecologically relevant process, by which one stressor (e.g., UV or ionizing radiation) may enhance the tolerance to other. We also discuss a possible role of low doses of ionizing radiation as inductor of POS-like responses in animals.

Thermal ecology of montane Atelopus (Anura: Bufonidae): A study of intrageneric diversity.

Harlequin frogs (Bufonidae: Atelopus) are among the most threatened frog genus in the world and reach very high elevations in the tropical Andes and the Sierra Nevada de Santa Marta (SNSM). Learning about their thermal ecology is essential to infer sensitivity to environmental changes, particularly climate warming. We report on the activity temperature and thermoregulatory behavior of three high-elevation species of harlequin frogs, Atelopus nahumae, Atelopus laetissimus and Atelopus carrikeri. The first two mentioned live in streams in Andean rain forests, whereas A. carrikeri inhabits paramo streams in the SNSM. We studied the thermal ecology of these species in tree localities differing in altitude, and focused on activity body, operative, substrate and air temperature. A main trend was lower body temperature as elevation increased, so that differences among species were largely explained by differences in substrate temperature. However, this temperature variation was much lower in forest species than paramo species. The Atelopus species included in this work proved to be thermoconformers, a trend that not extended to all congenerics at high elevation. This diversity in thermal ecology poses important questions when discussing the impact of climate warming for high-elevation harlequin frogs. For example, forest species show narrow thermal ranges and, if highly specialized, may be more susceptible to temperature change. Paramo species such as A. carrikeri, in contrast, may be more resilient to temperature change.