Newswise —  Animal behaviour: Ants push and shove to avoid traffic congestion (pp70-73)
Ants take alternative routes to avoid overcrowding on their most congested thoroughfares, an experiment published in this week's Nature shows. When traffic gets too jammed, ants shove others out of the way, causing them to travel by a different path.
Foraging ants prefer to ferry food along a single trail, which they reinforce with scent cues, say Audrey Dussutour and colleagues. The researchers tested this by giving black garden ants (Lasius niger) a sugar source that could be reached by a bridge divided into two equally wide branches. With branches at ten millimetres wide, traffic was much heavier over one of the two branches. But if the branches were narrower, causing increased congestion, the flow across the two branches was more even.
The effect was due to pushing and shoving where the branches diverged, the authors explain. Ants, if allowed, preferred to follow the route where scent cues were stronger. But if they came head-on with heavy traffic, the resulting collisions would send them along the alternative branch.
 &  Plant development: The ups and downs of leaf development (pp81-84 and 84-88)
Tiny fragments of 'microRNA' may help leaves to establish their polarity, according to two studies in this week's Nature.
The upper and lower surfaces of leaves are distinctly different. Upper surfaces are often waxy, waterproof and specialized for photosynthesis, whereas lower surfaces are commonly peppered with tiny pores, called stomata, giving leaves a site for gas exchange.
Two studies have found that a microRNA, known as miRNA165/166, is distributed unevenly between the two surfaces — there is more in the top surface of the leaf than the bottom. The effect is seen in the model plant Arabidopsis, as described by Catherine A. Kidner and Robert A. Martienssen, and also in maize, as described by Marja C. P. Timmermans and colleagues.
These microRNAs suppress expression of genes needed to form the structures found on the underside of leaves, causing the two surfaces to develop in their unique ways.
 Physics: Atoms served extra chilled (pp50-52)
Scientists reveal a new way to help atoms chill out in this week's Nature. This may prove important for the future of quantum computing, which aims to exploit non-classical physics for faster information processing.
Gerhard Rempe and colleagues used laser light to hold single atoms of rubidium in an optical cavity. Light from a second laser then cooled the rubidium down by carrying a bit of the atoms' energy with it as it escaped from the cavity. This method is at least five times better than the conventional way to cool an atom, the authors say.
Conventional laser cooling, which won a Nobel Prize in 1997, relies on first exciting the atom with a jolt of energy. When the atom relaxes again, the packet of energy that it spits out travels in random directions. Unfortunately, this process can destroy some of the quantum information held by atoms. Rempe claims that his 'cavity cooling' method is unique in that it preserves these quantum states.
 Ecology: Corals make mucus to recycle energy (pp66-70)
Coral reefs recycle energy by making mucus, according to a study in this week's Nature. The mucus feeds microorganisms and traps particles in the surrounding water, building up nutrients that are eventually filtered through the sandy sea bed and back into the reef.
The discovery answers the long-standing question of why corals bother to produce so much seemingly wasteful slime, say Christian Wild and colleagues. Algae that live on the coral make energy from sunlight and donate it to the coral, much as a tenant pays rent in exchange for living space. Researchers were unclear why corals would spend so much of this energy on making mucus.
The mucus helps to prevent nutrients from being lost from the reef altogether, Wild's team found. Some 56"80% of the mucus dissolves, feeding local microorganisms, whereas the rest forms whitish films and threads called flocs that gather bacteria, algae and small particles. These energy-rich cargoes are filtered through the seafloor sand and find their way into corals at the bottom of the reef.
 Climate: Cold lid on polar oceans (pp59-63; N&V)
A study in this week's Nature provides more evidence for the ocean's role in ancient climate change.
Daniel Sigman and colleagues have analysed deep-sea records of sediment composition from the Southern Ocean and the North Pacific. The records indicate that 2.7 million years ago — when temperatures plummeted and the Northern Hemisphere became glaciated — vertical mixing in the polar oceans was reduced. The thin surface layer of fresh water became sufficiently light compared to the saltier waters below to insulate the abyssal ocean, trapping carbon dioxide at depth. Removing this important greenhouse gas from the atmosphere accelerated cooling further.
As climate cools, salinity becomes increasingly more important for seawater density as the influence of temperature wanes. This mechanism might also apply in other cold episodes of the Earth's evolution.
