The coastal plain here runs level toward the Atlantic, interrupted by isolated plateaus that rise to roughly 200 or 300 metres and then stop, like tables left standing after the room was cleared. Elevation across the reserve spans sea level to about 400 metres. It rains more than 1,800 millimetres a year, temperatures swing between 19 and 28 degrees Celsius with an annual average of 24, and the air stays heavy enough that the forest never fully dries. Three rivers organise the terrain. The Serra runs through the reserve from west to southeast; the Maruim traces the northwestern boundary; and everything drains into the basin of the Una River, itself fed by the Aliança and the São Pedro. Where the original forest cover was stripped from the plateaus, erosion has accelerated - the rain that sustains the canopy also carves the ground it was holding down.

The IUCN reserves its most restrictive designation, category Ia, for strict nature reserves: places set aside to preserve the biota and their natural attributes without direct human interference. Una carries that label. There is no visitor infrastructure at its core, no managed recreation, no negotiated middle ground between conservation and use. The reasoning is straightforward. When a species exists in a single fragmented biome and the fragments keep shrinking, the most valuable thing a reserve can offer is to be left alone. In 2002 Una was folded into the Central Atlantic Forest Ecological Corridor, an attempt to knit isolated protected areas into something an animal could actually cross - because a tamarin population sealed inside 18,715 hectares is safe from chainsaws but not from its own narrowing gene pool.

Read the reserve's protected bird list and it starts to sound like an inventory of near-misses. The red-billed curassow, a heavy ground-dwelling bird with a scarlet bill. The banded cotinga, whose males are an improbable turquoise. Stresemann's bristlefront, among the most endangered birds in the Americas. The ochre-marked parakeet, the black-headed berryeater, the Atlantic black-breasted woodpecker, the band-tailed antwren, the striated softtail - eight species whose survival depends largely on forests like this one. The mammals include the maned sloth, the giant anteater, the cougar, and the golden-bellied capuchin, another primate endemic to this corner of Bahia. Even the invertebrates are notable: the Hercules beetle, one of the largest insects alive, and the butterfly Napeogenes rhezia. In the streams, a small characiform fish called Nematocharax venustus.

Protection does not stop at a boundary line, and neither does damage. Deforestation around the reserve, the clearing of riparian vegetation along stream banks, and cattle trampling those banks into mud have combined to change how water behaves here. Floods come harder and faster because there is less vegetation to slow them. The water carries more sediment, which smothers the streambeds where fish spawn and reshapes the channels the forest grew around. The effects on wildlife have been negative and measurable. It is the quiet form of habitat loss - not a bulldozer at the gate, but a slow degradation of the systems the reserve was drawn to protect, arriving from outside the line.

It is tempting to describe the golden-headed lion tamarin as charismatic, which it is, and leave the argument there. But the animal's real interest is what it reveals. Tamarins live in groups of two to eleven, most often four to seven, and every member of the group - not only the parents - helps carry and feed the young. They forage in the middle storey, which means they need forest with structure, not just trees. They eat fruit, flowers, nectar, insects, spiders, snails, frogs, lizards, bird eggs and small snakes, which means they need a forest with everything else still in it. You cannot preserve this one species in isolation. Protecting it requires protecting the entire architecture of the Atlantic Forest, which is precisely why a reserve for a monkey ends up sheltering woodpeckers, sloths, beetles and fish.