Many places on our planet are becoming hotter and drier, threatening the animals that live there with the stress of heat and dehydration. Scientists have long been fascinated by how temperature and dehydration individually affect animals, but these two factors also interact. As the air warms up, it can hold more water, meaning that a temperature rise generally makes the area drier. This may be bad news for insects, which can heat up and lose water faster because of their small size. So, what happens to insects when these two environmental factors team up? Jamieson Botsch, Jesse Daniels and Karl Roeder at the USDA-ARS North Central Agricultural Research Station, USA, and Jelena Bujan at the Ruđer Bošković Institute, Croatia, wanted to know how temperature and humidity interact to determine how fast Bombus impatiens bumble bee workers lose water.
First, Botsch and colleagues built small containers to regulate the humidity at different air temperatures. The researchers then weighed each bee to determine how much water the bee's body started with, then placed each bee into a container with low (<5%), moderate (∼50%) or high (>95%) relative humidity (the percentage of water the air can hold at a given temperature). The scientists then put each container into a dark room set to 20, 25 or 30°C and checked them every 2–4 h until at least half of the bees from each group were unresponsive. If a bee was found unresponsive, the researchers weighed it again and measured how much water was left inside it when it became unable to move. The team found that the warmer the temperature and the lower the humidity, the faster the bees dehydrated. This, they explained, was partly due to the higher ability of the warm air to pull moisture from the bees’ bodies.
Surprisingly, Botsch and colleagues found that humidity alone did not affect how long worker bumble bees lived before succumbing to dehydration. Instead, the air temperature determines how long a worker bumble bee can survive. Bees kept at 20°C air temperature survived twice as long as bees kept at 25°C and three times longer than bees at 30°C. In contrast, the team found that humidity, not temperature, influenced how much water a worker bumble bee could lose before becoming unresponsive. The worker bees in the driest air lost ∼17% more water than those in moister air. The team also found the bees from the different colonies had different sizes and body masses. This also influenced how long the insects survived and how much water they could lose, but not how fast the bees lost water.
With so much of our planet becoming warmer and drier, bees living in these environments may soon struggle to keep cool and hydrated. Not only do these changing environments threaten these animals directly but this shift can also increase the chances of drought. As many environments become warmer and drier – and droughts more common – flowers will likely become more scarce, threatening bumble bees with heat and dehydration as they are forced to make longer trips to find food. This study by Botsch and colleagues emphasizes our need to understand the relationship between humidity and air temperature and their consequences for animals. Bumble bees will struggle to survive if the world keeps warming, which will make our own food supply start to vanish as well.
