Springwatch Unsprung: Why Do Robins Have Red Breasts?. Jo Stevens

Чтение книги онлайн.

СКАЧАТЬ a dazzling fan. However, familiar British birds such as grouse, blackbirds and many species of duck, such as mallards, also have males that are more eye-catching than the females. Other birds have more subtle cues, such as the yellower chests of male blue tits that are particularly apparent when viewed in ultraviolet light, which birds, unlike humans, can detect.

      In 1859 Charles Darwin first proposed that male birds used elaborate plumage to impress females in his book On the Origin of Species. He argued that the males evolved ornaments such as beautiful feathers, or beguiling courtship rituals like the complex dances performed by birds of paradise, to woo the females, which then choose their partners according to their fitness. At the time, Darwin’s entire theory of evolution was controversial and met with much consternation, but the concept of female choice was especially scandalous at a time when women had little control over their lives in a patriarchal Victorian society.

      Anyway, back to robins! Robins are unusual because both males and females hold territories throughout the year and defend them vigorously. Their red chest feathers act as a warning to other robins rather than an attraction to potential mates. The male and female both have bright orange/red breasts that stand out amid the gloom of dusk and dawn when they are often active. They use their beautiful song and brilliant red chests to see off any intruders. A robin will find a high perch, fluff out the feathers to show as much red as possible then sing out its melodious war song. If the vivid red chest is their war paint, then their song is their battle cry.

      Most of the time, the opponent submits and a fight is avoided. If not, the owner will attack, and may even peck the other bird to death. Up to 10 per cent of robins might die in this way. Robins are so feisty that any object of a similar colour may be attacked, like the apocryphal red rag to a bull. We’ve had reports of assaults on orange lawnmowers, red socks and even someone gardening in a red jumper.

      From late December onwards, the robin’s thoughts turn to romance, but courtship can be confusing when both sexes look almost the same. The male may be a little brighter and slightly larger than the female but even robins may have difficulty telling the sexes apart. They have to take into account how a robin acts rather than rely on how it looks. During the breeding season, females approach their chosen male’s territory with care. Sometimes, the male mistakes his lover for a fighter and initially responds aggressively, singing at the ‘intruder’. She patiently listens and doesn’t sing back – instead she follows him around. After a few hours, her patience pays off and he finally accepts her. They set up home together and both fiercely defend their patch. Robins may look cute and appealing but they are gutsy little warriors.

      Topsy Turvy

      How do bats hang upside for so long without any adverse effects, such as poor blood circulation? The Chameleon

      Bats spend the daylight hours hanging upside down in a secluded spot, such as the roof of a cave or the inside of a hollow tree, safely hidden away from predators and inclement weather. Hanging up high has another advantage: bats can’t run and launch into flight like birds can; instead they drop into the air straight into flight.

      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 СКАЧАТЬ