[Terr_]

The article has three headings which I'd paraphrase as:

1. With their poor eyesight, if they were fast they'd half-blindly get into trouble and fall to their deaths or whatever, so it's kinda-sorta beneficial given that other constraint.

2. Being slow-by-design reduces calorie needs, meaning they can survive eating a smaller amount of stuff and stuff that other animals also don't want as much. This lets them fit into a low-competition niche.

3. Most nearby predators are looking for fast-moving targets, so being slow helps them avoid being detected.

[ggm]

This has chicken/egg qualities. Sloths may have poor eyesight because the energy burden and genetic expression behind better eyesight doesn't pay in their niche, but that could imply it's the downgrade option after they slow down, not the input condition which led to slow lifestyle.

Similar to why some small things become giants and some giant things become smalls, it all depends on the niche, and the input conditions before transformation through time and genes.

[bradley13]

The article doesn't mention it, but I wonder if there isn't a fourth:

4. Having little muscle mass, and with more than a third of their body weight being half-digested leaves, there's not much worth eating.

Surely a sloth is not the primary prey of any predator. More of a desperation move?

[ricksunny]

(not sure why 'surely')

[Aardwolf]

Sure but why did poor eyesight evolve, what was the evolutionary advantage?

They hide from predators like jaguars, ocelots and eagles, but why did no predator that grabs them from the trees evolve?

(Just trying to learn from them and their amazing ability to stay under the radar for 65 million years :D)

[Terr_]

> They hide from predators like jaguars, ocelots and eagles, but why did no predator that grabs them from the trees evolve?

Isn't that what the eagles already do? https://www.youtube.com/watch?v=i0zrah5JEBM

[plaidphantom]

A harmful attribute on its own can be advantageous when combined with others. Think of it as finding a new "local maxima" in the sum total of evolutionary traits. Being (probably randomly) near-blind may have led to being slow and careful, which was the temperament needed to survive climbing trees, which got them away from ground-based predators and found them a lot of leaves ground sloths couldn't reach.

Evolution is basically a random-walk algorithm to find combinations that work.