I read this whole article and it’s still not clear to me _why_ they’re slow.
I’m genuinely curious! It seems like this article just lists a bunch of facts about sloths. But doesn’t say why being nearly blind or having a slow metabolism gives any evolutionary advantages.
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.
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.
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.
> It seems like this article just lists a bunch of facts about sloths.
It's because there's no singular reason for it. Evolution is blind (pun not intended), doesn't have motivation or reasoning. I think the article makes a good job explaining the set of evolutionary forces which likely pushed sloths into this direction.
> But doesn’t say why being nearly blind
The article mentions that this mutation already happened to the sloth ancestors, which lived on the ground. It's possible that it provided some advantage to those, or perhaps it didn't actually matter much (perhaps they were nocturnal?)
Evolution happily trades everything away that would allow to escape the niche your stuck in. And one day, you are a koala, unable to recognize leaves when they are not on branches, a planktonwhale beaching yourselves, a panda who needs electric help to procreate, or a sloth to slow and calorie low to eat. Moral of the story: High effort Low Energy diets are a evolutionary swamp - never eat all your veggies.
> But doesn’t say why ... slow metabolism gives any evolutionary advantages
I imagine the author accidentally thought it was obvious. Having a slow metabolism is an evolutionary advantage in the same way not spending money is a financial advantage. It uses little energy and energy is usually the limit on what animals can do. If you don't spend much money, you don't need to earn much and can just exist. Being a high-earner-high-spender is probably a more efficient equilibrium, but spending nearly nothing works well enough to survive and if someone can spend less than they earn eventually they get wealthy.
If sloths don't do much, they don't need to eat much to balance out the energy expenditure. If they don't need to eat much, they can take less risks and tend to survive. Being slow is one aspect of their strategy of not needing to eat much.
I read this whole article and it’s still not clear to me _why_ they’re slow.
I’m genuinely curious! It seems like this article just lists a bunch of facts about sloths. But doesn’t say why being nearly blind or having a slow metabolism gives any evolutionary advantages.