Their bodies have a number of adaptations to deal with this topsy-turvy lifestyle. For example, they have specially adapted feet for hanging upside down. If we want to grip onto something with our hands we use our muscles to close our fingers around it, but the tendons in bats’ feet and legs work the opposite way round. Bats have to flex muscles to open their grip, and the talons shut when they relax. The weight of the bat locks the foot into a tight but effortless clasp. This means bats don’t use any energy when they hang upside down and their grip remains constant, even after death.
In most mammals, the knees bend forwards. This would be awkward for bats hanging against a surface, and their knees bend the other way. Many bats have a flap of skin between the legs which they use to catch bugs, and their backward-bending knees form a basket to hold their prey. Bats’ respiratory and cardiovascular systems are also adapted for the high energetic demands of flight. The heart is up to three times as large as that of a non-flying mammal of a similar size and they can increase their heart rate up to 1,000 beats per minute when flying.
Bats are also much smaller and lighter than us so they don’t experience the blood rush to the head that we would when hanging upside down. We have evolved to live upright and our circulation system reflects that. Our circulation system has one-way valves that prevent blood pooling in our feet and legs. Conversely, a bat’s large, powerful heart and specialised valves in its blood vessels prevent any blood pooling in its head. Evidently, bats have overcome any downsides to gravity as they can remain hanging upside down for weeks during hibernation with no adverse effects.
Feeling Sleepy?
Is it just mammals that yawn or do all animals yawn? Helks
Yawning isn’t unique to humans or even mammals but there can be confusion over what constitutes a yawn. Animals may show their wide-open mouths in a variety of situations. This is not necessarily a yawn but may be a gape. Opening the mouth fully can be part of a threat display used to show off fearsome teeth, such as in baboons. Birds may display open beaks during a courtship display.
A ‘real’ yawn is usually involuntary and starts with a large intake of breath, stretching the mouth wide, and then an exhalation. If the yawn is accompanied by a good stretch then it is delightfully known as ‘pandiculation’. Most vertebrates appear to do this to some degree. Despite being such a widespread behaviour and one that we experience regularly there is no obvious reason for or benefit of yawning. Yawning is seen as a sign of boredom or tiredness, but why would either of those states instigate a great, big yawn?
One long-held theory is that yawning is triggered by a lack of oxygen or a build-up of carbon dioxide – taking a large breath counteracts this by drawing in oxygen and expelling carbon dioxide. This seems to be hot air though. In experiments, increasing the levels of carbon dioxide or oxygen in a controlled atmosphere had little effect on how often people yawned. Another idea is that yawning is used to cool the brain. Yawning seems to be suppressed by an ice pack on the forehead or when the air is warmer than body temperature. Yet neither of these theories can account for why unborn babies yawn in the womb – even an eleven-week-old foetus has been shown to yawn. Yawning does appear to increase blood flow to the brain (the heart rate may increase) and promote alertness.
Have you yawned yet? One thing we do know about yawning is that it is highly contagious. It’s almost impossible to restrain a yawn once you’ve seen someone else yawning and even reading about it can trigger the reflex. This has led some biologists to believe that contagious yawning, is a sign of empathy, the ability to understand someone else’s state of mind and emotions. People with less developed empathy, such as children under about four years old and those with autism, don’t find yawning so contagious.
Yawning may have evolved to be contagious as a form of social bonding or group communication. A few other species also find yawning infectious and both chimpanzees and dogs that live in social groups will yawn in response to others. One theory is that this would allow the group to synchronise their rest periods or increase general alertness to compensate for one member’s tiredness. It just goes to show that there is plenty left to discover about human and animal behaviour, even with something as seemingly straightforward as a yawn.
Old Sparrow, New Tricks
How long do sparrows live? A few years ago, a male sparrow visiting the seed feeder in our yard was seen flying to the top of our wall with seeds for his fledglings for a few days, then the next day, when they were able to fly down to the ground, he perched at the feeder and dropped seed to them. He then seemed to get fed up with this process because it was too slow, so took to flying at the feeder and crashing into it with both feet up to shake a shower of seed down. The next year, we saw the same behaviour but then not for the next couple of years. So when it reappeared this year we wondered if it could be the same bird, or one of his descendants. Any ideas? Linda
Linda doesn’t mention whether the sparrow was a tree or house sparrow but either way it’s likely that the clever parenting she witnessed was performed by two different birds. The oldest recorded house sparrow lived for twelve years but their average lifespan is about three years. A tree sparrow is known to have lived for over ten years but typically they only survive for a couple of years. So, while it’s feasible that the original sparrow returned, it is unlikely. Instead, the new feeder-bashing sparrow could be evidence of an interesting phenomenon called cultural learning.
Cultural learning occurs when a new behaviour, like a novel feeding strategy, is transmitted from one individual to another within a population or from one generation to the next. This is particularly important in social birds such as house sparrows and starlings. Recent research suggests that birds living in large groups, such as sparrows, are better at solving problems because the individuals all have different experiences and skills to draw upon. We’ll never know whether Sue’s feeder-bashing sparrows learned the trick from the original sparrow or worked it out independently. However, if the behaviour had been passed on it doesn’t imply that the original feeder-bashing sparrow intentionally taught the new skill to others; they probably just imitated him.
The most well-known instance of cultural learning is the intriguing case of the milk thieves. In the good old days, when milk was delivered to the doorstep by a friendly milkman whistling a merry tune, an inquisitive blue tit or great tit discovered that those glass bottles contained fat-rich cream. Originally, milk was delivered in open bottles that birds could easily drink from, but later, when milk bottles were covered with silver foil, blue tits and great tits soon learned to peck through the foil to steal the creamy goodness underneath. The pecking of milk bottle tops rapidly became widespread. It took 25 years for the habit to grow from one observation to being seen in over 400 different locations. Whether each bird worked out how to do this independently or just watched and copied others is open to debate. The behaviour did not spread out in a wave from the original location but popped up sporadically, suggesting that it arose on several occasions. Blue tits and great tits often peel off bark looking for insects and grubs. They are naturally curious and have relatively large brains so it would not have been a huge step to try peeling off milk bottle tops. Of course, there’s no reason why birds would not have copied the behaviour from each other within each location, so it’s likely that a combination of inquisitive individuals and social learning was responsible for the stolen milk. Now that fewer people have milk delivered, this behaviour is rarely seen.
Run, Rabbit, Run
Why do rabbits have white, furry bottoms? Does this not make them easier for predators to spot? Lupodod
Rabbits certainly have their fair share of predators. Fully-grown rabbits are taken by foxes, cats, stoats and polecats