Watching the pair of nine-banded armadillos in this video root around the forest floor looking for food, you’d think that in addition to their snuffling and sensitive noses, the armored mammals would also be using their eyes to spot scurrying insects or the white flash of a grub in the dirt. And a three-toed sloth like the one in this other video should definitely be able to scan the skies for predatory birds when it is taking care of its vulnerable young, right? Well actually, as NatGeo contributor Ed Yong reports, scientists at UC Riverside believe the genes that build color-detecting cone cells in eyes are broken in sloths, armadillos, and several other mammal species descended from the same burrowing ancestor:
Armadillos have terrible vision. In 1913, American zoologists Horatio H. Newman and J. Thomas Patterson wrote, “The eyes [of the nine-banded armadillo] are rudimentary and practically useless. If disturbed an armadillo will charge off in a straight line and is as apt to run into a tree trunk as to avoid it.”
The three-toed sloth isn’t much better. “If an infant sloth is placed five feet away from its mother on a horizontal branch at the same level, at once the young sloth begins to cry, the mother shows that she heard it calling and turns her head in all directions. Many times she looks straight in the direction of her offspring but neither sight, hearing nor smell apparently avail anything,” wrote Michel Goffart in 1971. And more comically: “Infuriated male [sloths] try to hit each other when they are still distant by more than a metre and a half.”
Now, decades after these descriptions were written, Christopher Emerlingand Mark Springer from the University of California Riverside think they know why armadillos and sloths are so poor of sight. These animals have broken copies of the genes that build colour-detecting cone cells in their eyes. That leaves them with only rod cells, which have poorer resolution and work best in dim light. They see the world in coarse black-and-white, and they struggle to cope with bright light.
This discovery supports the idea the armadillos, sloths, and anteaters—a group collectively known as the xenarthrans—evolved from a burrowing ancestor that spent much (if not all) of its time underground. With light in short supply, these ancestral animals may have prioritised the sensitive rod cells over the sharp and colour-tuned cones.
They eventually re-surfaced and, in the case of sloths, even took to the trees. But they still retain traces of their burrowing past, including sturdy front legs, curved claws, and skeletal features that gave them a powerful digging stroke (the word “xenarthran” means “strange joints”). Anteaters use these traits to rip through ant nests, while sloths use them to hang from branches.
But they also carried their ancestors’ cone-less retinas. These, according to Emerling and Springer, might have constrained their evolution in important ways. With poor vision, they couldn’t take up many of the lifestyles that other mammals developed, like fast-running, active-hunting, or gliding. And armadillos “have minimal ability to see approaching cars when crossing roads, a fact all too familiar to residents of Texas,” says Emerling.
To read the rest of the original article, click here.