Possibly the best lay-by in the world juts out of a western escarpment of the Great Rift Valley in Tanzania. We stopped there on the road from Lake Manyara to the Ngorongoro crater. The opaque waters of Lake Manyara stretched out like spilt milk across the plains below, and we could clearly see thumb-sized pairs of giraffes courting on its shores, a wash of pink flamingos paddling in its shallows and herds of toy elephants munching at its greenery – it was as if a toddler's bedroom floor had stirred into life.
This God's eye perspective was all the more heavenly for the fact that we had recently met the animals up close. Lake Manyara was our first port of call on Tanzania's Northern Circuit of game reserves, a quartet of wildlife sanctuaries that also includes the Ngorongoro Conservation Area, the Serengeti National Park, and Tarangire National Park. Our six-day tour with the Arusha-based safari operator Kearsley was to take us to the first three of these, each geologically distinct from the next, and each with its own remarkable ecosystem.
Our guide and driver was a twinkly-eyed, softly spoken man named Mohamed, who preferred to go by his last name, Kachui. With a sharp eye for hidden beasts, and as sharp a knack for identifying them, he clearly had something of the cat about him. "Kachui means 'leopard'," he told us with a grin.
The wildlife appeared as soon as Kachui pulled our 4x4 onto the dirt road into the forests that fringe Lake Manyara. A baboon stared at us from the roadside. His family played nearby – a mother with a scrawny pink baby clinging to her back and another tumbling around her feet.
Around another bend, an elephant was sandpapering an itch on his leathery hide against a tree. Two giraffes peered at us over the backs of the elephants: a pair of aristocratic heads looking down their noses at the newcomers. There was a rustle in a bush. A tiny, Bambi-like creature with big blinking eyes and little devil horns emerged. "A dik-dik – named after the sound it makes when it is alarmed," said Kachui. The dik-dik wasn't alarmed by us, and neither were any of the animals that we encountered over the week: the sight of a green 4x4 had become as natural to them as the search for the next meal, it seemed.
Kachui pointed up into the branches at a pair of amorous toucans, then down to a lower branch where a kingfisher shone blue and orange in the sunlight. "Manyara is one of the best places in Africa for birdwatching," he confided. To underline the point, a line of speckled guinea fowl nodded their way across a clearing. "Learn about birds, and you'll never be bored," said Kachui.
There was no chance of that. Every turn brought a new vista and a new species: a pool of hippos; a monitor lizard resting on a branch; a family of hyrax in a cracked rock below the muscular contortions of a baobab tree. Only Manyara's much-hyped tree-climbing lions, which allegedly lounge about in acacia trees, hid themselves from view.
By the time we reached the Ngorongoro crater a day later, we were being spurred on by the promise of lions, cheetahs, buffalos, zebras, and wildebeests, although the park's few black rhinos had been moved to a private area out of view for their own protection. The black rhino is one of Africa's most endangered species, poached to near extinction for its horn. Fewer than 50 survive in Tanzania. A trip to the visitor's centre at the park gate supplied some other pertinent data. Some 2.5 million years ago, an enormous volcano on the western edge of the Great Rift Valley erupted in a massive explosion, leaving a crater 600m deep and 19km wide. Today, Ngorongoro is the world's largest unbroken and unflooded caldera and is home to the densest permanent concentration of wildlife on earth.
My imagination was fuelled: this, surely, was the Lost World. A land where ancient animals ruled, hemmed in by a tectonic ruin, into which humans ventured at their peril. Kachui set me straight on both counts. The animals were not, as I imagined, trapped. They were there by choice because everything they needed was right there. And humans have been venturing fearlessly into the crater for centuries. A group of Masai, resplendent in their red robes, were herding cattle across a stream when we reached the crater floor, proving Kachui's point.
After chilling with some zebras and wildebeest, hanging out at the hippo pool and skulking with a hyena, we saw our first lions – a group of females lying by a muddy pool, tired and bloodied after a kill. We sat watching them through binoculars for a while, excited by their every drowsy twitch, until Kachui heard word over the CB radio of two cheetahs on the prowl in a distant corner of the crater. We dashed to where three other 4x4s were parked. It took us a good minute to make out their slinky frames stalking a herd of impala about half a mile away. Half an intense hour passed before the cheetahs were seen by the impala, and gave up the hunt.
"The element of surprise is everything," Kachui explained. Like the cheetahs, we also moved along once the game was up; a curious trigonometry of tourists watching hunters watching hunted.
As daylight faded, we drove up to the caldera's rim, 600m above the animal kingdom, and stopped at another of the world's best lay-bys. This time, the animals were just tiny specks of movement on the crater floor. The final stop on our trio of game reserves was the largest of them all – the Serengeti, to view one of nature's great wonders, the migration of around 1.7 million wildebeest and about a million other animals. It took a day's drive across the vast park to reach its north-eastern frontier with the Masai Mara, the point where, in July, the migration seeks the rains and fresh grass over the border in Kenya. We rose early. Our first spot was the rare treat of a lioness cuddling its cub on top of a boulder. Then, from a hilltop, we saw the migration in its full splendour. Wildebeests wound like a trail of ants back across the plains.
Later, as our twin-prop plane took off from Klein's airstrip – nothing more than a cricket wicket in the wilderness – plumes of smoke rose from the south. Earlier, I had asked Kachui why certain chunks of grassland we had passed appeared charred. "Controlled burning," he'd replied. After the migration leaves for the north, the rangers torch selected areas to encourage fresh growth for the animals' return. I was amazed to discover that man was playing such a hand in one of nature's mysteries. Kachui explained that burning occurs naturally, when lightning strikes the dry grass, but that the weather patterns in recent years could not be relied upon for the task. Was climate change to blame? He couldn't be sure. But as Kachui and his 4x4, groups of Masai herdsmen, and the great beasts of the savannah shrunk to dots below us, it hit home that the fate of these ecosystems now lay in human hands.
Northern Tanzania is served by flights to Kilimanjaro airport via Nairobi daily with Kenya Airways (020-8283 1818; www.kenya-airways.com). An alternative route is direct to Dar es Salaam with British Airways (0844 493 0787; www.ba.com) and then connect with a light aircraft flight to Arusha or Manyara National Park.
The writer booked a customised package through the specialists Imagine Africa (020-7622 5114; www.imagineafrica.co.uk) and travelled with the Arusha-based safari operator Kearsley (00 255 27 250 8043; www.kearsleys.com).
A Northern Circuit safari usually takes between five and eight days and visits a selection of, or all of, Lake Manyara, the Ngorongoro Crater, the Serengeti and Tarangire. Prices vary depending on where you stay and how much you choose to see. A five-day tour of the Northern Circuit with Imagine Africa starts from $2,830 (£1,617) per person, including accommodation, guide, park fees and food but excluding international flights.
Each park has numerous nearby lodges to choose from. The writer stayed at E Unoto Retreat for Lake Manyara (www.maasaivillage.com) in a Masai-style bungalow (doubles from $175/£100 per person per night, full board); The Plantation Lodge, a stylish country retreat near the Ngorongoro Crater (from $200/£114 per person per night, full board; www.plantation-lodge.com); and the Suyan Camp (from $228/£130 per person per night, full board; 00 255 27 250 4118; www.asilialodges.com) which moves around the Loliondo area, on the eastern border of the Serengeti, following the migration.