QotW: How do bats roost upside down?

Is hanging upside down bad for a bat's circulation?...
03 November 2020

BAT

A sleeping bat.

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Question

Satish got in touch to say "How does a bat sleep all day, by hanging on a tree? Will being upside down not affect blood circulation?"

Answer

Phil Sansom asked bat expert Brock Fenton...

Phil - Every one of our questions of the week goes on our science forum - that’s nakedscientists.com/forum - and Evan from Australia responded pointing out that most bats are pretty small, so there is very little distance the top and bottom of their bodies, certainly compared to humans, making pumping the blood that little bit easier. But the next few responses quickly moved onto the topic of bat excrement, so I left the forum to speak to bat expert Brock Fenton.

Brock - Many bats hang upside down, and in this posture show no negative impact on patterns of blood flow. But these bats look at us and worry about whether our blood all rushes to our feet!

Phil - In either case, humans or bats, the circulatory system has the important job of maintaining blood pressure across the body.

Brock - If our blood pressure drops too much, we suffer from an oxygen shortage, get dizzy, and pass out.

Phil - To avoid this, bats, like all mammals, have valves that prevent blood from rushing to the head - but the next challenge is to go from roosting to flying...

Brock - In either situation, the circulatory systems are designed to deal with changes in posture. Bats are very small mammals that undergo many changes in body posture as they fly. Their circulatory systems are adapted to smooth out the impacts of the changes, as ours do when we run and jump and move around.

Phil - So that’s a small body, a specialised circulatory system, and plus, bats also have very large hearts and lungs for their size, and astonishing heart rates to power their rapid flight - that go up to 20 beats per second!

Brock - But a fundamental question is ‘why hang upside down?’ For bats the situation is complicated. While most species/individuals do hang upside down, some do not.

Phil - Those that do rely on a particular arrangement of tendons so that they don’t have to tense their feet to hang - in fact, clenching for them becomes the relaxed posture, and they have to use a muscle to unclench. Why have they evolved like this…?

Brock - Hanging upside down allows a bat to take flight by simply letting go and dropping … spreading its wings and taking flight. This is much cheaper than trying to jump into the air, or making a long take off run. Why do bats really hang upside down then? I think it’s because they are lazy!

Phil - Western University’s Brock Fenton, taking bats to task. Next time, from animals to plants - with this question from listener Kevin...

Kevin - Does watering my veggie garden in the morning mean that water uptake will coincide with the intake of sunlight, and give my vegetables the best chance? Or is it just as good to water in the evening?
Phil - Do you know? If so, please come plant the seed of the answer on our forum - that’s nakedscientists.com/forum. Or if you have a question you’d like us to answer, email chris@nakedscientists.com, use the webform at nakedscientists.com/question, or find Naked Scientists on Twitter or Facebook. That’s all - until next time, goodbye!

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