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By Stephen Luntz

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Wasps Develop Ant Control
Invasive wasps in New Zealand have adapted to conflict with ants by picking the ants up and removing them from food.

Vespula vulgaris is considered one of the world’s 100 worst invasive species, and its highest known density is in the beech forests of New Zealand’s South Island, far from its European home. Many species are in decline as a result of the wasp’s arrival, including parrots, which struggle to compete for honeydew.

Native ants, however, are providing more competition for the wasps as both like to feed on dead or dying insects on the forest floor. “Despite being 200 times smaller, the ants are able to hold their own by rushing at the wasps, spraying them with acid and biting them,” explains Dr Phil Lester of Victoria University, Wellington.

Lester set up baits of canned tuna to watch the ant/wasp interaction, and was astonished to see the wasps picking the ants up in their mandibles and dropping them a distance away. Although the ants are not injured by the fall they appear stunned, and do not always return to the bait.

The more ants present around the bait, the more the wasps engaged in the grab-and-carry routine. Interestingly, when ants were more common the wasps would carry them greater distances away from the bait, although Lester notes this is “maybe a 7 cm flight rather than a 3 cm flight” that takes around half a second.

“To the best of our knowledge this behaviour has never been observed before. Our results suggest that these insects can assess the degree and type of competition they are facing and adapt their behaviour accordingly,” says Dr Julien Grangier, co-author of a paper on the behaviour published in Biology Letters.

Unlike the ants, the wasps are solo foragers so they don’t gain a collective benefit from ant removal. The behaviour has never been described before, despite long observations in their native range.

Lester believes the wasps don’t kill the ants because they are “little chemical weapons factories” and puncturing an ant would release a larger dose of these chemicals than the ants can spray.

Although Lester says the finding has offered no ideas about how to fight the wasp, he says: “We’re at the point where anything we know is better than nothing”.

Sadness after Sex Hard to Predict
One in three women say they have experienced postcoital dysphoria, more commonly described as post-sex blues. There has been very little research into the topic, but A/Prof Robert Schweitzer of Queensland University of Technology’s School of Psychology and Counselling has made a start with work published in the International Journal of Sexual Health.

Crucially, postcoital dysphoria is not a reaction to bad sex or regrets about the choice of partner. Schweitzer quotes one woman as saying: “I did not associate the feeling with an absence of love or affection for my sexual partner, nor with an absence of love or affection from them towards me, because it seemed so unconnected with them”.

Schweitzer notes: “Under normal circumstances the resolution phase of sexual activity, or period just after sex, elicits sensations of well-being, along with psychological and physical relaxation”. Postcoital dysphoria, on the other hand, is associated with melancholy, anxiety and tearfulness. One woman said she felt “homesick”.

Even among women who report having experienced postcoital dysphoria, it was usually rare. However, 1.8% of women in the study said they experienced it most of the time. With a sample size of only 222 university students, the figure is not terribly precise.

Schweitzer notes that “people assume it must be associated with a history of sexual abuse or psychological distress,” but his study found only a weak correlation. In fact, none of the factors he tested for proved good predictors of whether, or how often, women would experience sadness after sex.

A Google search reveals tens of thousands of discussions of the topic, mostly on blogs, but Schweitzer says he could only find three publications in the scientific literature related to the topic, and none measuring prevalence.

Media coverage of Schweitzer’s research has led many women to express relief upon discovering that their experience is not unique, with one recounting that she repeatedly locked herself in the bathroom in tears.

Schweitzer says there is even less research into similar male experiences, although he notes that postcoital headaches are a recognised condition. He is also interested in researching cases of men who, even in committed relationships, can’t stand to be around their partners after sex.

The next step is to interview women who experience dysphoria frequently. Schweitzer says that once ethical approval for this is granted he will be keen to hear from women wanting to take part.

Ants Beat Algorithms
Argentine ants have been able to solve dynamic problems in ways beyond most computer algorithms. The findings have inspired hope for improvements in computer programs if we can work out the methodology used by the ants.

University of Sydney zoology student Chris Reid created a triangular maze based on the Tower of Hanoi game. The maze was then doubled to create a diamond shape, with an ant’s nest placed at one vertex and food at another.

The ants were able to find the quickest way through the maze, as expected. However, when Reid blocked the ants’ path and opened another route he saw something more surprising. While the ants initially pursued a path based on their previous route, it took only a few minutes before some discovered a quicker way, and less than an hour later the whole colony was using the most efficient way through the maze.

