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Comment by dboreham | Hacker News Reader

The tail seems like vestigial anatomy (originally useful in fish) that has three possible states in mammals: 1. Repurposed for a new function (tree swinging, signals) 2. Not much used but still in the repo (mice) 3. Garbage collected (apes).

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I'm curious about the "garbage collected" theory as there doesn't seem like a lot of selection pressure to remove tails simply due to them not being useful and consuming energy to grow. The appendix is a far more likely candidate for garbage collection as it has no obvious function and can easily cause death if it bursts.

I suspect that there's another reason that apes lost their tails - most likely related to walking on two feet.

Edit: According to this study (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6417348/), the tailed chap had difficulty/pain when sitting and during other activites

> The appendix is a far more likely candidate for garbage collection as it has no obvious function

Preserving the gut microbiome through diarrhoea and vomiting.

I didn't know about that, but it seems a poor trade-off to me. Acute appendicitis is the most common abdominal surgical emergency in the world.

Yet most people never suffer it, while I'd assume 100% of the population suffered bouts of vomiting/diarrhea. Looks like the trade-off is alright.

Yeah, I suspect a lot more people have experienced diarrhea than appendicitis. If preserving gut bacteria is important enough, then the appendix clearly does have a function.

But it might not be that important at all; do people with an appendectomy struggle more to recover from diarrhea?

In that study the tail was not innervated so there would be no way to voluntarily move the tail out of the way when sitting.

Tails are extremely useful to mice, especially for balance and climbing: https://www.theguardian.com/lifeandstyle/2019/apr/30/why--mi...

Be careful with assuming functionlessness, in general. Evolution does not like spending energy on sticky-out bits that are prone to injury, or good handles for predators to catch, unless they provide some serious advantage.

#1 covers a whole range of different possible functions, of course.

Balance and navigation (birds, cats, etc) is still somewhat related to the original function in fish.

Sexual attraction/signalling

Grabbing stuff

A third leg in kangaroos

Fly swatter (cows and other large grazers)

For kids to hold onto their parent (elephants)

I'm sure there's a lot of other creative applications of tails that I'm missing.

Kangaroos use their tails as a fifth leg - https://phys.org/news/2014-07-tale-kangaroo-tail.html

Also used to distract and confuse (deer and rabbits)

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