<p>Dietary breadth profoundly influences species' resilience to environmental change and extinction risk, yet the relative importance of environmental and demographic drivers remains poorly understood. We investigated temporal variation in dietary breadth of mantled howler monkeys (<i>Alouatta palliata</i>) over 10 years (2012–2021) in two groups translocated to the study site 8–10 years before observations began. During the study, population density increased 75% (from 0.20 to 0.35 ind/ha). We tested five hypotheses regarding environmental factors (resource scarcity, temporal heterogeneity in food availability, climatic stress) and demographic pressures (intraspecific competition) as drivers of dietary species richness, alongside compensatory foraging mechanisms. Based on 6828 hours of focal observations recording 56,540 feeding episodes, we quantified annual dietary plant species richness and resource use patterns while monitoring food availability and climate variables. Standardized dietary breadth declined 60% over the decade (from 3.97–5.84 to 1.59–2.49 species per 100 observation hours), with population density explaining more variation than any environmental factor. Bootstrap-confirmed correlations revealed a negative relationship between dietary species richness and population density, supporting the competition hypothesis. Contrary to the dietary breadth predictions of optimal foraging theory, we detected no relationships between dietary species richness and food availability, temporal heterogeneity, or climatic variables. Dietary contraction co-occurred with reduced tree use and smaller tree selection rather than compensatory intensification. These findings indicate that demographic pressures can impose fundamental constraints on dietary flexibility even in productive habitats, questioning assumptions that howler monkey&#xa0;dietary plasticity buffers populations against resource limitation.</p>