During naturalistic foraging, animals deploy complex decision-making strategies to acquire resources in the presence of uncertain environments. Here we focus on a probabilistic foraging task and how serotonergic (5-HT) neuromodulation affects mouse performance via optogenetic activation of the Dorsal Raphae nucleus (DRN). We show that it is necessary to account for the complexity of behavioral variability across mice in order to uncover the strategies mice deploy to perform the task and how serotonergic neuromodulation affects behavior. While most studies have quantified decision-making performance in terms of observables derived from lick rewards and failures, we found that these simple measures only poorly capture behavior and neuromodulatory effects. We thus take a holistic approach and define the high-dimensional "ethological fingerprint" of individual mice in terms of a library of both task-relevant and task-irrelevant behavioral measures, including licking dynamics, grooming, waiting, and uninstructed movement behaviors. We found that individual mouse fingerprints are maintained across days and weeks, are predictive of genotype, and their variability captures the complex effects of 5-HT activation. In particular, we found that 5-HT activation affects an animal's tolerance for failure and reaction times, fragments their licking behavior, and overall modulates their behavior in a complex interaction with the animal's internal state. Our results suggest that understanding decision-making in naturalistic and uncertain environments requires a holistic approach that accounts for the entirety of an animal's experience and variability across subjects. Only within this wider scope can one understand the effects of experimental manipulations such as serotonergic activation.