Burrowing snakes have far worse eyesight than their ancestors

An international team of scientists -- led by the Natural History Museum and the University of Plymouth -- carried out the first detailed analysis of gene sequence data for any species of the so-called "blindsnakes" (Scolecophidia), a group of small-eyed burrowers.

They found that seven of the 12 genes associated with bright-light vision in most snakes and lizards species are not present in scolecophidians.

This, they say, demonstrates extensive vision gene loss over tens of millions of years of evolutionary history, similar to that which has also been observed in burrowing mammals with reduced vision.

It also challenges the hypothesis that all snakes living across the world today evolved from extreme burrowers, because the vision genes lost in scolecophidians are present in most other living snakes. The researchers say it would be extremely unlikely for such genetic deficiencies to have been reversed through evolution.

Scolecophidians are dedicated burrowers and form one half of the oldest divergence in the snake tree. They comprise approximately 460 of the approximately 3,850 currently recognised living snake species, and likely diverged from their closest living relatives (Alethinophidia, which includes all other living snakes) more than 65 million years ago.

Scolecophidians are typically small, cylindrical, burrowing snakes with reduced eyes and small mouths, strikingly different from the more familiar alethinophidians, which include pythons, vipers, and cobras.

For this research, published in Genome Biology and Evolution, scientists generated new gene-sequence data for the dark-spined blind snake (Anilios bicolor) and the prong-snouted blind snake (Anilios bituberculatus), both of which are common across southern Australia.

They then compared their results with similar data for functional vision genes among other species of snakes and lizards.

Lead author Dr David Gower, Head of the Life Sciences Vertebrates Division at the Natural History Museum, said: "Our data and analytical results provide clear evidence for very substantial reduction of elements of the visual system of burrowing scolecophidian snakes. We already knew that snakes lost some vision genes and eye structures during their evolution from lizards, but most were nonetheless retained. It is highly unlikely that functional copies of a large number of vision genes were lost from the ancestral snake but subsequently re-evolved in most living snakes. As a result, our study strongly suggests that the ancestor of all living snakes was unlikely to have been as extreme a burrower as living scolecophidian snakes."

The same team of researchers published a study in May 2020, which revealed that the visual systems of sea snakes have undergone remarkable adaptation to life underwater for 15 million years.

Dr Bruno Simões, Lecturer in Animal Biology at the University of Plymouth, led that research and is senior author on the current study. He said: "The nature of the ancestral snake has been highly debated. It has been suggested that it had evolved a reduced visual system as a result of a possible burrowing ecology. Our research provides new information on the likely ecology and environment of the ancestral snake and suggests that the highly subterranean lives and reduced eyes of living scolecophidian snakes likely evolved as specialisms rather than being the retention of primitive characteristics. This study also shows convergent evolution between subterranean snakes and burrowing mammals with the loss of similar genes, especially those associated with bright-light and colour vision."