“Heat waves” experienced during larval life have species-specific consequences on life-history traits and sexual development in anuran amphibians

Extreme temperatures during heat waves can induce
mass-mortality events, but can also exert sublethal negative effects by compromising
life-history traits and derailing sexual development. Ectothermic animals may,
however, also benefit from increased temperatures via enhanced physiological
performance and the suppression of cold-adapted pathogens. Therefore, it is
crucial to address how the intensity and timing of naturally occurring or
human-induced heat waves affect life-history traits and sexual development in
amphibians, to predict future effects of climate change and to minimise risks arising
from the application of elevated temperature in disease mitigation. We raised
agile frog (Rana dalmatina; Bonaparte, 1840) and common toad (Bufo bufo; Linnaeus, 1758) tadpoles at 19 °C and exposed them to a
simulated heat wave of 28 or 30 °C for six days during one of three ontogenetic
periods (early, mid or late larval development). In agile frogs, exposure to 30 °C during early larval
development increased mortality. Regardless of timing, all heat-treatments
delayed metamorphosis, and exposure to 30 °C decreased body mass at
metamorphosis. Furthermore, exposure to 30 °C during any period and to 28 °C
late in development caused female-to-male sex reversal, skewing sex ratios strongly
towards males. In common toads, high temperature only slightly decreased survival
and did not influence phenotypic sex ratio, while it reduced metamorph mass and
length of larval development. Juvenile body mass measured two months after metamorphosis
was not altered by temperature treatments in either species.
Our results indicate that heat
waves may have devastating effects on amphibian populations, and the severity
of these negative consequences, and sensitivity can vary greatly between species
and with the timing and intensity of heat. Finally, thermal treatments against cold-adapted
pathogens have to be executed with caution, taking into account the thermo-sensitivity
of the species and the life stage of animals to be treated.