“Although inspired by nature, these computer algorithms are static and designed to solve a single, unchanging problem,” says Reid. “But nature is full of unpredictability and one solution does not fit all.”

The ants have evolved to be ready for this, while computer programs designed to mimic them have yet to catch up. “A small percentage of ants are always wandering off the path, and we think these have found the optimum path and others follow it.”

Intriguingly, when ants were allowed to explore the maze before the food was placed on the other side they were better able to adapt to changes in possible paths than when they entered the maze with the food already present.

Reid believes this is evidence that ants are more complex than previously thought. “It was believed the ants only produced one pheromone whether they had found food or not, and others just followed this path. Now we think there may be multiple pheromones produced depending on the circumstances.”

Argentine ants are so small that Reid says it is not possible to dissect them to examine whether they have multiple pheromone-producing glands for different sorts of travel. The quantities produced are also so small that he does not believe it is practical to collect enough for comparison by mass spectrometer.

Instead, he and his colleagues in the Behaviour and Genetics of Social Insects Lab are trying other ways to test ant behaviour, such as seeing if ants are capable of following a trail after a longer interval in some circumstances than others, which would suggest the use of different pheromones with different evaporation rates.

Reid says that Argentine ants were chosen because they were considered one of the simplest ants with poor eyesight and navigation skills. “They’re also so common we can dig them up from the backyard, and they don’t fight each other if we fuse colonies.”

Funding Shortage for Universal Flu Vaccine
A synthetic vaccine providing protection against all stages of the flu virus has achieved success in mice, but currently languishes without support to take it further.

In response to fears about the possibility of an avian flu pandemic, Dr Darren Miller of the University of Adelaide’s Gene Silencing and Expression Facility was funded to identify areas of RNA that are present in all strains of flu virus. The stretch he identified is believed to enable the virus to enter cells. Miller produced peptides to match this section and delivered them to the noses of mice.

Two weeks later the mice experienced 100% protection against the H3N2 strain of influenza, and 20% protection against the deadly H5N1 avian flu strain. While 20% protection may seem low, Miller notes that drugs stockpiled against a pandemic have even less success in similar tests. “With more time I think we could have got a higher rate of protection,” he says. H5N1 kills around 60% of people infected, so if it became easily transmissible even a low level of protection would save millions of lives.

Should a pandemic break out, Miller’s work may well find new sponsors but for the moment he has received little interest from the vaccine industry, which he attributes to the cost of changing over from the current system of annual shots.

Nevertheless, his synthetic vaccine offers several advantages. “A simple and totally synthetic universal vaccine – one that is not derived from an influenza virus and does not require annual reformulation – would have clear advantages in health clinics to control and prevent the spread of flu,” he says.

Nasal delivery also has advantages over the current intramuscular methods. It takes the vaccine directly to a common site of virus entry, and in a pandemic would remove the temptation to share needles.

Miller is seeking funding to conduct phase 1 clinical trials to establish the vaccine’s safety. While funding for larger trials might be hard to obtain, clearing phase 1 would mean that, should a pandemic break out, it would take less time to bring the vaccine to market.

Miller is also keen to do further research on mice to establish how long the immunity provided by the vaccine lasts.

Early Diagnosis for Motor Neuron Disease
An early test for motor neuron disease could extend the life of patients and offer researchers a greater opportunity to investigate new treatments.

“At the moment, we diagnose motor neuron disease using clinical signs, but it can take months to satisfy these criteria,” says Dr Steve Vucic of Neuroscience Research Australia.

Although tingling feelings in the hands and feet can be an early indication of motor neuron disease, they have many other causes. Even serious symptoms such as muscle wastage can be the result of what Vucic calls “mimic diseases”, some of which are treatable.

Although motor neuron disease is not curable, the drug riluzole offers some protection to undamaged neurons, slowing the course of the disease. It is anticipated that earlier prescription of riluzole would extend the life expectancy of patients significantly.

Electrical activity in the brain increases during the early stages of motor neuron disease. Vucic says the reason for this is not known, but suspects that cells that regulate brain electrical activity are early victims of the disease, with downstream effects where neurons responsible for muscle control are “excited to death”.

