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Monday, Dec. 4, 2000

NATURAL SELECTIONS

Ants find inheritance tax high

Easier to build a new colony than clean up the old


The maximum rate of inheritance tax in Japan is 70 percent, more than many people can afford to pay: If they inherit, they have to sell off land and property to pay the tax.

Researchers at Toyama University have discovered a similarly high cost to inheritance in an ant species. Kazuki Tsuji and co-workers at Toyama University studied an ant common all over the main Japanese islands, Lasius japonicus. In this species, a single queen presides over a colony of around 10,000 worker ants.

The nest is a complex structure that many thousands of ants have cooperated to build, and as such, like a family house in Tokyo, an established ant nest is a valuable structure. All the workers in the colony are female -- all sisters -- but they are sterile. Only the queen lays eggs.

When the colony gets to a certain size, fully sexual individuals are produced. These kings and queens have not only functioning ovaries and testes, but also wings. They fly from their natal nest in summer, mate and shed their wings. Males die, and females found a new colony. The new queen lays eggs and rears her daughter workers using her own energy reserves.

Meanwhile, back in the old colony, the original queen ages, and the number of eggs she lays to replenish the colony's workers decreases. The queen dies, and so does the colony.

Why doesn't a daughter queen inherit the nest, instead of flying to a new site, in soil or in a rotten tree stump, and alone founding a brand-new colony? In other species, where the colonies have multiple queens, the nest is used by succeeding queens. In termites too, nest inheritance is common. (Termites are social insects in the insect order Isoptera. Despite their Japanese name shiro-ari, white ants, they are unrelated to ants, wasps and bees, which are all in the order Hymenoptera.)

If offspring inherit the nest, the colonies have the potential to become immortal. This led Tsuji and colleagues to suspect that the nests of their species were not as valuable as they looked. But why not?

Worker ants spend their whole lives marching through dirt and detritus, and tramping it into the nest. They are omnivorous, hunting small insects and scavenging dead ones. They also feed on the honeydew produced by aphids. Could it be that during the lifespan of one queen (sometimes many years), the nest simply becomes too old and dirty to be worth inheriting?

Birds often build new nests for each brood of eggs that they lay, because the old nests are full of ticks and mites and other parasites. A similar infestation with parasites might be the reason why ants don't inherit their nests.

To test this idea, the researchers collected young founding queens of L. japonicus just after their dispersal flight. They were kept in tubes with water and one of three different soils: soil taken from a nest of L. japonicus; soil from areas where ants were not present and soil that had been sterilized at 150 degrees C. The researchers then measured the survival rate of the young queens. Queen mortality was significantly higher when she was kept in soil taken from old ants' nests than in the other two groups. Examination of the dead queens found that they had been infected by fungal pathogens.

In a similar experiment, worker ant cocoons were kept with the queen in each of the tubes. In natural conditions worker ants clean the queen. In the experiment their presence did seem to help the queens in the tubes with soil from ants' nests, but the effect was small. Scientists studying social insects have been unsure of the reasons why in some species a colony has multiple queens, while other species have single-queen colonies.

"The mode of colony foundation is thought to be associated with differences in life-history," said Tsuji. "If colonies are founded by single queens, then there is dispersal by winged females. But if colonies are inherited, there is likely to be a society with multiple queens."

Most scientists have hitherto believed that it is the effectiveness of different methods of dispersal that determines whether a society has one or many queens. What Tsuji and his colleagues have shown is that the risk of parasite infection is another reason why some species might disperse and others might stay and inherit the nest. In their study queens living in soil from old nests died from fungal infections from multiple pathogen species.

"It means that in this ant species inheriting the nest has high costs," said Tsuji, "just as there are high costs to some humans in Japan paying inheritance tax."



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