The researchers studied only female bats, which tended to catch more insects on warm nights than did the male bats. Eastern pipistrelle bats are incapable of long periods of sustained activity outside the roosting caves on very cool spring and fall evenings. Because of the long period of winter inactivity, eastern pipistrelle bats tend to consume more insects per day in the spring and fall than in the summer. Eastern pipistrelle bats can detect changes in barometric pressure within the caves that correlate closely with changes in temperature outside the caves.
In the spring and fall, eastern pipistrelle bats roost deep inside caves. They feed at night on flying insects and must leave the cave to catch their prey. Flying insects are much more abundant on warm nights than on cool ones. Researchers found that many more bats leave the caves on warm nights than on cool nights, even though the temperature within the caves where the bats roost remains virtually the same from one night to the next.
Which one of the following, if true, most helps to explain the researchers' findings?