Embryos and Tadpoles of the Eurythermal Baja California Chorus Frog (Pseudacris hypochondriaca) Show Subtle Phenotypic Changes in Response to Daily Cycling Temperatures.

AbstractEnvironmental thermal fluctuations influence fitness-related organismal traits. Investigations of ectothermic physiology must include cycling thermal regimes because such fluctuations are increasing with environmental change. We used the eurythermal Baja California chorus frog (Pseudacris hypochondriaca) to examine developmental responses to daily temperature cycles informed by recorded field conditions (15°C constant and cycles of 12.5°C⟷17.5°C and 10°C⟷20°C) and an extreme daily cycle (15°C⟷25°C). We measured survival, development rate, mass, and oxygen consumption rate (V̇o₂) upon hatch. The 15°C⟷25°C daily cycle decreased time to hatch, produced larger hatchlings, and reduced mass-specific V̇o₂. Lower mass-specific V̇o₂ in 12.5°C⟷17.5°C hatchlings indicated a minor effect of daily temperature cycles with a common mean temperature (15°C). We also measured size, thermal tolerance, V̇o₂, and swim speed at a common tadpole stage (hindlimb toe differentiation), with V̇o₂ and swim speed measured at 10°C, 15°C, 20°C, and 25°C. The 15°C⟷25°C tadpoles were smaller but showed higher thermal tolerance, mass-specific V̇o₂ at 25°C, and length-corrected sustained swim speed than the 15°C constant and 12.5°C⟷17.5°C tadpoles. The 15°C⟷25°C treatment with a higher mean temperature (∼20°C) drove most phenotypic differences in hatchlings and tadpoles. Compared to tadpoles in the 15°C constant treatment, tadpoles in the 10°C⟷20°C treatment had significantly higher thermal tolerance and moderate but statistically indistinguishable increases in swim speed, illustrating subtle effects of daily temperature cycles on tadpole phenotypes. Developing chorus frogs function well at and above maximum temperatures experienced naturally, and our results indicate a subtle "warmer is better" acclimation response for physiological traits in response to substantial daily thermal cycles.