56 Cooperative Attack

Key Idea: Working together in attack can help increase the chance of success especially if roles are allocated between the attacking members. Group attack is often used for hunting for food, but may be used by some species for raiding nests or territories in order to gain access to new resources (e.g. space or workers). Group attacks may be highly organised, with individuals taking specific roles.

Lionesses hunt as a coordinated group. Several lionesses hide downwind of the prey, while others circle upwind and stampede the prey towards the lionesses in wait. Group cooperation reduces the risk of injury and increases the chance of a kill. Only 15% of hunts by a solitary lioness are successful. Those hunting in a group are successful 40% of the time.

Adrian A. Smith Kalyan Varma

Many ant species, e.g. slavemaker ants (above left), raid other ant nests (called slaveraiding), killing workers and capturing grubs. The grubs are carried back to the home nest where they grow and tend the slavemaker ants' own young. Sometimes, however, the slaves rebel and can destroy the slavemaker nest. In his book Journey of Ants Edward O. Wilson, the world's leading ant expert, noted (not wholly tongue-in-cheek) that with ants "their foreign policy can be summed up as follows: restless aggression, territorial conquest, and genocidal annihilation of neighbouring colonies wherever possible. If ants had nuclear weapons, they would probably end the world in a week."

The Gombe Chimpanzee War

Group attacks between members of the same species and even the same social groups do occur. They usually involve disputes over resources or territory, but may be due simply to rifts in social groups. One of the most well recorded and startling examples of group fighting is the Gombe Chimpanzee War. Observed by Jane Goodall, the violence began in 1974, after a split in a group of chimpanzees in the Gombe Stream National Park, in Tanzania. The group divided into two, the Kasakela in the northern part of the former territory and the Kahama in the south. Over the course of four years, the Kasakela systematically destroyed the Kahama, killing all six males and one female and kidnapping three more females. The Kasakela then took over the Kahama territory. However, ironically, the territorial gains made by the Kasakela were quickly lost as their new territory bordered a larger more powerful group of chimpanzees, the Kalande. After a few violent skirmishes along this border, the Kasakela were pushed back into their former territory.

1. (a) Suggest two reasons for cooperative attacks:

(b) Suggest why cooperative attacks are more likely to be successful than individual attacks:

2. Chimpanzees often hunt cooperatively. Use the data below to plot the relationship between hunting success and group size.

No. hunters 1 2 3 4 5 6

Hunt success (%) 13 29 49 72 75 42