Vucic has employed threshold tracking transcranial magnetic stimulation (TTTMS) to measure the hyperexcitability of the cortex. He says this is not only faster than previous methods, but also more precise. “We’re in negotiations with a company that produces the hardware, and hope to have the test available in clinical and research settings in the next few years,” Vucic says.

Vucic believes that early diagnosis will also provide better opportunities to test novel drugs, including one he is about to start trials on with a co-author of the Clinical Neurophysiology paper that announced the success of TTTMS.

Nerves Regenerated
Roundworms have been observed repairing damage to their nervous system in a different manner to humans, raising the possibility that the method could be adapted for treatment.

Axons are long, electrically conducting structures that connect neurons to each other. In humans they do not recover from damage easily, and when regeneration does occur it seems to be by clearing away the damaged axons and growing replacements from scratch.

However, Dr Brent Neumann of the Queensland Brain Institute observed an alternative process in the roundworm Caenorhabditis elegans. Axonal fusion involves the creation of a bridge across the damaged section of an axon, and has previously been observed in earthworms, leeches and crayfish. However, roundworm axons are so small that Neumann needed ultraviolet lasers to conduct the dissections required for observations of axonal fusion taking place.

“We do not know yet if something similar occurs in humans, but if it is not in place it doesn’t mean we can’t make it happen,” Neumann says.

Neumann says that if human axons can be repaired in this way the implications are particularly positive for spinal injuries because the spinal cord is “a bundle of axons where the cell body is usually a long way away from where the damage occurs”.

Hope for Huntington’s Disease
Mice with Huntington’s disease have a reservoir of stem cells and precursor cells that are ready to turn into new neurons. Remarkably, the supply of such cells is much larger in mice that have been genetically manipulated to have Huntington’s disease than in other mice of the same age.

“Combined with previous findings which show that environmental enrichment and antidepressant treatment delayed both the onset and progression of Huntington’s disease in mice, these findings are encouraging,” says Dr Tara Walker of the Queensland Brain Institute (QBI).

A feature of Huntington’s disease is the decline of cognitive abilities like learning and memory. QBI Director Prof Perry Bartlett says this appears to be associated with a failure to produce new neurons in the hippocampus.

“We have come a fair way down the track in understanding how to activate the neuron precursors,” Bartlett says. For example, work led by Dr Anthony Hannan of the Howard Florey Institute has found that mental and physical exercise can delay the onset of Huntington’s disease (AS, March 2007, p. 14). Hannan is a co-author of the PLoS One paper revealing the extent of the neuron precursor reservoir.

While some neuron precursors are stimulated by exercise, other sorts respond to anti-depressant medication, while noradrenaline and synaptic activity work in other cases.

“Now we know that the capacity to generate neurons is retained in animals in even advanced stages of Huntington’s disease, further research will need to explore what stops this process from occurring,” Walker says.

Huntington’s disease affects 5–10 people per 100,000, but it is believed to provide a simplified model of more common age-related conditions such as Parkinson’s and Alzheimer’s diseases. Bartlett hopes that discoveries on the stimulation of neuron precursors for Huntington’s disease will be relevant for these more common conditions.

Rare Orchid Probed for Malaria Weakness
A study of Western Australia’s critically endangered underground orchid has revealed the genetic decay of its chloroplast, potentially offering insights for fighting malaria.

All orchids rely on a parasitic relationship with fungi during germination and seedling growth. While most photo­synthesise their own sugars later in life, a few species have lost their capacity to photosynthesise and rely on fungi throughout their lives.

The apparent advantage is that they can spend most of their lives underground, offering protection from grazing animals and dehydration. However, even these underground species flower above ground for pollination.

Rhizanthella gardneri has gone a step further. Even its flowers live under the soil, or at least a layer of leaf litter. The species relies on ants for pollination. Only 50 individuals are believed to remain in the wild in two populations east of Perth and Albany, respectively.

“We found that, compared with normal plants, 70% of the genes in the chloroplast have been lost,” says Dr Etienne Delannoy of the ARC Centre for Excellence in Plant Energy Biology. “With only 37 genes, this makes it the smallest of all known plant chloroplast genomes.”

At this stage the role of some of these genes remains unknown, while others are required for the chloroplast to maintain itself. It is hoped the findings will help find a weakness in the malaria parasite, a protist that has also lost its chloroplast genes.

R. gardneri is doubly parasitic, since the fungus from which it gets its food is dependent on broombush to do the initial photosynthesis.

