Muriquis (Brachyteles arachnoides) are endemic to the Atlantic Forest of southeastern Brazil, where an estimated 1200 individuals are distributed unevenly among only 19 populations. Despite the fact that over two-thirds of all muriquis live at low densities in large and protected, undisturbed forests, virtually nothing is known about the demography of these populations. Consideration of the mechanisms underlying the low extinction probabilities simulated for muriquis inhabiting the small, disturbed forest at the Estação Biologica de Caratinga (EBC), Minas Gerais, where long-term demographic and behavioral studies have been conducted, provides disturbing predictions about the viability of other populations living at lower densities. Local resource competition may be responsible for the female-biased infant sex ratio documented at the EBC under the female-biased dispersal regime exhibited by muriquis. At lower population densities, however, release from competition may lead to male-biased infant sex ratios, and correspondingly slower population growth rates. High densities at the EBC also result in frequent intergroup encounters, permitting muriquis there to avoid close inbreeding through extra-group copulations; but at low densities, opportunities to avoid inbreeding may be more limited. Finally, habitat heterogeneity at the EBC, particularly the greater dietary diversity supported by secondary and regenerating vegetation in disturbed forests, not only may support higher muriqui densities but also provide greater dietary buffers against catastrophic fruit scarcities compared to undisturbed forests. Comparative demographic data on muriqui populations inhabiting undisturbed forests are critically needed to evaluate these predictions and to develop informed management plans for this endangered primate.