Metabolic Flexibility in Response to Within-Season Temperature Variability in House Sparrows

Area of Science:

• Physiology
• Ecology
• Evolutionary Biology

Background:

• The climatic variability hypothesis (CVH) links phenotypic flexibility to environmental variability.
• CVH is typically studied across populations, not within-season variations.
• Winter climates often show higher variability than summer climates in north-temperate regions.

Purpose of the Study:

• To test the within-season extension of the CVH in house sparrows.
• To investigate if winter phenotypes are more flexible than summer phenotypes.
• To explore the mechanistic basis of metabolic rate variation.

Main Methods:

• House sparrows collected in summer and winter were acclimated to 24°C, 5°C, and -10°C.
• Basal metabolic rate (BMR) and summit metabolic rate (Msum) were measured before and after acclimation.
• Flight muscle and heart masses, and enzyme activities were analyzed.

Main Results:

• Cold acclimation increased BMR and Msum, with higher BMR in winter birds.
• Contrary to the hypothesis, winter birds showed decreased metabolic rates during acclimation, while summer birds increased theirs at cold temperatures.
• Muscle masses correlated with metabolic rates, but enzyme activities did not clearly explain variation.

Conclusions:

• The CVH's predictions may not apply to seasonal temperature variability within a population.
• House sparrows exhibit greater metabolic upregulation in summer than winter in response to cold.
• Initial metabolic rates at capture influence acclimation responses in captive birds.