The effect of sampling schedule on assessment of dietary measures: Evidence from blue monkeys (Cercopithecus mitis stuhlmanni)

Accurately assessing primate diets is important in studies of behavioral ecology and evolution. While previous research has compared sampling methods (scan, focal), we compiled a data set to examine how sampling schedule influences accuracy of dietary measures. We define sampling schedule as the combined distribution (random vs. consecutive) and frequency of sampling days within a given month. Under field conditions, time may be required to locate a study group, and we therefore also subtracted 1, 2, or 3 hours from the beginning of all non-consecutive days in each sampling schedule to mimic observation time lost to search. From a dense (near daily) 5-year record of feeding behavior derived from focal animal follows of adult females in five wild blue monkey (C. m. stuhlmanni) groups, we created data subsets matching various sampling schedules, and compared monthly dietary measures calculated from each subset to those based on the full dataset. These measures included (1) the proportion of observation time feeding on fruit, (2) diet composition (three top-ranked food items), (3) species richness of plant diet, (4) Shannon-Weiner Diversity Index based on plant species, and (5) Holmes-Pitelka Index expressing dietary overlap (plant species) with the previous month. We used linear mixed models to assess how frequency and sampling type (a combination of distribution and hours lost) relate to a subset’s deviation from the full dataset, where a smaller deviation (or higher chance of matching, for diet composition) implies greater accuracy. The data set presented here is the full data set from which subsets were extracted for these analyses.

Methods
Data were collected from 2012-2016 through field observation of 5 groups of wild blue monkeys (Cercopithcus mitis stuhlmanni) in Kakamega Forest, Kenya. On a near daily schedule, trained observers conducted 30 minute focal follows of adult females, balancing observation time across individuals and time of day (morning, midday, and afternoon periods). During follows, observers made instantaneous records (on the beep of a timer) of the subject’s activity at 1-minute intervals. Observers recorded a subject as feeding only when she put a food item in her mouth within 2 sec of the beep. Further details can be found in the methods section of the associated publication.