How Survey Design Affects Monkey Counts: A Case Study on Individually Recognized Spider Monkeys (Ateles geoffroyi)

Abstract

The fast movement and high degree of fission-fusion dynamics of spider monkeys (Ateles spp.) make them notoriously difficult to survey. We examined which aspects of survey design affect spider monkey sightings along transects in a group of individually recognized spider monkeys (Ateles geoffroyi) in Punta Laguna, Yucatan, Mexico. We calculated the number of monkeys and subgroups sighted per transect walk. Using generalized linear models, we found no effect of the number of observers, transect type (new vs. existing), walking speed, or time of day on individual monkey counts or subgroup counts. Recounting of individuals was relatively rare and occurred when transects were walked relatively slowly. We missed more young than adult monkeys. The group composition based on survey data was similar to the known group composition. Based on our findings we recommend that surveys performed on relatively flat terrain be conducted at speeds similar to or faster than the moving speed of spider monkeys to minimize recounting of individuals and that young:adult female ratios based on survey data be interpreted as conservative indicators of population health.The novel methods presented to determine sources of bias in population estimates are applicable to a wide range of primates that are difficult to survey.

Primate life history, social dynamics, ecology, and conservation: Contributions from long‐term research in Área de Conservación Guanacaste, Costa Rica

“…Since primates provide critical seed dispersal and pollination services in the dry forest and other tropical ecosystems (Chapman, 1995; Hogan et al., 2016; Valenta & Fedigan, 2008), the growing populations of these species may, in turn, contribute to the regeneration of the forest in ACG (Chapman et al., 2020). We do not have similar data for spider monkeys, as their high degree of fission–fusion dynamics makes it more difficult to assess population size with traditional techniques, but we continue to test different methods for collecting and analyzing spider monkey survey data, including drones with thermal cameras (Spaan et al, 2017; Spaan, Burke, et al 2019; Spaan, Ramos‐Fernández, et al 2019).…”

Anthropogenic habitat disturbance and food availability affect the abundance of an endangered primate: a regional approach

“…Given that the aim of our study was to examine the effect of different types of habitat disturbance and ecological factors on the relative abundance of spider monkeys rather than calculate population density, spider monkey counts were summed for the six surveys on the same m segment. A previous study on A. geoffroyi suggests that repeated counts of the same individual during surveys are rare (2.1% according to Spaan et al, 2017). We therefore expect any potential error introduced into monkey counts as a result of including recounted individuals to be minimal.…”

“…Each transect was walked at least twice in the morning (07:00-11:00) and twice in the afternoon (14:00-18:00), at least one month apart to increase independence between replicates of the same transect and to control for the effect of seasonality on spider monkey habitat use. Transects were walked at a speed of 1.0-2.6 km/hour (mean = 1.6 km/hour), in line with the recommended speed for this species to minimize observer impact on the distribution of the study animals (Spaan et al, 2017). To ensure consistency between surveys at different sites, the same person collected the data during all transect walks.…”

Assessing the effects of survey-inherent disturbance on primate detectability: Recommendations for line transect distance sampling

“…For this reason, we estimated group size during the survey. When comparing our observed and estimated group size (Table 2 ), we found that exact counts (i.e., observed group size) were consistently lower than estimates including acoustic detections, suggesting that rainforest habitat is particularly unsuitable in visually counting individual primates (Spaan et al, 2017 ). As it is difficult to accurately estimate the number of individuals from acoustic cues only, we suggest improving accuracy by combining visual and acoustic observations.…”