Quantifying Dietary Breadth in Howlers: How Much Sampling Is Enough?

Accurate characterization of dietary breadth is fundamental to understanding animal ecology, yet comparative studies based on inconsistent sampling protocols have often confounded measurements of behavioral variation. We address a critical question: how much observation effort is required to adequately characterize dietary breadth in mantled howler monkeys (Alouatta palliata)? Using a comprehensive ten-year dataset, we applied multiple complementary accumulation curve approaches (abundance-based, incidence-based, and time-based) to 56,540 feeding observations totaling 6828 observation hours across two groups. Sample coverage exceeded 0.99 for both groups, confirming near-complete sampling. Species accumulation curves revealed that characterizing dietary breadth to near-completion required approximately 5000-5200 observation hours. The first 1500-3000 h captured 85%-100% of dietary diversity, with accumulation rates declining dramatically (> 95%) beyond this threshold. Overall accumulation rates averaged 0.006-0.007 species per hour. Monthly incidence-based approaches required 7-8 years of continuous sampling to reach asymptotic levels. Despite occupying neighboring habitats, groups differed substantially in observation hours required to reach equivalent species milestones, highlighting how foraging selectivity affects sampling requirements. Coverage-based rarefaction enabled comparison at equivalent sampling completeness (0.999 coverage), revealing that Group 1 exhibited higher estimated richness (51.3 species) than Group 2 (36.2 species), though overlapping 95% confidence intervals precluded definitive conclusions about between-group differences. Our results provide practical sampling benchmarks for this species and demonstrate methodological approaches applicable to primate dietary studies. We recommend that dietary studies routinely report accumulation curves and sample coverage statistics to enable assessment of sampling adequacy.