A/Prof Mark Brundrett of the Wheatbelt Orchid Rescue Project paid tribute to the volunteers who assisted with the orchid’s location. “We needed all the help we could get since it often took hours of searching under shrubs on hands and knees to find just one underground orchid!”

He said: “The genome sequence is a very valuable resource as it makes it possible to estimate the genetic diversity of this Declared Rare Plant”.

Tiger Parrots Are an Ancient Lineage
Tiger parrots are an ancient branch of the parrot evolutionary tree rather than close relatives of either rosellas or certain Asian and African birds, as previously thought.

The finding comes from work conducted by Dr Leo Joseph using stretches of DNA that have previously been sequenced across many parrot species. Tiger parrots live in the New Guinea highlands, so tissue samples are not easily available, but Joseph was able to get the DNA required.

“This research has shown for the first time, for example, that tiger parrots represent a very early branch of the parrot evolutionary tree in Australia and New Guinea,” Joseph says.

Previously it had been thought that commonalities in the plumage of tiger parrots and some relatives of rosellas indicated that the birds were closely related. “We have shown that the New Guinea tiger parrots aren’t rosella-like parrots and that their resemblance in some aspects of their appearance to rosellas probably indicates some plumage characters that have been part of the evolution of parrots of Australia and New Guinea for a long time,” Joseph says.

“We affirmed that the Australian parrots are far from one cohesive group. They appear, instead, to be made up of about five different main branches of the parrot evolutionary tree.”

The study also helped place the night parrot and its recently distinguished close relatives the eastern and western ground parrot (AS, March 2011, p.7). The night parrot is a critically endangered species native to central Australian deserts and one of the few nocturnal parrots.

Joseph notes that many of the night parrot’s closer relatives are most active at dawn or dusk. “Sometimes the DNA relationships can make behaviour make more sense,” he says.

Temperatures Increasing in Fits and Starts
Climate change at Australia’s latitudes is not smooth, according to Prof Roger Jones of Victoria University’s Centre for Strategic Economic Studies.

Instead we have seen two sharp rises in temperature of 0.7°C in 1968 and 0.9°C in 1997–98. After each of these spikes, temperatures were relatively flat. The changes have been accompanied by changes in rainfall distribution.

The rest of the Southern Hemisphere between 24° and 44° experienced changes at the same time as Australia. Other latitudinal bands have also experienced sharp increases in temperature, but the timing has not always been synchronised.

Jones thinks he has found an explanation in the interactions between the atmosphere and oceans, but is keen to spend more time testing his theory before discussing it in detail.

It is not possible to predict the timing of the next spike, Jones says, but unrestrained greenhouse gas emissions make further rises inevitable. Moreover, he says “there’s some evidence that in the 21st century we’ll see both step changes and trending increases because we’re putting so much energy into the system”.

Plans for adaptation to climate change usually rely on gradual changes, both for predictability and to gather information as we go. However, Jones says we can no longer rely on this, which is why we have been unready for the 45% increase in the number of days of very high fire danger experienced in recent years. “We’re already seeing fire dangers we anticipated for 2050,” he says.

The first shift was associated with the drastic reduction in rainfall in the south-west of Western Australia, while the second represented the start of south-eastern Australia’s long drought.

Despite the past year, Jones thinks the change to a drier climate in the east is permanent.

Ancient Ice Sheet Melting Explained
A study of the drivers of Antarctic melting at the end of the last ice age has concluded that increased ocean warmth was more important than sea level rise. The research will contribute to modelling of future melting.

Ice shelves, the floating extension of ice sheets, are particularly susceptible to warming of the ocean beneath. However, another way of initiating ice retreat is for rising sea levels to float parts of the ice sheet that are grounded below sea level.

To distinguish the difference between the two effects, Dr Andrew Mackintosh of the Antarctic Research Centre at Victoria University, Wellington, combined detailed studies of the chronology of past events with computer modelling.

“The whole Earth started to warm around 20,000 years ago,” Mackintosh says. “It took the Antarctic ice sheets 6–8000 years to respond.” By this stage the glaciers that had covered Canada and northern Europe were in rapid retreat, raising ocean levels worldwide. Mackintosh says this has been considered the dominant factor in controlling the size of the ice sheets.

“We found that although the initial stage of retreat may have been forced by sea level rise, the majority of the ice loss resulted from ocean warming. Increasing ocean warmth seems to be the main driver of ice sheet retreat,” says Mackintosh.

