WHY WERE we surprised that a gorilla used a broken branch first to test the depth of a jungle pool and then as a walking stick to help her balance as she waded through the water? Chimpanzees, the gorilla’s close relative, regularly use the power of reason to improve their quality of life.

They break open nuts with stone “hammers”, refresh themselves with “sponges” made from leaves and construct “tools” from twigs with which they extract edible termites from hollow trees. Some of them communicate, in sign language, with their close relations — Homo sapiens.

Reluctance to admit that other primates can think is, at least in part, the result of a primitive desire to be associated with angels rather than apes. We want to be an exclusive species — superior to the “animal kingdom” over which (or whom) the Bible says man was given “dominion”.

But there is also a more immediate dilemma posed by the acceptance of lower primates’ power to reason. If their thought processes are like ours, how do we justify sinking electrodes in their brains, injecting them with deadly viruses and cutting them open to discover the results of “experiments”.

The old-fashioned excuse was that, being incapable of consecutive thought, they could neither anticipate their suffering nor imagine the consequence of the treatment. Tell that to Leah, the gorilla who imagined the consequences of getting out of her depth and rightly anticipated that she could keep her head above water with the aid of a walking stick.

Once we justify experiments on cognisant mammals, it becomes difficult — if not impossible — to draw a line between any of the species that walk upright and possess the power of reason. If chimpanzees, why not humans? It has been done before — not only in Nazi Germany but also by the armed forces of western democracies testing for radiation hazards.

So, where will it end? And, for that matter, where should it begin? Once we accept that animals that reason should not endure laboratory experiments, the argument against vivisection may become all-embracing. Aeschylus was killed by an eagle which, mistaking his bald head for an egg, thought that it had cracked open its breakfast with a stone. Low marks for recognition. But the bird-brain had worked out a way of getting what it wanted. Cogito, ergo no vivisection.