The amount of stress hormones in the blood of free-living cowbirds varies greatly. The researchers found significant variation in the rate at which glucocorticoid hormone levels varied among individual tit in response to changing ambient temperatures. The ability of bird populations to adapt to changing conditions, such as more frequent temperature extremes caused by climate change, may be facilitated by these individual variations.
A research project at the Max Planck Institute for Biological Intelligence studied birds for several years. The study’s findings were published in the journal Proceedings of the Royal Society B Biological Sciences. Hormones control vital bodily functions in many animals, including birds. They help animals regulate metabolism and food intake, thus supporting them in maintaining body temperature within a certain range. Glucocorticoids are stress hormones that coordinate many functions that help animals cope with changes in their environment.
On cold days, they are produced in greater quantities and help the body use its reserves of carbohydrates, fats and proteins to produce heat. At higher temperatures, the level of glucocorticoids decreases and thus the conversion of energy into body heat. In an environment with widely fluctuating temperatures, the stabilization of body temperature by hormones is increasingly important. Small warm-blooded animals such as birds must respond quickly to temperature fluctuations to maintain their body temperature. As a result of climate change, many habitats are undergoing major changes in environmental conditions and extreme temperatures are becoming more common.
But how much does climate affect the hormonal balance of birds?
Do individuals cope differently with climate change?
To answer these questions, data must be collected over several years. The leader of the research group, Michaela Hau, and two colleagues determined the level of glucocorticoids in a population of horse chestnuts in southern Bavaria over the course of five years. They correlated their measurements with environmental temperatures and found, as expected, that hormone levels were higher at cooler temperatures. However, there were also big differences in the reactions of individual birds to temperature fluctuations.
Large individual differences
“We observed for the first time in free-living vertebrates that some individuals show a more pronounced adaptation of glucocorticoid levels to environmental temperature than others,” says Michaela Hau. “This variation among individuals may allow populations to cope with a wide range of environmental conditions.” Further studies are needed to determine whether the observed hormonal differences between individuals lead to differences in heat production or resistance to heat loss. It is also unknown whether this individual variation is associated with increased reproductive success or survival.
“If the strength of glucocorticoid changes has a heritable component and individuals with certain hormonal responses produce more offspring or survive longer, natural selection can change the composition of populations in subsequent generations,” explains Michaela Hau. “Our work is therefore an important basis for understanding whether and how animals can adapt to climate change.”
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