Temporary increases in body temperature help birds save water in the heat

Alexander R. Gerson, Andrew E. McKechnie, Ben Smit, Maxine C. Whitfield, Eric K. Smith, William A. Talbot, Todd J. McWhorter and Blair O. Wolf

 

Desert dwelling birds are ideal candidates for investigating the mechanisms that allow animals to persist in the world’s hottest deserts.

As temperatures approach and exceed body temperature (~40°C for a resting bird), birds increasingly rely on evaporative cooling to maintain body temperature below lethal levels. But these birds also need to conserve water in these dry, desiccating environments.  To limit water losses, birds routinely allow their body temperature to increase above resting levels, sometimes to as high as 45°C. This regulated increase in body temperature, termed hyperthermia, has been observed in many species of birds and mammals, and three mechanisms have been proposed to explain how hyperthermia could reduce water losses.

• Birds could maintain body temperature above environmental temperatures which would allow heat to be lost to the environment without evaporation.
• Minimizing the difference between environmental temperature and body temperature by allowing body temperature to increase requires lower rates of evaporative water loss to maintain sub-lethal body temperatures.
• As temperature increases, heat can be stored in the body rather than being dissipated, which causes the body temperature to increase. Since this heat is not dissipated evaporatively, no additional water is used for evaporation.

To partition the benefits of hyperthermia among these mechanisms, we measured metabolic rate, rates of water loss, and body temperature continuously during heat exposure in 33 species of birds from the deserts of South Africa, South Australia, and the Southwest United States.  We show that how hyperthermia saves water during heat exposure differs among species. Small songbirds tend to maintain body temperatures above environmental temperatures until the latter approach lethal limits (~45°C).  Songbirds use this strategy because they cannot evaporate water very rapidly, and their high surface area to volume ratios mean they gain heat at a rate too high for them to tolerate once environmental temperature exceeds body temperature.

Once temperatures exceed body temperature, most larger songbirds and doves  use hyperthermia to reduce the gradient between body temperature and air temperature. By minimizing this gradient, less water is needed to maintain a constant but increased body temperature.  Doves are highly efficient at evaporating water, and therefore do not increase body temperature to the same degree as passerines, owls, and quail. Burchell’s sandgrouse from South Africa were unique among all the birds tested because they are large, but still allowed body temperature to rise significantly, resulting in significant water savings.

By identifying differences in the benefit of hyperthermia across many species from three of the Earth’s major desert regions, we have identified important differences in heat susceptibility and water economy. Understanding the mechanistic basis of heat tolerance is essential for birds living in hot environments that are projected to get hotter as a result of anthropogenic climate change.

Read the paper here.