Sloth and armadillo vision pseudogenes suggest history of underground lifestyle

“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.” [1]

“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.” [2]

“Infuriated male [sloths] try to hit each other when they are still distant by more than a metre and a half.” [2]

It doesn’t take an expert to read these quotes and recognize that sloths and armadillos have terrible vision! Taking advantage of this knowledge, I recently published a paper [3] where I examined the genes required for cone-based (i.e., bright light) vision in the nine-banded armadillo (Dasypus novemcinctus), Hoffmann’s two-toed sloth (Choloepus hoffmanni) and an extinct ground sloth (Mylodon darwinii).

What I discovered is that all three species have deleterious mutations in several cone-based genes, rendering them rod monochromats. Rod monochromats are predicted to have zero capacity for color vision and an inability to see in bright light conditions.

We had access to DNA for some more sloths and armadillos, so I sequenced fragments of a gene crucial for cone function (PDE6C). I confirmed that Linnaeus’ two-toed sloth (Choloepus didactylus), the pale-throated three-toed sloth (Bradypus tridactylus), big hairy armadillo (Chaetophractus villosus), six-banded armadillo (Euphrates sexcinctus), southern three-banded armadillo (Tolypeutes matacus), and giant armadillo (Priodontes maximus) are also rod monochromats.

 

Additionally, several armadillos and all four sloths share deleterious mutations, respectively, suggesting that this gene (PDE6C) was inactivated in common ancestors of these lineages.

F1.large

Examples of deleterious mutations in various cone genes.

Why would sloths and armadillos lose their cone-based vision? Rod monochromacy has been found in animals that dive or live deep in the ocean or underground. The prevailing hypothesis is that natural selection wouldn’t maintain the ability to perceive bright light in animals that have lived in dark conditions for millions of years. If you live in complete darkness, what good to you is color vision?

In my last post I described how armadillos, sloths and anteaters, collectively known as xenarthrans, are more genetically similar to each other than to other mammals. An anatomical feature that unites them are extra articulations in their spine (“xenarthrous articulations). This extra bracing of the spine is useful for digging. Additionally, xenarthrans have a pelvis fused to their spine

_original

long curved claws

xenarthran_claws

and a second spine (=ridge) on their shoulder blade which allows for extra muscle attachment for digging

14683989565_898866722c_b

The retention of these traits in modern and extinct xenarthrans suggests that their last common ancestor was a digging animal. Further evidence of this is that the earliest known xenarthran had limb bones that were proportioned for digging.

Add all of this up: I proposed that the earliest xenarthrans were subterranean, and most remerged to the surface later and adapted to new environments. This would explain the absence of cone-based vision and is corroborated by shared anatomy and the fossil record.

Questions for Creationists

Why would God create sloths and armadillos with such terribly poor daytime vision, especially when so many of them are active during the day? Why would He create them with cone pseudogenes when it would have been simpler to create them with no cone genes at all? Why do two-toed, three-toed and extinct sloths share the same deleterious mutations in a cone gene (PDE6C)? Why would God create sloths with features related to digging when they live in trees?

References

1. Newman HH. 1913 The natural history of the nine banded armadillo of Texas. Am. Nat. 47, 513–539. 

2. Goffart M. 1971 Function and form in the sloth, vol. 34. Oxford, UK: Pergamon.

3. Emerling CA, & Springer MS. 2015 Genomic evidence for rod monochromacy in sloths and armadillos suggests early subterranean history for Xenarthra. Proc. R. Soc. B282(1800), 20142192.

National geographic reporting on my paper: http://phenomena.nationalgeographic.com/2014/12/24/sloths-and-armadillos-see-the-world-in-black-and-white/

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3 thoughts on “Sloth and armadillo vision pseudogenes suggest history of underground lifestyle

  1. Pingback: Biogeography: Xenarthrans originated and have (mostly) lived in South America | Evolution For Skeptics

  2. Pingback: Where did the glyptodonts go? | Evolution For Skeptics

  3. Pingback: Ontogeny recapitulates phylogeny: Fetal sloth teeth point to evolutionary history – Evolution For Skeptics

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