By maintaining their body temperatures at only a degree or so above that of
the air outside, hibernating mammals such as dormice can dramatically reduce the
energy they expend. During hibernation they remain in a death-like sleep for
days at a time. Just as most animals have a body clock that induces a daily
rhythm of activity, hibernating mammals seem to have one set on an annual
cycle.

Irving Zucker of the University of California at Berkeley and colleagues from
Stanford University noted that a small part of the hypothalamus called the
suprachiasmatic nucleus (SCN) is unusually active during hibernation compared to
the rest of the brain. The researchers looked at the effect on hibernation of
removing the SCN in laboratory-reared golden mantled ground squirrels (
Spermophilus lateralis).

They kept squirrels whose SCNs had been removed, along with normal animals,
at a constant temperature of 6.5 °C over two and a half years. The normal
squirrels followed their usual annual cycle, but four of the eight altered
squirrels hibernated continuously throughout the study. The remaining four
followed an annual cycle, but with longer hibernation seasons and shorter
intervals between them (Proceedings of the National Academy of
Sciences, vol 93, p 9864).

“One of the ideas we have is that the SCN inhibits hibernation for part of
the year, when animals need to be building up their fat stores,” says
Zucker.

They picked corticosterone as the most likely culprit because stress causes
the adrenal glands to produce more of this hormone. Corticosterone molecules can
also cross easily from mother to fetus through the placenta.

To test their hunch, Arnaud Barbazanges and his colleagues, who work for the
French medical research organisation INSERM at the University of Bordeaux,
repeatedly stressed female rats in their last week of pregnancy by placing them
in a clear Perspex tube in bright light for short periods. In half the mothers
the adrenal glands had been removed, and replaced by a pellet that released
corticosterone at a steady rate. The remainder had normal adrenal glands.

When these rats’ pups grew up, the researchers put them through the same
stressful experience as their mothers for 30 minutes, and measured how long it
took for their corticosterone levels to return to normal. They all had high
levels half an hour after being restrained in the tube, but only those born to
stressed mothers with intact adrenals still had the same high levels after two
hours (Journal of Neuroscience, vol 16, p 3943).

ĐÓ°ÉÔ­´´s already knew that a negative feedback loop in the brain usually
responds to increased corticosterone by turning off the adrenals. But when
levels of corticosterone are consistently high, the brain’s receptors become
desensitised, so that it takes longer for the negative feedback mechanism to
work. Now Barbazanges and his colleagues have shown that corticosteroid
receptors in the brains of the rats are less sensitive even when they had been
exposed to high levels of stress-induced corticosterone in the womb.

Researchers working with humans have linked stressful experiences in early
life with depression and anxiety in adults. If, as seems likely from the French
experiments, these effects can begin before birth, then the best start in life
is a stress-free gestation.

The answer is yes, according to John and Colleen Marzluff of the Sustainable Ecosystems Institute in Meridian, Idaho, and Bernd Heinrich of the University of Vermont in Burlington. They studied ravens, Corvus corax, in the pine forests of western Maine. By night ravens roost in groups, flying off each morning to scavenge the carcasses of deer and other large animals. When the birds find a new carcass they visit it over several days until the bones are picked bare.

In 1973, the Israeli biologist Amot Zahavi suggested that communal roosts function primarily as information centres, allowing birds newly arrived in an area to acquire local knowledge about the best sources of food. But the idea has been difficult to prove. “Up to now, there’s only been anecdotal evidence,” says John Marzluff.

Marzluff and his colleagues caught a number of birds and kept them in an aviary long enough for their knowledge of available carcasses to become out of date. They placed fresh carcasses in the study area and then released some of the captives to join a roost containing birds that had fed at one of these carcasses earlier in the day. The following morning each of the released birds followed their roost-mates to these carcasses and joined in the feast.

Other captive birds were given a harder task. They were tipped off about a rich source of food by releasing them near a newly placed carcass just before dusk, to see if they could influence the next day’s foraging party. Of 26 birds released, just one returned to the carcass the next day, bringing with it a flock of 30 ravens from a roost 2 kilometres away. Two others returned several days later with a gang of fellow foragers (Animal Behaviour, vol 51, p 89).

Perched in a treetop hide at dusk, the researchers frequently saw birds that had returned to the roost from various directions suddenly soar into the air, circle over a large area and head off to another roost that turned out to be nearer to the next morning’s breakfast. They suspect that this behaviour may signal to other ravens that one of the soaring birds knows the location of a feast.

