Aerobic scope and cardiovascular oxygen transport is not compromised at high temperatures in the toadRhinella marina

Abstract

SUMMARYNumerous recent studies convincingly correlate the upper thermal tolerance limit of aquatic ectothermic animals to reduced aerobic scope, and ascribe the decline in aerobic scope to failure of the cardiovascular system at high temperatures. In the present study we investigate whether this ʻaerobic scope modelʼ applies to an air-breathing and semi-terrestrial vertebrate Rhinella marina (formerly Bufo marinus). To quantify aerobic scope, we measured resting and maximal rate of oxygen consumption at temperatures ranging from 10 to 40°C. To include potential effects of acclimation, three groups of toads were acclimated chronically at 20, 25 and 30°C, respectively. The absolute difference between resting and maximal rate of oxygen consumption increased progressively with temperature and there was no significant decrease in aerobic scope, even at temperature immediately below the lethal limit (41-42 o C). Haematological and cardiorespiratory variables were measured at rest and immediately after maximal activity at benign (30°C) and critically high (40°C) temperatures. Within this temperature interval, both resting and active heart rate increased, and there was no indication of respiratory failure, judged from high arterial oxygen saturation, P O2 and [Hb O2 ]. With the exception of elevated resting metabolic rate for cold-acclimated toads, we found few differences in the thermal responses between acclimation groups with regard to the cardiometabolic parameters. In conclusion, we found no evidence for temperature-induced cardiorespiratory failure in R. marina, indicating that maintenance of aerobic scope and oxygen transport is unrelated to the upper thermal limit of this air-breathing semi-terrestrial vertebrate.

Hormonal and metabolic responses to upper temperature extremes in divergent life-history ecotypes of a garter snake

“…Given that neither VȮ 2 nor glucose concentration plateaued at high temperatures and that lactate concentration did not escalate with temperature, these snakes are not limited by oxygen acquisition or transport at temperatures near their thermal maximum. This result is in accordance with recent studies that refute the OCLTT hypothesis in free-living stages of terrestrial ectotherms (Fobian et al, 2014;He et al, 2013;McCue and Santos, 2013;Overgaard et al, 2012; but see Shea et al, 2016). Our data do not address other possible mechanisms that may contribute to thermal tolerance limits, including protein denaturation, membrane fluidity, enzymesubstrate interactions, mitochondrial failure and failure of neural processes (reviewed in Angilletta, 2009;Schulte, 2015).…”

Baroreflex regulation affects ventilation in the Cururu toad Rhinella schneideri

“…_ V E was affected by temperature from 25 to 35°C (127% increase, Q 10 =2.27), mainly as a result of increases in V T (Q 10 =1.57). Thus, increased _ V E at high temperature matches an elevated metabolic rate up to 105% (thermal interval range from 25 to 35°C) reported in a closely related species, R. marina (Overgaard et al, 2012). High temperature exposure (35°C) in R. schneideri elicited cardiovascular adjustments to support an increased metabolic demand in which baseline f H rose 100% relative to that at 25°C, while blood pressure remained unchanged.…”

Oxygen- and capacity-limited thermal tolerance: bridging ecology and physiology

“…Such evidence is required when investigating a role for oxygen in thermal limitation. A later study of the same species by Overgaard et al (2012) found a typical exponential increase in resting oxygen consumption, which entered the steep phase beyond 30°C, in line with a transition to pejus range (see Fig. 2).…”