The bad news is that some parts of the ocean around Antarctica are now warming rapidly. “Our findings suggest that a substantial contribution from melting ice sheets to global sea level rise in the near future is likely, especially if this oceanic warming observed in some areas spreads to the remainder of the Antarctic perimeter,” Mackintosh observes.

It is already known that ocean temperatures are more important than sea level changes for short-term effects on ice melting. “This research makes our understanding of long-term ice sheet changes more compatible with what we see today from satellite observations,” Mackintosh says.

The research was published in Nature, but Mackintosh adds: “As a caveat, though, we also show that the response of an ice sheet margin can vary significantly between locations, depending on the geometry of the land beneath the ice. This makes it challenging to predict the actual response of the ice sheet without detailed computer modelling.”

Email Trail Reveals How Employees Handle a Crisis
A study of internal emails prior to Enron’s collapse has offered insights into the behaviour of corporations in crisis. Prof Liaquot Hossain of the University of Sydney’s School of Civil Engineering believes the lessons learnt could be useful in detecting when other corporations are falling apart, offering a chance to head off disaster.

Enron was one of the biggest corporate collapses in world history, a case study in fraud that sounded an unheeded warning of the Global Financial Crisis. The corruption was so comprehensive that federal agencies released the entire dataset of internal Enron emails sent in the years leading up to the collapse.

This provided Hossain with an unprecedented opportunity to study behaviour in an organisation under siege. “We found it went from a highly centralised network to a largely decentralised and distributed system,” Hossain says. Although externally Enron appeared to be a highly successful company, staff were aware there were problems and started increasingly relying on networks they trusted rather than messages from the top.

“As Enron approached disintegration in late 2001, more people communicated with their colleagues and at a higher frequency,” says Hossain. “During this peak crisis period there was also a jump in the number of cliques forming within the company. This increase in communication is consistent with organisational theory that purports cohesiveness is greater under conditions of great anxiety.”

Privacy concerns mean that investigators do not normally have access to the contents of internal emails. However, Hossain believes this should not stop his research being put to use testing the health of other institutions.

“Normally it is possible to find out who has emailed who,” Hossain says. This may be enough to reveal an unexpected level of stress that may indicate deeper problems. “If you can make sense of how a structure is changing you can see if this is positive or negative.”

The observations Hossain has made for Enron fit with theories of organisational disintegration stretching back to the 1970s. He is now doing similar studies on how health organisations responded to the H1N1 influenza outbreak and how communications structures change for firefighters during major bushfires.

Sand Shrimp Invasion
An East Asian sand shrimp has been discovered for the first time in Melbourne’s Port Phillip Bay. A collection of specimens from near the mouth of the Yarra in 2008 revealed three shrimps that did not resemble local species. After some investigation these were identified as Crangon uritai.

“The invertebrate fauna of our bay have been very well studied over the years, so this really was a surprising find,” said Dr Joanne Taylor of Museum Victoria. “Before this chance discovery, this particular species of shrimp has never been reported outside of East Asia.”

The shrimps may have arrived in ballast water from container ships, and flourished in the warm pool created by the Newport power station.

New Chief Scientist Appointed
Prof Ian Chubb has been appointed as Australia’s new Chief Scientist following the controversial resignation of his predecessor, Prof Penny Sackett, midway through her term. A neuroscientist by training, Chubb has had a distinguished career in higher education and research, and recently retired after a decade as vice-chancellor of the Australian National University.

In a statement Prime Minister Julia Gillard said Chubb would effectively engage with industry, researchers and the wider community as part of important scientific debates.

Science Minister Senator Kim Carr said Chubb “ also understands that government needs frank and objective advice and communities need strong advocates.”

Chubb began his 3-year term on 23 May.

Paracetamol Risk in Pregnancy
Childhood use of paracetamol raises the risk of asthma (AS, March 2011, p.8), and now the same goes for paracetamol use during pregnancy.

A metastudy of six published studies found that paracetamol use during pregnancy increases the risk of asthma in children aged 2.5–7 by 21%.

“Rates of asthma have been increasing around the world for several decades,” notes Prof Richard Beasley of the Medical Research Institute of New Zealand. “Definitive further research into the effect of paracetamol use in pregnancy on the risk of asthma in offspring is urgently required so that appropriate recommendations for pregnant women can be made.”

