17 November 2012 | NewScientist | 17

OUR ancestors began eating grass half a million years earlier than thought, soon after they started leaving the trees. Early hominins, living 3 to 3.5 million years ago, got over half their nutrition from grasses, unlike their predecessors, who preferred fruit and insects.

This is the earliest evidence of eating savannah plants, says Julia Lee-Thorp at the University of Oxford. She found high levels of

carbon-13 in the bones of Australopithecus bahrelghazali, which lived on savannahs near Lake Chad in Africa. This is typical of animals that eat a lot of grasses and sedges (PNAS, DOI: 10.1073/pnas.1204209109).

Previously, the oldest evidence of grass-eating was from 2.8 million years ago. The 4.4-million-year-old Ardipithecus ramidus, an early hominin, did not eat grass.

Chilling effect of a black hole moon

HOW do you cool a black hole? Just add a small moon.

Stephen Hawking predicted that black holes should emit a small amount of radiation, giving them a temperature that depends on their mass. But to find it, physicists also need to know the surface gravity, which so far cannot be calculated unless the black hole is sitting still.

To expand Hawking’s theory to moving black holes, Samuel Gralla and Alexandre Le Tiec at the University of Maryland in College Park modelled a scenario in which a moon is orbiting a black hole at the same speed that the hole rotates. The moon seems to hover in place, and the black hole appears stationary (arxiv.org/abs/1210.8444).

The team then used existing equations to find its temperature. A partner would cause the black hole to wobble, lowering surface gravity and cooling it a little.

Your password is incorrect. The punishment is death

MOST parents are security conscious, but one Australian bird takes it to a new level. Superb fairy-wrens (Malurus cyaneus) often find their nests hijacked by cuckoo chicks, so they have started using password protection.

The egg of the Horsfield’s bronze cuckoo (Chalcites basalis) looks like the wrens’ eggs but hatches earlier, at which point the cuckoo chick ejects the wren eggs. The idea is that the mother wren will raise the cuckoo as her own. But in 40 per cent of cases, the wren detects the cuckoo, abandons it and starts another nest.

Wondering how the wrens do it, Sonia Kleindorfer of

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When hominins went herbaceous A. bahrelghazali may have eaten roots and tubers, rather than tough grass blades. Adding these to their diet may have helped them leave their ancestral home in east Africa for Lake Chad.

The question is whether hominins moved onto savannahs permanently, or went between woodland and savannah when it suited them, says Rick Potts at the Smithsonian National Museum of Natural History in Washington DC. “I would vote for the latter.”

Flinders University in Adelaide, Australia, and colleagues recorded their calls. Mother wrens sang a call made up of 11 elements while incubating the eggs, but stopped when they hatched. One of the elements was a note unique to that bird. When the wren chicks begged for food, they repeated this note (Current Biology, doi.org/jqh).

Kleindorfer thinks the note is a password, which reassures the mother that the chicks are her own. “The password identifies ‘nest membership’,” she says. The cuckoo chicks have less time to learn the password because they hatch earlier than wrens. The two species are engaged in an “acoustic arms race”, Kleindorfer says.

“This is a way of avoiding parasitism that we hadn’t really thought of before,” says John Endler of Deakin University in Geelong, Australia.

MEN with partners increase the space they feel comfortable with between themselves and an attractive woman if exposed to the bonding hormone oxytocin.

René Hurlemann at the University of Bonn in Germany and colleagues gave men either a sniff of oxytocin or a placebo before asking them to choose the ideal distance for an interaction with a woman. The distance that they felt was comfortable significantly increased after sniffing oxytocin, but only for men in relationships.

The team conclude that oxytocin discourages partnered but not single men from getting close to a female stranger (Journal of Neuroscience, DOI: 10.1523/jneurosci.2755-12.2012).

Oxytocin changes men’s behaviour

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