Half asleep
Question: Is it true that some animals sleep with only half their brain at a time? If so, which animals do this and why?
Answer: Yes, it鈥檚 true. So-called unihemispheric sleep happens in animals when one side of the brain shows waking activity while the other side is asleep (an electroencephalographic recording of brain activity under these circumstances shows slow synchronous waves).
Unihemispheric sleep is a characteristic of several bird species and may be present in some reptiles. Not many mammals can sleep unihemispherically; only aquatic mammals such as dolphins, porpoises, toothed whales, and certain species of seal are known to alternate sleep between brain hemispheres.
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Unihemispheric sleep may have several functions in aquatic mammals. For example, unihemispheric sleep allows these mammals to swim to the surface to breathe. Discovering that dolphins keep one eye open during unihemispheric sleep has also led to the suggestion that they may monitor their environment during this type of sleep. Why humans and other mammals are not able to sleep in this manner is not known.
Fiona Baker
Wits Sleep Laboratory
Wits Medical School
Johannesburg
Answer: The bottlenose dolphin shuts down half its brain along with the opposite eye while sleeping. The other half stays awake at a low level of alertness to watch for obstacles and predators and to allow the dolphin to rise to the surface for air. The dolphin can reverse this process, awaking the brain鈥檚 rested half and resting the active side.
Birds employ this half-brain strategy to escape being eaten while asleep. When ducks and penguins sleep in a group, those on the edges keep their outward-facing eye open and vigilant. Wild birds may shuffle their positions so that ones that have already slept with one eye open eventually push into the middle to get more complete rest.
Johan Uys
Bellville, South Africa
Answer: Birds have mastered what humans can only dream of: letting one half of the brain sleep while the other half stays alert. Many birds seem to sleep with one eye open, but until recently no one had been able to prove the ploy was designed to help guard against predators. Now Niels Rattenborg and colleagues at Indiana State University in Terre Haute say birds are deliberately keeping one half of their brain awake as a safeguard.
The researchers lined up four groups of four mallard ducks, Anas platyrhynchos, and videotaped them while they slept. Just as they predicted, birds on the edge of a group were two and a half times as likely as the birds on the inside to keep one eye open-usually the outward-facing eye. Shown a video of an approaching predator, the birds detected it within a fraction of a second. Electroencephalograms showed the hemisphere opposite the open eye was alert, while the other hemisphere was getting high-quality sleep (Nature, vol 397, p 397).
Alison Motluk
Toronto, Canada
Peat ponds
Question: While walking in Scotland鈥檚 Cairngorm mountains, in an area where there are thick deposits of peat, I noticed that many stagnant ponds seemed to be covered by a film that displayed the same swirling patterns of rainbow colours seen when oil floats on water. Do hydrocarbons form in the peat or is there another explanation?
Answer: The colourful, oily-looking substance is in fact an ultrathin film of mixed ferric iron (Fe(III)) and ferrous iron (Fe(II)) oxide precipitates. The mechanism for the formation of the film is quite simple. Pond scum precipitates are seen in peat swamps and geologically similar environments such as coal deposits and some mangrove swamps. These places are characterised by carbon-rich deposits with trace amounts of other minerals including iron-bearing phases.
In the simplest case, iron (III) oxide phases, such as the common minerals haematite and limonite, dissolve readily in near-neutral water under the highly reducing conditions created by the organic-rich environments. As these waters interact with the atmosphere, the soluble ferrous iron is oxidised by atmospheric oxygen at the water-air interface.
While ferrous iron can be highly soluble at neutral pHs, ferric iron is essentially insoluble, and rapidly forms a precipitate. Hence, the thin films represent a complex precipitated mixture of iron (III and II) oxides. The impressive, often iridescent colours appear to be a result of the mixed oxidation states of iron in the resulting precipitate. Other mechanisms, such as anthropogenic oxidation of iron sulphide-which is relatively uncommon-can also account for the elevated ferrous iron in the water, but the mechanism of pond scum formation is the same.
Jeff Taylor
Principal Environmental Chemist
Earth Systems
Victoria
Answer: Iron bacteria may also help reduce iron compounds to iron oxides. The bacteria live at the air/water interface and, as they grow, they deposit a film of iron oxide on the under-surface of the water. This reflects light, which interferes with that reflected from the surface of the water, giving rainbow-like interference colours. If allowed to continue undisturbed, and if the nutrient resources are available, the films will thicken and become brown, to look like rust. The communities of these bacteria are common in undisturbed slightly anoxic sites, and the colours they produce are often misinterpreted as reflecting an oil spillage.
David Patterson
Department of Biological Sciences
University of Sydney
Answer: Many suspected oil spills have been reported to the Alaska Department of Environmental Conservation in swamp areas. As your correspondent suggests, few of these turn out to contain oil.
The water in these peaty bogs often contains high levels of reduced metals such as iron but they may also contain manganese. The quick field test for oil is to stir the pond with a stick. Metal oxide films will break into irregularly shaped platelets while oil will swirl around but still have an intact surface film.
Rosemary Antel
Seattle, Washington
This week鈥檚 questions
Flying V: I read a while ago that there are several competing theories as to why geese fly in a 鈥淰鈥� shape. Does anybody know the definitive answer?
Bruce Shuler
Plymouth, Michigan
Preservation order: I want to keep some copies of newspapers and magazines published after the 11 September attack on the World Trade Center in New York. Newspapers quickly turn yellow or become brittle and curly. Short of hiring a hermetically sealed library, what鈥檚 the best way to prevent this?