"They propose an elegantly simple mechanism that could very well explain some of the major climatic shifts in the geologic past" , says Roger FranÃ§ois in an accompanying News and Views article.
 Physics: The highs and lows of superconductivity (pp53-55; N&V)
A study in this week's Nature offers insights into the enigmatic behaviour of the so-called high-temperature superconductors. The research may aid the discovery of ever more efficient superconductors.
Superconductors are energy efficient materials that can conduct electricity without resistance when cooled below a certain temperature (the transition temperature). No material has yet been found that superconducts at room temperature: the record (still a chilly minus 150 degrees Celsius) is currently held by a family of copper oxides, which are made of superconducting copper oxide layers, interspersed with blocks of insulating material. As the number of superconducting sheets next to each other goes up, so too does the transition temperature — but only up to a point. If more than three superconducting layers are next to each other, then the transition temperature begins to go down again.
Sudip Chakravarty and colleagues have identified three factors that combine together to explain this trend — the ability of electrons to hop from one superconducting sheet to the next, the distribution of electrons between the superconducting sheets and the ordering of charge within the layers.
"That we might now understand why the transition temperatures in these curious layered materials have been limited is exciting indeed — and from this understanding, the prospect of designing materials with still higher transition temperatures looks more realistic," says Piers Coleman in an accompanying News and Views article.
 Health and Medicine: Getting the broader picture (pp73-77)
If you want to judge absolute distances with accuracy, you need to look at the broader picture, researchers say.
Humans can accurately judge an object's distance up to 20 metres away, as long as it is seen against a flat terrain. But if the same object is more than 3 metres from us without the ground surface for reference, our accuracy wanes. In this week's Nature, Zijiang J. He and Teng Leng Ooi and colleagues reveal how our visual systems cope with long-distance judgements.
The process begins close to — the visual system scans the immediate surroundings and builds up a template of the local ground surface. It then applies this template further afield, helping the brain to generate a more global frame of reference. With the broader picture in view, distances can be judged more accurately.
 Finally..: Ant parasites forgo multiple mating (pp35-36)
One partner is enough for a lazy social parasite, a Brief Communication in this week's Nature shows. Ant queens that live in luxury at the expense of a related species' workforce take only a single mate, in contrast to the multiple partners taken by a bona fide queen.
The discovery shows that multiple mating is only worthwhile for socialite ants such as Acromyrmex echinatior, say Jacobus Boomsma and colleagues. A. echinatior queens take around ten partners, producing a genetically diverse group in which workers are less susceptible to disease and males are less vulnerable to infertility.
Queens of its related parasite Acromyrmex insinuator, which invaded 44% of colonies at the authors' study site in Gamboa, Panama, do not need to produce a nestful of healthy workers. So they forgo the trouble of finding several partners and make do with just one.
ALSO IN THIS ISSUE"¦
 Crystal symmetry and the reversibility of martensitic transformations (pp55-59)
 Hybrid fracture and the transition from extension fracture to shear fracture (pp63-66)
 Inactivation of hCDC4 can cause chromosomal instability (pp77-81)
 A non-B-DNA structure at the Bcl-2 major breakpoint region is cleaved by the RAG complex (pp88-93; N&V)
 Preferential cis"syn thymine dimer bypass by DNA polymerase ? occurs with biased fidelity (pp97-100)
GEOGRAPHICAL LISTING OF AUTHORS"¦
The following list of places refers to the whereabouts of authors on the papers numbered in this release. For example, London: 4 - this means that on paper number four, there will be at least one author affiliated to an institute or company in London. The listing may be for an author's main affiliation, or for a place where they are working temporarily. Please see the PDF of the paper for full details.
CANADAOntario Toronto: 7
GERMANYBremen: 5Bremerhaven: 5Dresden: 1Garching: 4Leipzig: 10Potsdam: 6
JAPANSuita: 14Saitama: 14Toyonaka: 14
(HASHEMITE KINGDOM OF) JORDANAqaba: 5
UNITED KINGDOMLondon: 9
UNITED STATES OF AMERICACalifornia Los Angeles: 7, 13 Pasadena: 10Florida Tallahassee: 5Kentucky Louisville: 8Maryland Baltimore: 12Nebraska Omaha: 13New Jersey Princeton: 6New York Cold Spring Harbour: 3, 12 New York: 12 Stony Brook: 3North Carolina Research Triangle Park: 14Pennsylvania Elkins Park: 8Texas College Station: 11 The Woodlands: 11
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