Few creatures on Earth are as cute as the black lion tamarin, and few have as dramatic a story line. Pug-nosed and diminutive, with a comic fringe of hair, these monkeys dwell in trees in small tracts of forest in southeastern Brazil. Or they did until 1905, when they were declared extinct. No one saw a black lion tamarin again in the wild until 1970, when the great Brazilian primatologist Adelmar Coimbra-Filho came across a tamarin family in a little reserve in the far western corner of São Paulo State. Later, in the 1990s, researchers turned up a small set of isolated, inbred populations scattered over a wide region. Since that time, the Brazilian team of Claudio Padua and Cristiana Martins of the Institute for Ecological Research has been engineering tamarin migration and mating, doing everything they can to bring back to health one of the world's most distinctive primates.

Although they are no larger than house cats, tamarins have brains that are surprisingly big for their size and a family life organized like our own. They live in groups anchored by an adult male and adult female, along with some younger animals, including their offspring, and an occasional male from a neighboring group. When a mother bears young, she usually produces twins, and although members of the group share in their upbringing, it is most often the father who carries them around in the trees, where the families feed on fruits, insects, lizards, and birds' eggs.

Unhappily for the lion tamarins, their tree-bound niche began to disappear after the Portuguese landed in Brazil and began clearing forest to make room for Rio de Janeiro, São Paulo, and the many settlements and farms of southern Brazil. As is the case for so many threatened species, the breakup of their habitat sounded the death knell for tamarins, depriving them of the continuity of forest they require to remain abundant and safe from potential threats in any single vicinity. The animals avoid predators by hardly ever coming down from the trees, so even a narrow logging road through a forest can begin the breakup by preventing them from moving from one patch of forest to another.

A simple solution was to build bridges across roads, allowing the monkeys to move from one forest to another. With some lumber and a day's work by Padua and his colleagues, habitats that had been separated became continuous again, improving opportunities for migrating and mating.

The next step was to broaden the distribution of the population. Padua and his team captured a family of black lion tamarins in the Rio Claro Farm (a forest managed by the timber company Duratex) and moved them to forest where black lion tamarins formerly lived. The first group consisted of an adult pair and a young male and infant daughter. The second had an adult pair, another adult male, one adolescent male and a female, and an infant son. After a year, the moves were declared a success: Not only had 80 percent of the tamarins survived, but they had also produced new offspring. The young adult male of the second group left his birth family and successfully entered the other recently translocated group.

So far, so good. The black lion tamarin recovery team had learned that the animals could adjust to the new forest, even if the insects there tasted a little different or the trees were a slightly different size. Still, the problem of limited gene flow in a small population remained. As tamarin numbers dwindled, the population had lost the genetic diversity that holds the key to resilience to various threats, like being able to withstand droughts or infectious diseases.

The number of black lion tamarins in the wild had fallen to fewer than 1,000, with most isolated fragments hosting well under 100 monkeys. In a population with so few individuals, an unhealthy mutant gene will more often find itself paired with the same mutant gene in an offspring because the parents are related and of similar genetic makeup. This double dose reduces the offspring's chances of surviving and reproducing, further reducing the species' chance of survival.