The ants, Crematogaster nigriceps, prune the trees which provide their homes to stop them touching neighbouring trees. Any overlap provides a bridge for other ant species to march on to the tree and evict its residents.

The team, working at the Mpala Research Centre, Kenya, staged battles between rival ant colonies which revealed that C. nigriceps have good reason for their horticultural skills - they lose about three-quarters of their conflicts.

Professor Maureen Stanton, at the University of California, Davis, led the study and said: "Our field results suggest that this selfish pruning behaviour has evolved because it increases the life span of C. nigriceps colonies, even though it removes all the host tree's flowers and stops the tree from reproducing."

Vast areas of savannah in East Africa are dominated by Whistling Thorn trees (Acacia drepanolobium). These plants produce swollen thorns which ants bore into and use to house workers and raise young. These "tree-houses" are popular, with less than one percent of the trees over 50cm tall being unoccupied.

But four different species of ant can colonise the trees and the scientists noted that: "The canopy architecture varies significantly among trees occupied by the different ants."

Nipped in the bud

Only one type of ant is found on each tree and those swarming with C. nigriceps have far more branches than others but do not spread as widely.

The ants had been seen nipping buds and preventing flowering but to prove that the ants were controlling the tree's architecture, the scientists banished them from some trees. After two full growing seasons, the empty trees had 25% fewer branches than control trees on which the ants were still working.

This showed the ants were practising topiary but the reason for their time-consuming efforts was not known.

"Competition for host trees leads to violent conflicts between the four species of acacia ant. We have observed a number of take-over raids in which colonies stream onto trees and attempt to dislodge the workers and their brood from inside the swollen thorns," reported the scientists.

So, the team staged dozens of inter-species ant battles to see if C. nigriceps was particularly prone to invasion.

War games

They tied together branches of adjacent trees which had different colonies living on them and waited six months. C. nigriceps was forced out more often than any of the other ants. Their unique pruning skills therefore seem to be used to keep their tree away from others and prevent any invasions.

"This avoidance strategy allows these ant colonies to persist longer, almost like fugitives in hostile neighbourhoods," said Professor Stanton.

The scientists believe the ants detect their nasty neighbours through pheromones which drift across on the breeze.