title: The dual rod system of amphibians supports colour discrimination at the absolute visual threshold.
abstract: The presence of two spectrally different kinds of rod photoreceptors in amphibians has been hypothesized to enable purely rod-based colour vision at very low light levels. The hypothesis has never been properly tested, so we performed three behavioural experiments at different light intensities with toads (Bufo) and frogs (Rana) to determine the thresholds for colour discrimination. The thresholds of toads were different in mate choice and prey-catching tasks, suggesting that the differential sensitivities of different spectral cone types as well as task-specific factors set limits for the use of colour in these behavioural contexts. In neither task was there any indication of rod-based colour discrimination. By contrast, frogs performing phototactic jumping were able to distinguish blue from green light down to the absolute visual threshold, where vision relies only on rod signals. The remarkable sensitivity of this mechanism comparing signals from the two spectrally different rod types approaches theoretical limits set by photon fluctuations and intrinsic noise. Together, the results indicate that different pathways are involved in processing colour cues depending on the ecological relevance of this information for each task.This article is part of the themed issue 'Vision in dim light'.
authors.affiliation: Department of Biology, Lund University, Sölvegatan 35, 22362 Lund, Sweden.
authors.forename: Carola A M
authors.initials: CA
authors.lastname: Yovanovich
authors.affiliation: Department of Biosciences, University of Helsinki, PO Box 65 (Viikinkaari 1), 00014 Helsinki, Finland.
authors.forename: Sanna M
authors.initials: SM
authors.lastname: Koskela
authors.affiliation: Department of Biosciences, University of Helsinki, PO Box 65 (Viikinkaari 1), 00014 Helsinki, Finland.
authors.forename: Noora
authors.initials: N
authors.lastname: Nevala
authors.affiliation: A.V. Zhirmunsky Institute of Marine Biology, Far Eastern Branch of the Russian Academy of Sciences, ul. Palchevskogo 17, 690041 Vladivostok, Russia.
authors.forename: Sergei L
authors.initials: SL
authors.lastname: Kondrashev
authors.affiliation: Department of Biology, Lund University, Sölvegatan 35, 22362 Lund, Sweden.
authors.forename: Almut
authors.identifier: http://orcid.org/0000-0003-3937-2808
authors.initials: A
authors.lastname: Kelber
authors.affiliation: Department of Biosciences, University of Helsinki, PO Box 65 (Viikinkaari 1), 00014 Helsinki, Finland kristian.donner@helsinki.fi.
authors.forename: Kristian
authors.identifier: http://orcid.org/0000-0001-7833-4236
authors.initials: K
authors.lastname: Donner
mesh_terms: D000818:Animals; D002025:Bufonidae / Q000502:physiology*; D003118:Color Perception; D055253:Color Vision; D008124:Locomotion; D008297:Male; D052586:Mating Preference, Animal; D011235:Predatory Behavior; D011898:Ranidae / Q000502:physiology*; D017949:Retinal Cone Photoreceptor Cells / Q000502:physiology*; D012684:Sensory Thresholds
keywords: Bufo; Rana; behaviour; colour vision; photoreceptors; visual threshold