In 2007, UNESCO inscribed the Rainforests of the Atsinanana as a World Heritage Site - thirteen separate forest blocks scattered across eight national parks down Madagascar's eastern flank, recognised as a global treasury of evolution in isolation. Zahamena is one of them. The designation is more than an honour. These forests are the surviving fragments of a vast woodland that once cloaked the entire eastern third of the island, and the species packed into them exist nowhere else on the planet. Ninety-nine percent of Zahamena remains under forest cover, from humid lowland canopy where trees rise fifteen metres with emergents punching to twenty-five, up through the cooler, denser montane growth that crowns the ridges. The altitude changes the forest as you climb, and the forest changes the life within it.

Zahamena's numbers read like an inventory of Madagascar's endemic riches: 112 bird species, of which 67 are found nowhere but this island; 48 mammals, including 13 kinds of lemur; 62 amphibians and 46 reptiles. The indri sings here too - the locals call it babakoto - its black-and-white form moving through the canopy. But the park's true rarities are harder to glimpse. The Madagascar serpent eagle, once feared extinct, patrols these slopes. So does the Madagascar red owl, vorondolomena in the local tongue, a russet-feathered ghost of the night. On the forest floor lurks the masobe gecko, a small, big-eyed nocturnal lizard, and in the leaf litter the red-tailed newtonia flits between branches. To find any of them, you have to go where almost no one does.

Two peoples live around Zahamena, and both have shaped the land they border. The Betsimisaraka - their name means roughly the many inseparable - are Madagascar's second-largest ethnic group, long settled along the humid eastern coast and its forested hinterland. The Sihanaka, whose territory bends toward the great lake to the west, are rice farmers and fishers whose name carries the sense of people of the marshes. For both, the forest has always been both larder and frontier, a source of timber, game, and new fields, and the pressure of that need is exactly what protected areas like Zahamena exist to balance. The park is not wilderness emptied of humans; it is a line drawn between a forest that must endure and the communities that live at its margins.

Zahamena does not give itself up easily. It sits in genuinely rugged terrain - the broken rock faces and undulating ridges of Madagascar's mountainous interior - roughly 40 kilometres northeast of the town of Ambatondrazaka and about 25 kilometres east of Lake Alaotra. Reaching the interior means trekking, and the forest meets you with rain, leeches, and slopes that rise steeply from one ecological zone into the next. Tree ferns unfurl in the understorey; screw pines and pandanus cluster in the wetter hollows; rosewood and ebony stand in the canopy above. The difficulty is the point. The very inaccessibility that frustrates visitors is what has kept Zahamena intact while easier forests fell - a stronghold held by geography as much as by law.