How does a frog see the world?

What the research shows

The claim that frogs see colour close to darkness rests on behaviour. Yovanovich et al. (2017) used the frog's tendency to jump towards light, offering a choice between blue and green at falling intensities. The animals kept choosing blue over green down to starlight levels, close to the limit set by photon noise itself. The explanation lies in two rod types: a rare blue-sensitive rod peaking near 433 nm and the ordinary rhodopsin rod near 502 nm. The names are confusing, as the authors themselves note. Histologists call the 433 nm cell the "green rod" because of its appearance and the 502 nm cell the "red rod", whereas the chart on this page labels them by the light they respond to.

Daylight detail is less well documented, and no measurement exists for frogs of the genus Rana. The closest figure is anatomical: Pushchin (2019) estimated about 2.1 cpd in air and 2.8 cpd in water from the retina of the Japanese tree frog, Hyla japonica. The page's 2 cpd, roughly 20/300, is an estimate based on that value. Temporal resolution is low, unquantified and, as in other ectotherms, dependent on temperature. A screen faces an awkward limit: the skill that makes frogs remarkable operates at light levels far below anything a display emits, so a brightened image can indicate that blue and green stay distinct but cannot recreate the photon-starved conditions in which the frog achieves it.