Des Thompson of Scottish Natural Heritage in Edinburgh, who studies ravens in the Highlands, says that they are highly intelligent and social birds. “If any bird species is going to show evidence of information transfer, then it’s the raven,” he says. But another interpretation is that foraging ravens could simply follow dominant birds. Thompson points out that banding together in roosts may also provide protection against predators. “I think this is the closest we are likely to get to an answer,” he says, “but as a Scot I’d have to say, case not proven.”

In the long term, sex should be worthwhile because the recombination of genes between males and females increases the chance that some offspring will have novel traits that allow them to flourish in changing circumstances. But if the first sexual individuals had lower reproductive rates, might they have been wiped out by the sheer numbers of competing asexual types?

Robert Dunbrack and his colleagues at the Memorial University of Newfoundland in St John’s, decided to test this question in a laboratory experiment. They put individuals of two strains of the red flour beetIe, Tribolium castaneum, into two jars of flour containing low concentrations of the insecticide Malathion.

From one jar, the researchers removed all offspring from each generation and replaced each of them with three adults from a reservoir of the original parent population, which had never been exposed to Malathion. This tripled the strain’s reproductive rate but ensured that, like a lineage of asexual carbon copies, it was unable to adapt to the insecticide.

In the other jar, all beetles were allowed to remain in the population and continue to breed. Those whose genetic make-up happened to be best able to cope with the Malathion would presumably leave the most offspring. Twice a week, Dunbrack and his colleagues Carla Coffin and Robert Howe shared out the flour between the two jars according to the number of beetles in each – in effect forcing the two strains to compete for the flour.

To begin with, things looked bad for the beetles that were being allowed to evolve. In one experiment the population fell to as few as 10 individuals, while the nonevolving population grew to nearly 1000. But after five generations (20 weeks), the situation was dramatically reversed. In every experiment, using either strain in the evolving role, the evolving population bounced back and outcompeted the nonevolving population, which was eventually completely eliminated (Proceedings of the Royal Society B, vol 262, p 45).

The study is the first experimental proof that sex can pay off within a few generations, says Dunbrack. “There’s a lot of suggestive field work showing differences in survival between sexual and asexual forms, but it’s very difficult to find sexual and asexual clones that are competing,” says Dunbrack. “Using this experimental technique, we’ve been able to address and control all the factors directly.”

People with brain damage occasionally suffer from bizarre defects, such as being unable to associate elephants with the colour grey while having no difficulty perceiving and naming either colours or elephants. Rather than having a single file marked “elephant” in our mental filing system, we seem to have a network of linked databases each specialising in a particular kind of information. But when it comes to identifying the brain areas responsible, brain damage can provide only a rough guide: cases are rare, and the damaged areas are often large.

More detailed studies can be done by measuring blood flow in the brain, using the scanning technique known as positron emission tomography (PET). This allows researchers to identify peaks of activity in different regions of normal people’s brains while they carry out particular mental tasks. Alex Martin and his colleagues at the NIMH decided to use the technique to investigate knowledge about colour and movement, because the areas of the brain that perceive these attributes are already well known.

They showed people black and white line drawings of a number of objects, and asked them to name either a colour or an action associated with each one. For example, a picture of a pencil might yield the responses “yellow” and “write”.

Both tasks caused the brain to work harder in the prefrontal, parietal and temporal lobes of the cortex, the outer layer of the brain that deals with higher functions such as perception and language. Some of this activity, however, was for subtasks such as word finding, which come into play when naming both actions and colours. So Martin and his colleagues computed the difference in brain activity between the two tasks. Brain regions that responded equally to both tasks yielded a difference of zero, while those that responded strongly to one task and not the other showed a high value.

Their analysis revealed that the lower surfaces of each temporal lobe contained an area that was most active when the subjects thought about colour. This is close to a region other PET studies have pinpointed as critical for colour perception. Similarly, an area near the junction of the left temporal and occipital lobes was most active when thinking up an action word – and this was immediately next to the motion perception area (Science, vol 270, p 102).

“What’s really important here is that knowledge is organised in a way that’s predictable from what we know about the organisation of perception,” says Martin. The results also support the idea that perception of an object almost instantaneously makes available everything we know about it. “We always see things in terms of the meaning they convey,” says Martin. “That’s how we can identify them so quickly.” (see Diagram)

James Steele of the University of Southampton and Simon Mays of English Heritage’s Ancient Monuments Laboratory examined skeletons excavated from a churchyard where most graves date from the 11th to the 16th century. They measured the difference between the lengths of the main bones of the right and left arms of each skeleton. As people make more use of the dominant arm for load-bearing tasks, the bones tend to grow slightly longer on that side during childhood and adolescence.

