‘By bugs, you mean pathogens,’ said Amelia. ‘That’s the collective term for viruses and bacteria.’

‘Smart-arse.’ I smiled.

She gave me a questioning look.

‘Just put the thing on!’ I said, snapping the elastic round the back of my head.

Masked, we looked like a gang of surgeons about to perform an operation; if I’d been a chimpanzee I’d have run a mile. But Bingo’s troop didn’t seem to mind. If anything, by the time we’d put on the masks and stepped forward into the trees they’d been cavorting through, they’d calmed right down. Some were still milling about on the ground, but most were back up among the branches, ignoring us.

I’ve seen chimpanzees before, at Whipsnade Zoo, and even though I was only about seven I’d found the fact that they were stuck in an enclosure a bit depressing. This was different. They might have been used to humans, but it felt as if we were visiting them on their terms. They seemed content enough to hang – literally – in the trees around us for the time being, but if they had decided to move off, we had no way to stop them and wouldn’t have been able to keep up with them. That, and the fact that we’d searched for so long to find them, made me really focus on what I was seeing.

The youngster nearest me was picking at the shoulder of the one next to him, parting the long dark fur carefully in search of something or other. He rolled sideways to look more closely at what he was doing, and his short back legs stuck out towards me. I zoomed in on him. How thickly padded his feet were, how pronounced and distinctive the paleness of his face. And those ears. They really did stick out like saucers, comical brackets either side of a serious brow. His fingers mesmerised me most. They were so unhurried and precise. The ape he was grooming had his eyes half closed. The pair of them, sitting right there, not ten metres from us, looked blissfully content.

Having cottoned on to the idea that Amelia knew a fact or two about the natural world, Caleb now started telling her what he knew about chimpanzees. It wasn’t much. ‘You know they’re five to eight times as strong as a man, pound for pound,’ was the best he could come up with. As it happens, Amelia, Xander and I had already speculated about whether that clichéd statistic was true, back at the hotel, and three minutes on Google had dispelled it as a myth. They’re strong, but not that strong. And anyway, looking at these chimpanzees, bumbling about among the vines and sitting placidly together, emphasised their gentleness, not their strength, for me.

I waited for Amelia to correct Caleb, but amazingly she just said, ‘Hmm.’

I fought the urge to say something myself, and won by concentrating instead on what was in front of me again. A female chimpanzee on the ground off to my left had picked up a piece of wood. After inspecting it she brought it down smartly on what I at first thought was a stone balanced on the tree root in front of her. In fact it was a nut. Once, twice, a third time she hit the nut, her face set in concentration, until the shell broke apart, at which point I swear her expression changed to one of satisfaction. I took a photograph of it. Delicately she swept the crushed shell off her anvil-root, retrieved the nut itself and ate it. And then she picked up another nut and repeated the process.