Ephemeral Sexual Dichromatism in Zebrafish (Danio rerio) Sophie Hutter, Attila Hettyey, Dustin J. Penn & Sarah M. Zala Konrad Lorenz Institute of Ethology, Department of Integrative Biology and Evolution, University of Veterinary Medicine, Vienna, Austria Correspondence Sarah M. Zala, Konrad Lorenz Institute of Ethology, Department of Integrative Biology and Evolution, University of Veterinary Medicine, Vienna, Savoyenstraße 1a, 1160 Vienna, Austria. E-mail: sarah.zala@vetmeduni.ac.at Received: April 24, 2012 Initial acceptance: June 3, 2012 Final acceptance: September 24, 2012 (M. Herberstein) doi: 10.1111/eth.12027 Abstract Secondary sexual displays may be overlooked in many species, especially when they are ephemerally expressed or imperceptible to human senses. Zebrafish (Danio rerio), like many schooling fish, do not appear sexually dichromatic, but previous anecdotal observations indicate that sexual col- ouration is expressed briefly during courtship (ephemeral nuptial colour- ation). Our goals were to compare colour estimates of male vs. female zebrafish using digital photography in situ, computer software and human observations. We found that both sexes changed their colour estimates during spawning (dark and light stripes) and that some sex differences (light stripes) were larger or only became apparent during this time. We also found that individual males that appeared more colourful and con- spicuous to the human eye engaged in courtship more often than less con- spicuous males. We detected differences in the colour estimates between wild-derived vs. a laboratory strain of zebrafish and reduced individual variation in the laboratory strain. This is the first study to systematically and objectively quantify body colour in zebrafish by utilizing colour esti- mates, although further studies are needed to determine the underlying mechanisms and signalling functions of this sexual dichromatism. Sexual dimorphism is widespread (Darwin 1871; Hedrick & Temeles 1989; Shine 1989), and sexual selection studies have generally focused on highly sexually dimorphic species, and especially those that display conspicuous, sexually dimorphic colours (sex- ual dichromatism) (Price et al. 2008), as such traits seem explicable only by sexual selection (Darwin 1871). Species that show no obvious or only minor sexual dichromatism to human observers receive less attention, although even subtle sexual signals could still have important functions (Endler 1991). An increasing number of studies are discovering sexual dimorphisms in many species, which are cryptic to human senses, including ultraviolet colouration (Cummings et al. 2003), ultrasonic vocalizations (Nyby & Whitney 1978; Musolf & Penn 2012), scent- marking, pheromones and other chemical signals (Penn & Potts 1998; Zala et al. 2004), and electrical communication (Crawford 1991). Sexual dichroma- tism can also go undetected when such traits are expressed temporarily, such as only during courtship (ephemeral nuptial colouration) (Kodric-Brown 1998). Very rapid colour changes are common in fishes, for example, and in males they often signal motivation and intention and can also be used during courtship behaviour (reviewed in Kodric-Brown 1998), male–male competition and female choice (Kekalainen et al. 2010). In schooling fish, sexually dichromatic displays might be ephemeral rather than permanent to avoid standing out in the crowd (Ko- dric-Brown 1998). However, ephemeral colour dis- plays have received relatively little attention in sexual selection studies compared with more permanent col- ours (Kekalainen et al. 2010). Consequently, compar- ative analyses on sexual selection that do not examine the expression of traits during courtship or that rely on human perception alone can potentially overlook sexual dichromatism and other sexual dimor- phisms, as even sexual dimorphisms in size can be cryptic to human observers (Zefania et al. 2010). Our goals were to compare body colour estimates of male vs. female zebrafish (Danio rerio) to determine whether Ethology 118 (2012) 1208–1218 1208 Ethology