They found that in 81 per cent of the skeletons, the right arms were longer than the left, in 3 per cent they were equally long, and in 16 per cent the left arms were longer. Rather than falling into two distinct groups of right and left-handers, the Wharram Percy peasants showed a continuous range of differences in arm length (International Journal of Osteoarchaeology, vol 5, p 39).

Steele and Mays argue that the relatively high degree of left-handedness or ambidexterity found at the Wharram Percy site represents “a distribution of hand-use preferences which was either unconstrained by, or resistant to, cultural pressures for conformity”.

They point out that the pattern of asymmetries is almost identical to that obtained by Marian Annett of the University of Leicester and her colleagues who timed contemporary British subjects as they carried out a peg-moving task with first one and then the other hand, and found a rate of left-handedness of about 15 per cent. Taken together, the studies provide evidence for a rate of “biological” left-handedness that has remained remarkably stable for centuries.

Claude Wischik of the Cambridge Brain Bank Laboratory has investigated the properties of tau, which in its abnormal state forms insoluble tangles in the brain cells of Alzheimer’s patients. Last week he told an international symposium in Cambridge that the abnormal, tangle-forming protein can convert normal tau to the abnormal form.

Alzheimer’s disease manifests itself in the brain in two ways. As well as the tau tangles, sufferers’ brains contain plaques of a protein called amyloid. But scientists do not know whether either the tangles or the plaques cause dementia.

Wischik decided to see whether there was a correlation between the severity of the disease and the quantity of tau tangles in the brain. He looked at the brains of patients who had been suffering from Alzheimer’s when they died, and compared them with the brains of people who died mentally normal. The number of tangles in the brain correlated well with the extent of the patients’ dementia. Wischik also found that the proportion of normal tau fell from 97 per cent in the normal patients to 17 per cent in those with severe Alzheimer’s.

Normal tau helps maintain microtubules, which form the “skeleton” that gives nerve fibres their shape. Wischik suspected that in Alzheimer’s patients the normal protein is converted into the abnormal form. So he took the abnormal protein, which is shorter than the normal version, and mixed it in a test tube with normal tau and enzymes found in nerve cells which digest proteins.

He found that the abnormal protein bound very strongly to normal tau. The enzymes were unable to break down the abnormal tau, but they cut the ends off normal protein molecules which had bound to abnormal tau, converting them to the abnormal form. These in turn became “magnets” to which other normal tau molecules bound. “You can’t break out of it once it starts,” says Wischik. The abnormal tau gradually clumps together to make the tangles. Wischik is uncertain what starts the process, but he says there may be many triggers, both genetic and environmental.

If the same process occurs in the body, and the tangles do cause dementia, slowing the conversion of tau to the abnormal form could slow the progress of the disease. Wischik has identified four compounds which can stop tau binding to itself. He now intends to test these compounds in cultured nerve cells.

Robert Barton of the University of Durham, working with Andrew Purvis and Paul Harvey of the University of Oxford, has completed a survey of the brains of 132 species of primates, bats and insectivores. Previously, most researchers studying brain evolution have measured the overall size of the brain relative to the body – often assuming that larger brains mean higher intelligence. Barton and his colleagues suspected that this was not the whole story, and set out to look at the brain’s sensory subsystems, relating these to the mammals’ lifestyles.

Several general trends emerged. For example, they found that in animals that are active in daylight the parts of the brain that deal with eyesight are more highly developed than they are in nocturnal animals, and those dealing with sense of smell are less well developed.

Barton admits that these findings seem obvious. But he says: “This is just the start in unravelling the adaptive significance of species differences in the brain.” The researchers found that sensory differences alone – reflected in the relative size of the brain areas devoted to vision and smell – often explained most of the variation in brain size among related species (Philosophical Transactions of the Royal Society B, vol 348, p 381).

The study has already thrown into question some theories about brain evolution. Biologists had previously thought, for example, that fruit-eating primates had larger brains than their leaf-eating relatives because they needed to be especially bright to find patchily distributed fruit trees. But the new study has shown that differences in the size of the brain areas involved in visual processing account for most of the difference in brain size between fruit-eating and leaf-eating primates. Barton argues that fruit eaters have bigger brains simply because they need good colour vision to find ripe fruit. Seeing in colour, he believes, may require much more neural processing than monochrome vision.