Study

Visual Pigments, Ocular Filters and the Evolution of Snake Vision

Simões, Bruno F.; Sampaio, Filipa L.; Douglas, Ronald H.; Kodandaramaiah, Ullasa; Gower, David J.

Molecular Biology and Evolution, 2016

Peer-reviewedcomparative studylaboratory

What they did

Visual opsin gene sequences and lens spectral transmission were characterised across a wide sample of snakes spanning nocturnal, diurnal and fossorial habits, and mapped onto the snake phylogeny.

Studied in: Serpentes
Sample size: visual opsin genes and lens transmission across dozens of snake species

Snakes retain three visual opsins but have lost several found in other reptiles, consistent with an ancestral period of dim-light or burrowing life. Diurnal, visually hunting lineages have independently shifted pigment sensitivities and evolved lenses that block ultraviolet, while several nocturnal and burrowing lineages retain UV-transmitting lenses.

What it means

The authors' reading, and ours. Where they differ, that difference is the point.

How the authors put it

Snake vision has been repeatedly and independently retuned to habit, from a reduced ancestral starting point.

How NatureHQ reads it

The reduced starting point is the interesting part, and it is a piece of evidence for something otherwise hard to test: the ancestor of snakes appears to have lived somewhere dark. What snakes have done since is rebuild vision several times over from a diminished toolkit, which is why a diurnal tree snake and a burrowing blindsnake are not two points on one scale but two separate reconstructions. "Snakes have poor eyesight" is therefore a statement about some snakes only.

Limitations

Opsin genes and lens transmission constrain what an eye can detect; they do not establish acuity or what the animal does with the signal.
Behavioural discrimination experiments exist for very few snake species.
Ancestral reconstruction depends on the phylogeny assumed.