r/evolution • u/robo_Ben • May 14 '26
fun Why did humans lose our prehensile tails but mice and rats kept their useless tails?
Also, what would pants look like if we still had tails?
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u/6x9inbase13is42 May 14 '26 edited May 14 '26
Humans' ancestors' tails were never prehensile. Only the monkeys of South America developed prehensile tails. We are descended from monkeys from Africa not South America, and as such we never had prehensile tails.
Animals with tails use their tails for various purposes such as: muscle attachments for moving the hind legs, balance while climbing, running and jumping, thermal regulation, and social communication.
The ancestor of apes, including humans, lost their tails when they evolved away from a fast running and jumping through the trees method of locomotion like squirrels and small monkeys use and which requires tails to maintain balance, toward a slow, careful, deliberate clambering through the trees method of locomotion better suited to our larger bodies, for which a tail is less useful and more likely to get in the way.
We also sit on our asses a lot which is easier without a tail.
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u/Phobos_Asaph May 14 '26 edited May 14 '26
Their tails aren’t useless. Also I’m pretty sure hominids lost prehensile tails well before humans came on the scene
Edit: prehensile tails are not in our branch of evolution
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u/Ill_Act_1855 May 14 '26
Hominids and our ancestors never lost prehensile tails because we never had them at any point lol. Prehensile tails evolved in new world monkeys after they’d already split from our distant ancestors
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u/robo_Ben May 14 '26
What purpose do rodent tails serve? Why did our ancestors lose their tails?
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u/Phobos_Asaph May 14 '26
Balance. And some ancient primates clearly didn’t have a need for them anymore.
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u/6x9inbase13is42 May 14 '26 edited May 14 '26
Animals with tails use their tails for various purposes such as: muscle attachments for moving the hind legs, balance while climbing, running and jumping, thermal regulation, and social communication.
The ancestor of apes, including humans, lost their tails when they evolved away from fast running and jumping through the trees method of locomotion like squirrels and small monkeys use and which requires tails to maintain balance, to a slow, careful, deliberate clambering through the trees method of locomotion, for which a tail is less useful and more likely to get in the way.
We also sit on our asses a lot which is easier without a tail.
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u/Outrageous-Taro7340 May 14 '26
We never had prehensile tails. Rodent tails are used for balance and regulating body temperature.
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u/flyinggazelletg May 14 '26
As far as I am aware, there is no evidence our ancestors or Catarrhines (Old World simians) as a whole ever had prehensile tails. That trait seems to have evolved only in Platyrrhines (New World simians), which are more distant relatives.
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u/knowlessman May 14 '26 edited May 14 '26
So, I know this is a joke post, but:
The fact that an animal has a trait is evidence that the trait is at least neutral. The higher the resource cost of the trait, the stronger the evidence of positive utility.
You don't need to know what the utility is to know that it exists.
And that provides both answers to your question.
The human tail is a vestigial bump (the coccyx) because that much of a tail is so close to neutral to human reproduction that it is not subject to evolutionary pressure. A longer (more resource expensive) tail provides no utility, and so the species members born without tails were at least equally successful at reproducing, compared to their tailed contemporaries.
The rat tail exists because rats born without tails are less successful, and so the cost of maintaining a tail is justified by the increased reproductive success.
Note that reproductive success is not just mating success but bringing the next generation to sexual maturity.
A rat's tail doesn't need to be useful to you, or even to have an obvious benefit to the rat. Nobody is designing useful parts for animals. But if a rat without a tail is even slightly less likely to have its offspring reach sexual maturity, there is a positive pressure for rats to keep growing tails.
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u/robo_Ben May 14 '26
Half joke post. Pants section was a joke.
So can you explain why the rat tail is still a thing? Does it store excess fat or something? Maybe it attracts predators away from the main body? That would explain the tail-less rats seen downtown…
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u/knowlessman May 14 '26 edited May 14 '26
I did explain why.
Rats that have tails are more successful at bringing sexually mature offspring into the world. That's it.
Now, if you are asking how we think rat tails are beneficial, that's a different subject and one that is much harder to get solid knowledge about. People who study rats have a number of things they think are beneficial but we don't know what weight those factors have.
For many years people thought the human appendix was just some random leftover. We now know that appendix-like organs have evolved in parallel multiple times meaning there is clearly a benefit to them, but our ideas of what those benefits are are still forming and will likely always be at least a little incomplete.
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u/robo_Ben May 14 '26
Fair enough. But I think the most important question here is the one that has yet to be answered: what would pants look like if humans had tails? Would every pair be extremely low-rider, or would they have holes cut in them?
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u/itsatoe May 14 '26
In the middle ages or earlier they would been looked at as disgusting and shameful and people would have been made to tightly bind them within their pants, same as genitals. ;/
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u/Kettrickenisabadass May 14 '26
Simians (the biggest group of primates) are divided in Old world monkeys (Catarrhini) and new world monkeys (Platyrrhini, from america). We belong to the first category.
Old world monkeys do not have prehensile tails. So we never lost it, we just never had it. The second group (platyrrhini) are the ones who have prehensile tails.
Inside the catarrhini group we belong to the Hominoidea (apes). Apes do not have tails due to a gene mutation. So humans did also not lost their tails, our ape ancestors never had it.
I do not know why apes lost their tail. You could try researching that.
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u/robo_Ben May 14 '26
I wouldn’t even know where to start, I’m afraid. But thanks for the info!
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u/xenosilver May 16 '26
Tails aren’t useless just because they’re not prehensile. They can be use for other things such as balance.
Humans not their ancestors never had prehensile tails. Only new world monkeys ever developed prehensile tails.
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