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

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СКАЧАТЬ route, following chemical cues in the mucus, back to its home scar. The mucus also increases the amount of algae that settles and grows, so in effect the limpets are ‘farming’ their food. There’s much more to the humble limpet than meets the eye.

      Oyster Beds

      How do shoreline creatures ever sleep? And do oysters and other such shellfish dream? Tedbun

      This is a deceptively simple question that’s actually fiendishly difficult to answer. That’s because sleep is something of a biological enigma and biologists grapple even to agree on a definition of what constitutes sleep or why it’s necessary. If you think about it, it makes no sense for an animal to stop paying attention to the world around it when there are so many potential dangers. So there must be a really good biological reason for it beyond simply resting – we just haven’t worked out what that is yet!

      All animals go through periods of activity and inactivity, even those that live on a shoreline and are bashed about by waves. Physiologically, sleeping animals usually have certain brainwave patterns, appear unconscious and have relaxed muscles (except some birds and marine mammals that would fall out of the sky or drown if they relaxed – see the later question about swifts sleeping on the wing). It’s fairly easy to recognise when mammals and birds are asleep, but what about animals such as oysters and shellfish? Many researchers have relied on assessing the brain activity of animals to determine if they are asleep or not, but what happens if an animal has only a simple nervous system and no brain? How can you tell if an invertebrate, such as a fruit fly, is asleep or just stationary? Well, biologists have devised experiments to investigate exactly that. We’re all familiar with the fuggy-headedness that comes with a lack of sleep – our cognitive abilities such as learning become impaired, and the same is true for other animals. When deprived of sleep, fruit flies take longer to learn and become more forgetful. And, just like us, some invertebrates that are denied sleep will try to catch up by sleeping longer at the next opportunity. Studies on honeybees, cockroaches, cuttlefish and crayfish suggest that they all experience a kind of sleep, and some biologists suspect that a sleep-like status is universal to all animals.

      What about dreaming? In humans, dreams usually occur during one stage of sleep called REM (rapid eye movement) sleep, a state with particular brain activity that most mammals and some birds also experience. It’s difficult to prove but some higher mammals do appear to dream – just ask any dog owner who has watched their sleeping hound twitch and whimper after some imaginary squirrel or bone! However, there’s no concrete proof of fish, reptiles or any invertebrates falling into REM sleep. So oysters and other shellfish may be lulled to sleep by the sound of the ocean but they probably don’t dream.

      Shenanigans in the Shrubbery

      Can you explain a strange behaviour we’ve recently witnessed? Dunnocks live in our pyracantha hedge by our back door and feed and drink below it. One dunnock, presumably female, stands still and presents its rear end – tail erect and feathers well fluffed up – and allows its partner to gently peck its bottom repeatedly. This happens time and time again. Is this birdie foreplay? Mr and Mrs H

      Behind the dull grey and brown plumage of the dunnock (also known as a hedge sparrow, though it’s not a sparrow at all) lies a story of sexual intrigue and behaviour worthy of any soap opera. Their name means ‘little brown one’ but this shy, reclusive bird has a rather colourful and promiscuous lifestyle.

      In most species of birds, monogamy is the norm – at least on the surface, though quite a few will skulk off for what are quaintly called ‘extra-pair copulations’. Dunnocks, however, have evolved a system where infidelity is par for the course. They use various mating strategies, including love triangles and secret affairs as well as good, old-fashioned monogamy.

      Some females breed with two (or more) males, which is a relationship known as polyandry. She mates with both simultaneously, often producing a brood with mixed paternity. By being promiscuous, the female is maximising the chances of her chicks surviving because both fathers will bring food to the nest. Each father will adjust his feeding rate according to how exclusive his relationship was with the mother – the more matings he had, the more likely he is to be the father and the more food he will provide for the nestlings.

      Some males will mate with several females (polygyny) and there are even more complicated arrangements among several males and females (polygynandry). A male does best by mating with several females while a female is better off with several male partners. Clearly, this may lead to a battle of the sexes, with males and females competing for the greatest reproductive success.

      The specific relationships that occur in each area appear to be determined by several factors. Firstly, the sex ratio in the population may be skewed after a harsh winter to have more males than females – males outcompete females for food so the females are less likely to survive. Secondly, areas with dense bushes allow the females to sneak off for ‘a bit on the side’ more easily. Lastly, each male and female defends its own territory, and if food is scarce the territories need to be larger. The bigger the females’ territories are, the harder it is for a male to monopolise mating rights to more than one female. Where food is plentiful, females’ territories are more compact and a male has a better chance of patrolling them and successfully seeing off any rivals.

      These sneaky sexual shenanigans lead to some interesting courtship behaviour. In a sense, what the Hanhams witnessed was foreplay. By presenting her rear end and twirling her tail coquettishly the female is inviting the male to mate with her. The male, however, wants to ensure his paternity so he pecks at the female’s cloaca (her genital opening) to make her eject any sperm from previous matings. He may continue pecking at her for several minutes until he’s satisfied that she has got rid of any other male’s sperm and he can be sure that it will be his genes making it through to the next generation. The male may spend a huge amount of time trying to guard his female and protect his paternity rights by preventing her mating with other males. Despite his best efforts, the female will regularly escape as soon as his back is turned and solicit matings from his rivals. Mating activity can become quite frenzied, the avian equivalent of a Benny Hill sketch. Males may mate up to 100 times a day, but blink and you’ll miss it – the action is all over in just a tenth of a second.

      Tough Love

      While I was walking my dog round the pond near my house the other day, I noticed a moorhen with three chicks on the water. As I watched them the adult moorhen began to peck the chicks and make them go under the water. I was just wondering if it was teaching them to dive or if it was trying to kill them? Charlotte

      Moorhens are notoriously violent parents, often ‘tousling’ their chicks, pecking at them or holding them underwater. It can look quite brutal and it’s probably not a pleasant experience for the chick! Why would a parent be so aggressive towards its own offspring? It seems to be a case of tough love to deal with sibling rivalry.

      Moorhens often have large broods of eight or more eggs. They hatch over several days so the ones that hatch first are larger and have an advantage in securing their parents’ care over the smaller ones. Moorhens are ‘nidifugous’; that is, they leave the nest soon after hatching but the chicks are still partly reliant on their parents for food. Most waterfowl, such as mallards, will simply lead their chicks to food and let them get on with it, but moorhens take great care in feeding their young. However, there is intense competition between the chicks for their parents’ СКАЧАТЬ