This installment investigates the ways in which those animals that live in social groups interact with each other. The solitary eagle is contrasted with whooper swans landing in Scotland after a 1,600-kilometre journey from Iceland. Once arrived, they must battle for territory with those already there, and pairs or families are usually victorious. Attenborough uses a group of farmyard chickens to demonstrate a pecking order. A pride of lions is shown co-operating to subdue a buffalo. Afterwards, each animal peacefully awaits its turn at the carcass. Baboons live in troops of up to 150, and their complex dominance hierarchy is examined in detail. Vampire bats display mutual assistance by regurgitating blood for any neighbour that has missed out on a night's feeding. Dwarf mongooses live in family groups of around a dozen. While some look for food or sleep, others are always posted on the lookout for predators and quickly raise the alarm if necessary. Meanwhile, some of the most extreme co-operation is demonstrated by the underground naked mole rat, whose 80-strong clusters are divided into workers (who tunnel perpetually), soldiers (who only act when danger threatens), and a single queen for breeding. Leafcutter ants are shown transporting their food deep below ground: it has to be planted in a special fungus to remove its indigestible cellulose, and each stage of the operation is carried out by a different caste of individuals.