It's now mid-morning and the sun is beating down from directly overhead. Coming from a northerly latitude, it always amazes me just how quickly the sun rises here in the tropics, it goes straight up vertically from the horizon in double quick time. The 'golden hour' of sunrise here is compressed into a 'golden half hour'. And then the sun seems to slow right down and be overhead for hour after sweltering hour. At 6:30 am, when we set off from the boat dock at Porto Jofre, it was beautifully cool. The sun was just rising, and we were all keen to start another day searching for jaguars on the massive Cuiaba River here in Brazil's Pantanal. But now we are very hot and getting just a little desperate for a sighting. As the temperature soars, our energy levels slump.

We've now been out in the boat for over four hours, we are wilting in the heat, and we have yet to see a jaguar. Fred, our guide, tells us that the temperature in the sun is over 50oC (122oF) . 'What's the temperature is in the shade?' I ask, thinking that is a more normal way to measure the temperature. Fred retorts: 'It's 42oC (108oF) in the shade, but I gave you the temperature in the sun, because we are in the sun!' He's right. We are in the full blast of the tropical sun, in an open boat, cruising slowly along this airless river. Our boat driver is on the radio, speaking in Portuguese. I can't understand a word, but the urgency in his voice sounds promising. 'Hold on to your hats,' Fred shouts.

We have become accustomed to this message over the last few days. We hold on to everything as the driver guns the engine and our powerboat accelerates away. With its huge 200bhp engine pushing us through the water, the speed builds and builds. We now know the 'hold onto your hats' drill: it means another boat has radioed in a jaguar sighting, and we are on our way. Our driver is a true expert, weaving his way along the twisting curves of the river at top speeds of more than 65kph (40mph). On the water it seems alarmingly fast. The boat tilts as we swoop around the river's tighter bends, and then we accelerate again on to the straighter sections. This is a real-life roller coaster! Great fun, and with the rushing air bringing welcome respite from the heat, we enjoy a few minutes of exhilaration and wonderful coolness.

We know we have arrived when we see a small armada of boats ahead of us, milling around close to the riverbank. Slowing right down, we approach quietly.