ORIGINAL PAPER Orangutans (Pongo abelii) and a gorilla (Gorilla gorilla gorilla) match features in familiar and unfamiliar individuals Jennifer Vonk • Jennifer Hamilton Received: 8 November 2013 / Revised: 19 February 2014 / Accepted: 3 March 2014 / Published online: 13 March 2014 Ó Springer-Verlag Berlin Heidelberg 2014 Abstract Great apes can perceive images as representa- tive of corresponding real-life objects. Coupled with the potential advantages of identifying specific members of one’s species and mounting evidence for individual rec- ognition in other non-humans, it seems likely that great apes would have the ability to identify conspecifics in photographs. The ability of four orangutans and a gorilla to match images of individuals of their own and a closely related but unfamiliar species was examined here for the first time. First, the subjects matched photographs of familiar conspecifics taken at various time points in a delayed matching-to-sample procedure (Experiment 1). Second, they matched different photographs of unfamiliar individuals of a different species (Experiment 2) at above chance levels. These results suggest that the subjects matched photographs by matching physical features, not necessarily by recognizing the identity of the individuals depicted. However, they also quickly learned to select photographs of familiar individuals when these photo- graphs were paired with photographs of unfamiliar indi- viduals of their own species (Experiment 3), and three subjects showed transfer to novel images of familiar and unfamiliar individuals. Thus, the findings support the idea that subjects attended to physical features to identify individuals that they could categorize on the basis of familiarity. Keywords Individual recognition Á Matching Á Gorilla Á Orangutan Á Familiar Á Unfamiliar Introduction In group-living animals, such as primates, it is critically important to differentiate familiar group members from unfamiliar individuals and to recognize specific individuals within the group. The ability to recognize group members can assist an individual in complying with dominance structures inherent within a group, maintaining group cohesion (Cheney and Seyfarth 1992), engaging in affilia- tive interactions, and selecting a mate (Johnston and Bullock 2001; Rendall et al. 1996). Studies from the field have compiled ample evidence for the recognition of broad categories such as familiar versus unfamiliar, and related versus unrelated conspecifics in group-living primates (Cheney and Seyfarth 1980, 1992, 1999; Rendall et al. 1996; Silk 1999). Non-primate mammals also appear to recognize group members. Pallid bats (Arnold and Wil- kinson 2011), for example, will call more frequently in response to playbacks from roostmates than from unfa- miliar bats. Bottlenose dolphins also appear to show increased orientation and approach behaviors in response to vocal playbacks from familiar individuals even after being habituated to unfamiliar dolphin playbacks (Bruck 2013). Non-group-living animals likewise appear to dis- tinguish between familiar and unfamiliar conspecifics. Orangutans will look more at unfamiliar conspecifics, unless they have not seen familiar individuals for a long period of time, in which case they will look longer at the old acquaintances (Hanazuka et al. 2013). Thus, it seems that many species have the ability to parse individuals into categories of familiar and unfamiliar. Playback experiments have also provided suggestive evidence that discriminations are made at an even finer level in a variety of species, where group members may perceive particular vocal signatures as representative of the J. Vonk (&) Á J. Hamilton Department of Psychology, Oakland University, 2200 N Squirrel Rd., Rochester, MI 48309, USA e-mail: vonk@oakland.edu 123 Anim Cogn (2014) 17:1089–1105 DOI 10.1007/s10071-014-0741-4