Behavioural evidence of dichromacy in a species of South American marsupial Eduardo de A. Gutierrez, Beatriz M. Pegoraro, Bráulio Magalhães-Castro, Valdir F. Pessoa * Laboratory of Neurosciences and Behaviour, Institute of Biological Sciences, University of Brasilia, Brazil article info Article history: Received 5 November 2010 Initial acceptance 8 December 2010 Final acceptance 8 February 2011 Available online 21 March 2011 MS. number: A10-00769R Keywords: behaviour colour vision Didelphis albiventris marsupial white-eared opossum Colour vision in marsupials is a controversial issue, especially among the genus Didelphis (Didelphidae, Didelphimorphia). While behavioural tests have diagnosed these animals as trichromats and electro- physiological studies have diagnosed them as monochromats, recent molecular genetics studies provide evidence for dichromatic colour vision, having found two classes of cone opsins in a species of this genus. This study examines the colour perception of a male and female white-eared opossum, Didelphis albi- ventris, through a series of tasks involving a behavioural paradigm of discrimination learning. Both opossums succeeded in discriminating pairs of stimuli consisting of Munsell colour cards presented in random brightness values that are assumed to be easily discriminated by dichromats and trichromats (e.g. blues versus oranges). However, both subjects failed to discriminate between colours that are expected to be easily discriminated only by trichromats (e.g. greens versus oranges). The opossums were also unsuccessful in distinguishing a colour against itself (e.g. oranges versus oranges), demonstrating that discrimination was based only on visual cues. These results are consistent with recent predictions based on molecular genetics suggesting that the genus Didelphis is routinely a dichromat. Ó 2011 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Five classes of photopigments are present in vertebrates; one rod class and four cone classes. However, most mammals have lost two types of cone photopigments and are classified as dichromats (Ahnelt & Kolb 2000; Hunt et al. 2009; Jacobs 2009). Excluding cave- living and fossorial animals, which have regressed vision relative to most mammals, the exceptions to dichromatism among mammals are concentrated in the order Primates. In this order a homogenous trichromatism (found in males and females) has evolved in all catarrhine primates (Old World monkeys, apes and humans) (Surridge et al. 2003) and in the howler monkey (Alouatta)(Jacobs et al. 1996; Araújo et al. 2008). Also, platyrrhine monkeys have X-chromosome opsin-gene polymorphism, resulting in more varied colour vision arrangements (Rowe & Jacobs 2004; Jacobs 2007). Finally, there is monochromatism found in the owl monkey (Aotus) (Jacobs 1993a) and in the prosimian bushbaby Galago garnetti (Wikler & Racic 1990). Further exceptions to dichromatism in mammals also include the monochromatic vision in three nocturnal carnivores (Procyon cancrivorus, Procyon lotor , Potos flavus)(Jacobs & Deegan 1992; Peichl & Pohl 2000), several rodents (Jacobs 2009) and some pinniped and cetacean species (Peichl et al. 2001). However, recent studies with Australian marsupials suggest that trichromacy is not unique to primates. Using microspectrophotom- etry (Arrese et al. 2002) and microspectrophotometry and immu- nohistochemistry (Arrese et al. 2005), Arrese and collaborators identified three classes of cone photopigments in four species of Australian marsupials that are representative of the two major taxonomic divisions, the diprotodonts and the polyprotodonts (Springer & Murphy 2007). Behavioural tests were also performed for the fat-tailed dunnart, Sminthopsis crassicaudata (Arrese et al. 2006), a species previously diagnosed as trichromats by micro- spectrophotometry. This study showed that S. crassicaudata also behaved like trichromats. However, the determination of amino acid sequences of cone visual pigments in S. crassicaudata and in the stripe-faced dunnart, Sminthopsis macroura, indicated that while both species have long-wavelength-sensitive (LWS) pigments with a predicted maximum absorption (l max ) of 530 nm and ultraviolet- wavelength-sensitive (UVS) pigments with a predicted l max of 360 nm (Strachan et al. 2004), there was no evidence of a third cone photopigment in either species. Furthermore, a recent in vitro expression study (Cowing et al. 2008) has expanded these results. Cowing and colleagues showed that although no cone opsin sequences for the middle wavelength cones could be extended, two genes for rod opsins (RH1) were found, which raises the possibility of RH1 being expressed in cones, leading to trichromacy. Colour vision in marsupials remains to be satisfactorily resolved, not only among the Australian clades but particularly among the order Didelphimorphia, the American marsupials (see Table 1). Jacobs (1993b) and Jacobs & Williams (2010), performing electro- retinographic measurements in the Virginia opossum, Didelphis vir- giniana, obtained responses for only one cone photopigment with a maximum sensitivity of about 560 nm. Previous colour discrimi- nation behavioural tests were also conducted for D. virginiana by * Correspondence: V. F. Pessoa, Laboratory of Neurosciences and Behaviour, Institute of Biological Sciences, University of Brasilia, Brasilia, DF 70.910-900, Brazil. E-mail address: vpessoa@unb.br (V. F. Pessoa). Contents lists available at ScienceDirect Animal Behaviour journal homepage: www.elsevier.com/locate/anbehav 0003-3472/$38.00 Ó 2011 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.anbehav.2011.02.012 Animal Behaviour 81 (2011) 1049e1054