When I was a teenager, one of my passions was cave exploring. Southern Indiana where I lived at the time, is honeycombed with caves of all kinds and descriptions, and the thrill of crawling around in mud and water to be where no one has been before can become an addiction for a teenage boy. One of the experiences that I remember with very little fondness is crawling into an extremely small area and suddenly finding myself surrounded with bats who were crammed into such a small area I could not see the rock they were hanging on to at all. Why bats congregated in massive numbers did not occur to me at the time, but this massing of bat bodies is a design feature that bats use to solve a major problem they face.
The caves bats live in are cool places. When female bats get ready to have their young, both they and the young bats need warm places to assure successful birthing and growing. Studies by scientists have shown that a 15 degree rise in temperature can reduce the gestation period by many days and reduce the time between birth and flight by as much as 50%. The way bats solve this problem is by grouping themselves into tight areas where their body temperatures may raise the roost temperature by well over 15 degrees. In Indiana this temperature increase is critical. If baby bats are three weeks late in reaching the flying stage, they do not have time to eat enough bugs to gain enough fat to make it through the winter.
Coming up with evolutionary scenarios to explain this might appear to be pretty easy. Bats that congregate survive, and bats that do not die. The fact is that all bats seem to have this behavior. Red bats cluster in broad leafed trees. Seminole bats roost in clumps of Spanish moss. Wood bats cluster in hollow trees. In many of the locations, bats would not have to have this behavior to survive because the temperatures of the environment are high enough to avoid it. It appears that this is a genetically designed characteristic that allows bats to live in all kinds of temperatures and environments, and still have the ability to survive and produce offspring. We would suggest this design is not a chance proposition, but is intelligently determined.-Reference: Natural History, June, 2000, pages 18-19.
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