As is often the case for animals living in extreme environments, desert birds face a choice between two competing physiological needs: in their case it's staying cool (which uses up a lot of body water) versus staying hydrated. Birds deal with this trade-off in different ways: some species make daily visits to reliable watering holes, sometimes flying considerable distances to quench their thirst, while other species hardly drink at all and conserve every drop they can get from meals of juicy insects. Zenon Czenze, a postdoctoral fellow at the University of Pretoria, South Africa, and colleagues investigated the implications of these different drinking habits for the thermal physiology of desert birds.
The team hypothesized that the heat tolerance of desert birds co-evolved with their dependence on surface water sources and, correspondingly, how much water they could commit to fending off the desert heat. Animals naturally lose water across their skin and when they breathe, a process known as evaporative water loss. Some animals, including sweating humans and panting dogs, turn this loss into their gain by getting rid of a little bit of body heat as the water turns into vapour, a process termed evaporative cooling. Czenze and colleagues predicted that birds that regularly imbibed fluids would take greater advantage of evaporative cooling to dump excess heat. With this additional cooling capacity, the team also expected that drinking species would tolerate hotter temperatures than species that rarely frequent watering holes.
To test these ideas, the researchers rounded up 12 species of songbirds with either drinking or non-drinking lifestyles from the Nama Karoo shrubland in South Africa. They implanted the animals with temperature sensors and measured their body temperature, metabolic rate and evaporative water loss rate at progressively higher temperatures.
As the temperature climbed, every species of bird examined increased their rate of evaporative water loss by panting. The species that refrained from drinking increased their rate of evaporative water loss 8-fold, whereas drinking species increased it 12-fold, showing that drinking species had more scope for evaporative cooling than species that drink little. Drinking species also had greater heat tolerance than non-drinking species and could withstand air temperatures of 52°C or higher, while the non-drinking species maxed out at ‘only’ 50°C. In addition, the species that tolerated the hottest temperatures were also those that showed the greatest increase in water loss, cementing the link between filling up on surface water and heat tolerance.
Almost everything comes down to conserving water or staying cool in the desert and arid-zone birds approach this trade-off in different ways. Regularly drinking species capitalize on predictable water sources to supercharge their body's air-conditioning compared with their non-drinking relatives, highlighting the close interrelationships between diet, movement ecology and thermal physiology in desert birds.
