Metabolic cold adaptation in the Asiatic toad: intraspecific comparison along an altitudinal gradient

The metabolic cold adaptation (MCA) hypothesis predicts an increase in metabolic rate and thermal sensitivity of poikilotherms from cold environments as compared to those from warm environments, when measured under standardized conditions. This compensatory response is also expected to evolve in life history and behavioral traits if the reductions in these phenotypic traits at low temperature involves in a reduction in fitness. We investigated the extent to which the level of energy intake (measured as feeding rate), energy turnover (measured as standard metabolic rate, SMR) and the energy budget (energy allocation to growth and physical activity) are influenced by climatic conditions in three populations of the Asiatic toad (Bufo gargarizans) distributed across an altitudinal gradient of 1350 m in the Qionglai Mountains of Western China. We found a similar thermal reaction norm of SMR at both population and individual levels; therefore, the data did not support the MCA hypothesis. However, there was a co-gradient variation (CoGV) for mass change rate in which the high and medium altitudinal populations displayed slower mass change rates than their counterparts from low altitudes. Moreover, this CoGV pattern was accompanied by a low feeding rate and high physical activity for the high- and medium-altitude populations. Our results highlight that adjustments in energy intake and energy allocation to behaviors, but not energy allocation to metabolism of maintenance, could act as an energetic strategy to accommodate the varied growth efficiency in Asiatic toads along an altitudinal gradient.

Automated summary

The study found similar thermal reaction norms of standard metabolic rate (SMR) in different altitudinal populations of Asiatic toads in the Qionglai Mountains, which does not support the metabolic cold adaptation (MCA) hypothesis. However, there was a co-gradient variation (CoGV) for mass change rate, with high and medium altitude populations showing slower mass change rates and lower feeding rates but higher physical activity compared to low altitude populations. This suggests that adjustments in energy intake and energy allocation to behaviors may play a role in the energetic strategy of Asiatic toads along altitudinal gradients.