Emergence of new technology has broadened our understanding of species behaviour, even those that are common and well-studied. We report the use of a thermal camera (TC) on a diurnal Neotropical primate, the brown howler monkey (Alouatta guariba). We describe the undocumented nocturnal activity, and present other contributions of the TC for the routine activities applied in the field. We were able to quickly locate groups and observe rare behaviours, such as interspecific interactions and the process of immigration. The TC allowed the location of fresh stool on the ground, ideal for non-invasive hormone measurements and seed dispersal studies. This device can be used to address many questions, particularly those combining behaviour and hormonal measurements, but may be unhelpful for surveys. We endorse its employment as it can be an invaluable tool for studying the behaviour of Neotropical primate species, even diurnal ones.