From darkness to daylight: cathemeral activity in primates

“Ecologically speaking, the diurnal world and the nocturnal world are completely different”. With this sentence Charles-Dominique (1975) introduced his historical paper describing the ecological interpretation of nocturnality and diurnality in primates. In fact, animals have to find different solutions to cope with these two opposing sensory environments which, consequently, have greatly influenced their evolution. Thus, from physiology to sociality several traits characterize the two choices of life, and they may tell us the activity phase of an animal. Generally speaking, the need to orientate, searching for food or escaping from predators, during the two phases of the 24-hour cycle improved a unique perception of the environment, while hampering others (Martin, 1990; Halle, 2006). So, a fine-grained sense of olfaction and/or a good hearing perception give a nocturnal animal the possibility to survive, while diurnal animals can solve the same problem with good vision during the hours of daylight. In evolutionary terms, the ancestral condition of