Hacker News Reader

Comment by cperciva | Hacker News Reader

How do they know the reasons beavers build dams?

Obviously it's a shorthand, but the interesting thing here is that different biologists come up with different implicit reasons. To an evolutionary biologist, the reason for a behaviour is how it grants an evolutionary advantage, so to them the reason for building dams is to create deep water which provides protection from predation.

But behavioural biologists look at what triggers particular behaviours, and we actually know this one thanks to experiments: Beavers hate the sound of running water. If you set up a speaker and have it play running water sounds, beavers will pile sticks on top of it! There's no grand dam-building plan; just mindlessly placing sticks wherever the noise is coming from.

View on HN

> we actually know this one thanks to experiments: Beavers hate the sound of running water. If you set up a speaker and have it play running water sounds, beavers will pile sticks on top of it

That isn't actually evidence that they don't like the sound. They might like placing sticks where they hear it.

One has to wonder what our "hate running water" thing is - ie something we feel compelled to do which seems perfectly sensible to us but is really just an evolutionary gimmick that some other species would think is weird.

Being predisposed to distrust people who look different is one example from the recent past. Back when people lived in frequently-warring tribes, someone who looked different (either due to genetic differences or cultural differences) was far more likely to kill you.

As far as I'm aware, cats don't care at all what colour fur other cats have.

Actually in hunter-gatherer societies inter-tribal violence is extremely rare, with intra-tribal violence being way more common. One has to remember that in hunter gatherer societies concepts like territory and property are not as important as in settled societies, and population densities are incredibly low. It's a very rare occasion when you interact with anyone outside your immediate community, and when you do it's almost certainly an interaction with a neighboring community that you've been collectively living near and interbreeding with for longer than anyone can remember. Violence in these societies tends to be part of personal feuds between individuals.

Encountering someone from far enough away that they would be part of a distinct genetic or cultural group was basically unheard of. That's not to say it didn't happen at all, there would be migrations and smaller groups would potentially travel long distances out of necessity, but it simply was not a part of normal life. The idea that a fear of different looking people provided any meaningful benefit in the past simply does not pass the sniff test.

Right, when I said warring tribes I meant the period around 4-8 kya where valuable territories were being settled.

That would not have an affect on human evolution. The time period was too short, too recent, and that was a series of independent local events.

Further, wars were almost always between regional neighbors then as well.

I'm not convinced. The black death had an effect on human evolution -- a single event with high mortality can be all it takes to shift the genome.

But that is directly due to the level of death. We have no evidence that half of the tribes in an area were wiped out.

I can see how that is a mechanism for fixing leaks, but not for building a dam from scratch.

The noisiest spot in the absence of a dam is going to be where water is moving the fastest -- which is the spot with the smallest flow cross section, which is also the best place to drop sticks to impede the flow.

ya but why do they hate the sound of running water

maybe it sounds like no protection from predation?

Because the beavers who didn't were more likely to die.

And they were more likely to die because there wasn't a body of water and pile of sticks nearby to hide and live in.

Reply on news.ycombinator.com