Field biologists adopted the term habituation from physiology, as the relatively persistent waning of a response as a result of repeated stimulation that is not followed by any kind of reinforcement (Thorpe, 1963). Repeated neutral contacts between primates and humans can lead to a reduction in fear, and ultimately to the ignoring of an observer. Historically, the techniques and processes involved were rarely described, as habituation was generally viewed as a means to an end (Tutin & Fernandez, 1991). As we become increasingly aware of the potential effects of observer presence on primate behaviour, and especially the potential risks of close proximity with humans, it behoves us to measure as much about the habituation process as possible. However, most recent studies that have quantified primate behaviour in relation to habituators have focussed on great apes (see, for example, Ando et al., 2008; Bertolani & Boesch, 2008; Blom et al., 2004; Cipolletta, 2003; Doran-Sheehy et al., 2007; Sommer et al., 2004; Werdenich et al., 2003), with little information available for other primate taxa (but see Jack et al., 2008).
There are limits to what studies of unhabituated primates can achieve: it is difficult to observe at close range, so subtle or cryptic behaviour such as facial expressions and soft vocalizations may be missed, and even individual identification may be difficult, resulting in analyses based only on age–sex classes.