‘Pretty obvious,’ said Caleb. ‘Shall we get on with it?’

As Innocent had predicted, the walking was fine to begin with, but we were all subdued. Xander did his best to lighten the mood by teeing up questions for Amelia to answer (a silverback gorilla weighs about 200 kilograms, stands ***

1.8 metres tall, and has an arm-span of two metres; they construct a new nest of flattened branches and leaves each night as they move around their territory in search of plants to eat and are one of the only primates to sleep on the ground; there are fewer than one thousand mountain gorillas living in the wild, though that’s an improvement on twenty years ago, and so on) but impressive as her mental factsheet was, it couldn’t lift the heavy presence of Caleb. He wasn’t outwardly angry, in fact he barely spoke, but everything about him put the rest of us, except Amelia possibly, on edge.

The fact of him, buzz-cut hair, stupid lime-green boots, brand-new machete and all, up front with his head down, pulsing with discontent, was maddening. Look at where we were and what we were doing: hiking through one of the most stunning landscapes on earth in search of one of the rarest and most impressive animals alive. How could he sour that experience? I forced myself to ignore him and take in the detail of the day.

It was cooler, but no less humid, as we climbed, and the clouds boiled up above us, unleashing lush curtains of rain that turned the path to mud. As we made our way higher into the mountains the glossy greenery of ferns and bamboo and gallium vines pressed in. Among them was something that managed to sting me through my trousers. Not knowing what it was made my heart race. Amelia put me at ease by saying, ‘Ow!’ almost immediately after I’d been stung. When I told her something had also bitten me, she laughed and said, ‘The chances of two spider bites or snake strikes in ten seconds is infinitesimal. So it was a plant. I’ve not read about any deadly stinging vegetation in the jungle. Therefore we’ve probably just brushed against a pumped-up sort of nettle. We can relax.’

‘Thanks,’ I said. It came out more sarcastic than I meant. The truth was that the sting hurt, which made it hard for me not to think of what worse dangers might be lurking just out of sight.

22.