In the meantime Beasley recommends that pregnant women should use care in regard to paracetamol use, saving it for times of significant pain or discomfort.

Give Grandma a Video Game
Women aged 56–84 benefited from using a Nintendo Wii Sports twice a week, La Trobe University’s School of Public Health has concluded.

“Older persons are at risk of isolation and have low levels of physical activity, both of which are associated with negative health outcomes. In this study, we explored the feasibility of incorporating the Nintendo Wii into a Planned Activity Group setting to assess the physical and psychosocial effects,” says Dr Dennis Wollersheim.

The machines simulate sport and are much more active than traditional video games. Wollersheim also described them as “more connected to the real world”.

Participants reported greater physical well-being, and felt that their mastery of the technology improved their communication with grandchildren.

GM Not a Tourism Deterrent
A survey of 515 first-time overseas visitors to New Zealand has found that the introduction of genetically modified rye grass would not put them off visiting in future. Only 1.9% agreed that the modified grass would discourage them, while another 5.9% thought this was possible.

“Whatever the issues regarding whether or not to introduce GM pasture, it seems safe to conclude that potential damage to our clean green image in the eyes of overseas visitors planning to come here should not be a factor,” says A/Prof John Knight of the University of Otago.

Don’t Waste Disasters
Management of solid waste is crucial to recovery from many natural disasters, a University of Canterbury study has concluded. PhD student Charlotte Brown found that disasters generate up to 15 years of a community’s solid waste in a few days, and recovery can be affected if mechanisms to handle this are not in place.

Speed is not always the answer, however. “It was alleged that debris removal after the 2008 earthquake in Sichuan, China, was so fast that many bodies were not recovered and were buried with the waste,” says A/Prof Mark Milke of Canterbury Civil Engineering and Natural Processes.

Hurricane Charley produced 76 million cubic metres of waste, and Milke says that only good public communication as to how the waste will be disposed of, and when and by which means, can avoid negative reactions.

Farmer Wants Advice
Agricultural workers are at high risk from heart attacks as a result of lack of knowledge about emergency medical services, according to Dr Tim Baker of Deakin University’s Centre for Rural Emergency Medicine.

Two-thirds of participants in a study of Victorian farmers believed it was safe to travel to hospital by car while possibly having a heart attack. When asked where their nearest emergency department was, 10% named a town without one. Rates of obesity were high in the sample, suggesting the information might well be critical.

“This group of farmers is at considerable risk of experiencing acute coronary events, yet many would make decisions about when and how to seek medical help for chest pain that are at odds with what they should be doing,” says Baker.

“For example, one-third would not seek help unless they thought the pain was caused by a heart attack. Furthermore, if chest pain was experienced during the night, around one in ten would wait until morning before seeking treatment.”

Baker notes that the more remotely located a person is, the more they are likely to benefit from acting quickly in response to chest pain.

The research was published in Emergency Medicine Australasia.

Biomarkers for Pancreatic Cancer Survival
Two proteins are strongly predictive of the prospects for patients with pancreatic cancer. People with both S100A2 and S100A4 in their cancers on average survive less than 1 year after surgery. Those with neither average almost 3 years.

Researchers from the Garvan Institute presented their findings based on 372 pancreatic cancer patients to the American Society of Clinical Oncology Gastrointestinal Cancers Symposium in San Francisco.

“We know that the operation benefits about 20% of people particularly well, and obviously we would like to be able to predict who they are likely to be,” says Prof Andrew Biankin. “At the moment, we make decisions about when to operate based on very indirect measures, such as CT scans, which aren’t really sensitive enough.”

The absence of the two proteins would encourage doctors and patients that it was worth taking the 3–5% risk of death from surgery complications.

Weight Lowers Dementia Risk
Researchers from the University of Western Australia have found a lower rate of dementia among slightly overweight elderly men.

Prof Osvaldo Almeida says the results were unexpected as there is good evidence that overweight or obese younger adults have an increased risk of cognitive decline as they get older.

Almeida says one possible explanation is survivorship bias, where people who are overweight later in life are unusually healthy compared with normal weight people in the same age group.

Another explanation is physiological changes as people age, causing the body to protect itself from degenerative effects. “The fat tissue behaves a little like an endocrine organ and releases a number of substances into the bloodstream that have different actions on the body,” Almeida said.

People who are slightly overweight in later life have lower mortality rates than those with normal weight.