ORIGINAL PAPER Visual discrimination of rotated 3D objects in Malawi cichlids (Pseudotropheus sp.): a first indication for form constancy in fishes V. Schluessel • H. Kraniotakes • H. Bleckmann Received: 3 April 2013 / Revised: 26 July 2013 / Accepted: 5 August 2013 / Published online: 28 August 2013 Ó Springer-Verlag Berlin Heidelberg 2013 Abstract Fish move in a three-dimensional environment in which it is important to discriminate between stimuli varying in colour, size, and shape. It is also advantageous to be able to recognize the same structures or individuals when presented from different angles, such as back to front or front to side. This study assessed visual discrimination abilities of rotated three-dimensional objects in eight individuals of Pseudotropheus sp. using various plastic animal models. All models were displayed in two choice experiments. After successful training, fish were presented in a range of transfer tests with objects rotated in the same plane and in space by 45° and 90° to the side or to the front. In one experiment, models were additionally rotated by 180°, i.e., shown back to front. Fish showed quick asso- ciative learning and with only one exception successfully solved and finished all experimental tasks. These results provide first evidence for form constancy in this species and in fish in general. Furthermore, Pseudotropheus seemed to be able to categorize stimuli; a range of turtle and frog models were recognized independently of colour and minor shape variations. Form constancy and catego- rization abilities may be important for behaviours such as foraging, recognition of predators, and conspecifics as well as for orienting within habitats or territories. Keywords Cognition Á Visual discrimination Á Object rotation Á Learning Á Behaviour Introduction Despite the lack of a neocortex, fish show excellent com- petency in many cognitive skills (Brown et al. 2011), with vision being an important sensory system in many species (Guthrie 1986). In a wide range of diverse experiments, fish have been shown to possess good visual discrimination abilities matching those of other vertebrates and inverte- brates (Von Frisch 1914; Vallortigara 2004; Srinivasan 2010; Brown et al. 2011). Previous studies include exten- sive research on model organisms such as the goldfish but have also been assessing species such as sticklebacks, swordtails, cichlids, and damselfish, demonstrating suc- cessful detection and recognition of geometrical 2D as well as 3D shapes (Schaller 1926; Herter 1929, 1930; Hager 1938; Meesters 1940; Mackintosh and Sutherland 1963; Sutherland 1964; Mark 1966; Wyzisk 2005; Wyzisk and Neumeyer 2007; Siebeck et al. 2009; Schluessel et al. 2012; Gierszewski et al. 2013). Further studies have shown that fish, like primates and humans, can categorize objects, such as fish and snail images (Schluessel et al. 2012), can be deceived by optical illusions, such as Mu ¨ller-Lyer illusions (Herter 1930), can perceive illusionary contours such as Kaniza and Nieder-Wagner figures (Wyzisk 2005; Wyzisk and Neumeyer 2007), can recognize partly occlu- ded or fragmented objects in a process called ‘amodal completion’ (Sovrano and Bisazza 2008, Sovrano and Bisazza 2009; Darmaillacq et al. 2011), and can distinguish images of individual letters and various other shapes from their mirror image counterparts (Gierszewski et al. 2013). Redtail Splitfins (Xenotoca eiseni) and zebrafish (Danio rerio) use environmental geometry and landmarks inde- pendently in different orientation contexts (Lee et al. 2012), a result supported by previous findings on the use of geometrical and feature information by fish in differently V. Schluessel (&) Á H. Kraniotakes Á H. Bleckmann Institute of Zoology, Rheinische-Friedrich-Wilhelm Universita ¨t Bonn, Poppelsdorfer Schloss, Meckenheimer Allee 169, 53115 Bonn, Germany e-mail: v.schluessel@uni-bonn.de 123 Anim Cogn (2014) 17:359–371 DOI 10.1007/s10071-013-0667-2