Comparing Population Census Methods for Howler Monkeys ( Alouatta palliata ) in a Fragmented Rainforest

Introduction Accurate population density estimates are important for guiding conservation management practices for threatened animal species. Aims This study tests the efficacy of population census methods at a long‐term study site for the mantled howler monkey ( Alouatta palliatta ), La Suerte Biological Research Station (LSBRS) in Costa Rica. This monkey is a group‐living Neotropical primate with long distance vocalizations that is declining throughout its range. Methods We compared the visual count and long calling acoustic triangulation census methods to determine population count and number of social groups. Results We found 14 monkey groups according to both population census methods, indicating the accuracy of the acoustic triangulation method at determining group number. Visual count data showed that the 1.36‐km 2 study area within a 3‐km 2 forest fragment had a population density of 125.7 individuals/km 2 , one of the highest population densities ever recorded for this species, although mean group size of 11.6 (±4.7 SD) monkeys/group at LSBRS remained within the 10–15 monkeys/group range found at other sites. Discussion The observed mean group size at LSBRS fell within the commonly reported range for the species, suggesting that the acoustic triangulation method alone may provide a reasonably accurate estimate of population density for sites with average monkey group sizes and/or where mean group sizes have previously been quantified using visual count surveys. Recommendations We recommend collecting both types of census data where possible, while suggesting that acoustic triangulation censuses can be used on their own when financial resources are limited. Visual counts are necessary to confirm group number and size, and to enable detailed demographic studies.