Freeze tolerance in neotropical frogs: an intrageneric comparison using Pristimantis species of high elevation and medium elevation

Abstract

AbstractParamos are high-elevation tropical Andean ecosystems above the tree line that display variable temperature and frequent freezing spells. Because a significant anuran community lives in this environment, physiological protection against freezing must characterise individuals in this community. Antifreeze protection has been studied in amphibians from other communities, and it is likely that Paramo anurans rely on the same underlying molecules that convey such protection to Nearctic species. However, given the pervasive presence of freezing spells in the Paramos year-round, the processes of activating protection mechanisms may differ from that of seasonal counterparts. Accordingly, this study investigated cryoprotection strategies in high-elevation tropical frogs, using as a model the terrestrial and nocturnal genus Pristimantis, specifically P. bogotensis, P. elegans and P. nervicus from Paramos, and the warm ecosystem counterparts P. insignitus, P. megalops and P. sanctaemartae. We focused on freeze tolerance and its relationship with glucose accumulation and ice formation. Under field conditions, the highest elevation P. nervicus exhibited higher glucose concentration at dawn compared to noon (1.7 ± 0.6 mmol/L versus 3.5 ± 1.32 mmol/L). Under experimental thermal freeze exposure for 2 hours between −2 and −4 ºC, the glucose concentration of the three Paramo species increased but physiological diversity was evident (P. nervicus 126%; P. bogotensis 100%; and P. elegans 55%). During this test, body ice formation was assessed calorimetrically. The species with the highest body ice formation was P. bogotensis (17% ± 5.37; maximum value: 63%; n = 8), followed by P. nervicus (5% ± 3.27; maximum value: 11%; n = 5) and P. elegans (0.34% ± 0.09; maximum value: 1%; n = 4). The study shows physiological diversity both within a genus and across the amphibian community around the freezing contour. Overall, Paramo species differ in freezing physiology from their low-elevation counterparts. Thus, climate shifts increasing freezing spells may affect the structure of communities in this zone.