Death & glory: A story of ants, ageing and altruism

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The Independent Online

King Solomon is said to have told sluggards to look to the hard-working ant and be wise. Aesop, too, extolled the virtues of the humble ant in his fable explaining why the insect's constant toiling through the summer months would make for an easier winter compared with the fortunes of the lazy, singing grasshopper.

Now there is another reason to admire the tiny, colonial denizens of the insect world. Ants not only work hard and are prepared to lay down their lives for their fellow ants, they also take bigger risks for the good of the colony as they get older – and they can even assess how much time they have left in life.

Dawid Moron and his colleagues at Jagiellonian University in Poland have carried out a set of laboratory experiments showing that ants have the ability to gauge the end of their lifespan and to use their assessment of imminent mortality to take bigger risks with their ageing lives.

It is well established that worker ants tend to take greater risks as they get older. Scientists have shown that this behavioural trait benefits the colony because certain risky activities, such as foraging far from the nest, are best done by ants coming to the end of their useful lives – it doesn't pay to put young workers in high-risk jobs.

As a result, younger ants tend to do housekeeping chores around the nest, which is inherently safer than travelling further afield. One remaining question, however, was whether ants had some internal mechanism that told them how old they were and how much time they had left before dying.

Dr Moron believed that it might be possible to manipulate an ant's lifespan artificially, and to observe changes to its risk-taking behaviour as a result. His study, published in the latest issue of the journal Animal Behaviour, did just this by increasing the concentration of carbon dioxide in a chamber housing an ant's nest.

High concentrations of carbon dioxide increase the acidity of the blood and curtail an ant's lifespan. The technique of shortening life expectancy is also gradual and predictable – and one which the ant should be able to exploit to estimate how much time it has left before dying.

As the scientists predicted, the worker ants in the colony began to forage further afield earlier than they would have done if they had been brought up in a low carbon dioxide atmosphere. "This implies that ant workers adjust their threshold for engaging in risk foraging according to their life expectancy," Dr Moron said.

The findings are further evidence of the apparent altruism of the ant. These workers are not only prepared to sacrifice their lives to serve and protect their queen, they also have the ability to make careful calculations of just how much risk they should take based on their current life expectancy.

Dr Moron studied a species of ant called Myrmica scabrinodis, one of more than 8,800 known species of ants, and there is every reason to suspect that the same ability to assess life expectancy occurs in other kinds of ants that live in social colonies with caste-based hierarchies.

In evolutionary terms, worker ants are the supreme altruists because they dedicate their lives to bringing up the offspring of another individual – their queen – while remaining sterile themselves. This is also true of many other social insects, which perplexed Charles Darwin and generations of biologists who came after him because it appeared to contradict the central idea of evolution – leaving behind as many of your own offspring as possible.

It was also at the heart of philosophical discussions of human altruism. As Darwin said in his 1871 book The Descent of Man: "He who was ready to sacrifice his life, as many a savage has been, rather than betray his comrades, would often leave no offspring to inherit his noble nature."

Many different kinds of animals, other than ants, are known to be altruistic and the issue of how this could evolve in a world of selfish genes remained unresolved until about 40 years ago with the work of the late William Hamilton of Oxford University.

It was Hamilton who showed that the altruism seen in ants and other social insects could be explained by something called kin selection. In other words, a female worker ant is prepared to lay down her life for her sister queen because of the unusual genetics of social insects which make sisters more related to one another than they are to their own offspring.

In essence, it pays the female worker ant to forego reproduction, leaving that activity for her queen and sister, and to dedicate her celibate life to rearing and protecting her sister's offspring. Indeed, Hamilton and Edward O Wilson of Harvard University, demonstrated that it was the peculiarly close relatedness of female ants – they share 75 per cent of their genes rather than the 50 per cent of ordinary sisters – that allowed social colonies with sterile workers to evolve in the first place.

It would also explain the extraordinary diversity of ant colonies. The army ants of South America for instance can have 700,000 members in one, nomadic colony. The leaf-cutter ants farm fungi by growing them on miniature fields of pulped leaves, and the slave-maker ants raid the nests of other ants to enslave their young.

But would the altruism of the simple ant explain human altruism? One of the greatest acts of human altruism is the near-total devotion of parents to their children, which can be at least partly explained by the kin-selection idea. Most people show the greatest kindness to their own children, followed by the children of their closest relatives.

But of course it cannot explain the more conscious acts of true altruism that people often show to complete strangers. It may be because this form of altruism is part of an unwritten rule of reciprocal altruism – I'll scratch your back now if at some future point I can expect you to scratch mine.

Human altruism may be far more complex, but the humble ant has at least given us a hint of how our own unselfish behaviour first evolved.

The most prolific species on the planet

* The combined weight of the earth's ants outweighs that of humans, and they have the highest population of any animal on earth.

* They are prolific: the only places without an indigenous ant population are Antarctica, Greenland, Iceland and a handful of remote tropical islands.

* Ants know how to look after their monarchs. A queen ant can live for up to 30 years, while male workers generally survive for only a few weeks.

* African driver ants are so vicious they have been known to kill humans. Although cases are rare, babies or already unconscious adults are occasionally found killed by the creatures, which attack in swarms of up to 100,000.

* There are more than 12,000 different species of ant, ranging in size from 2mm to 25mm (about an inch).

* They are surprisingly strong and fast. Ants can lift 20 times their own body weight and run so fast that if we could move as quickly for their size as ants, then we would be as fast as racehorses.

* Ants cannot actually chew or swallow their food. Instead they squeeze out the juice.

* They have two stomachs: one for their own food, and one for other ants in the colony.

* Ants are remarkably tidy. Some worker ants take on the job of taking the nest's waste to an outside rubbish dump.