The dual rod system of amphibians supports colour discrimination at the absolute visual threshold

Research output: Contribution to journalArticle


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.


  • Carola A M Yovanovich
  • Sanna M. Koskela
  • Noora Nevala
  • Sergei L. Kondrashev
  • Almut Kelber
  • Kristian Donner
External organisations
  • University of Helsinki
  • A.V. Zhirmunsky Institute of Marine Biology, RAS
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Zoology


  • Behaviour, Bufo, Colour vision, Photoreceptors, Rana, Visual threshold
Original languageEnglish
Article number20160066
JournalPhilosophical Transactions of the Royal Society B: Biological Sciences
Issue number1717
Publication statusPublished - 2017 Apr 5
Publication categoryResearch