An article introducing a book by Hiroshima University professor Toshihiro Yamada sparked discussion on 5ch’s Science News+ board. Commenters questioned the very premise that “mammals are short-lived,” asking what standard that claim was even based on — but others pointed out that, once adjusted for body size, birds live 3 to 9 times longer than mammals of the same size, and examples of long-lived animals like the bowhead whale (over 200 years) and the Greenland shark (around 400 years) kept piling up. If birds, descended from dinosaurs, managed to stay long-lived, why did mammals alone evolve toward short lifespans? Theories flew back and forth — high body temperature and metabolism, selective pressure from predation, mechanisms that halt growth — without converging on a single answer.
Are mammals short-lived because of “the dinosaurs”? …The startling theory that over 100 million years of predation shrank their lifespans
Toshihiro Yamada, Professor at Hiroshima University
Bamboo that blooms once every 120 years, 7,000-year-old coral, jellyfish that rejuvenate themselves…
Are dinosaurs actually long-lived? I guess they were huge, so…
3AnonymousOct 7, 2026 21:16
Birds apparently live 3-9 times longer than mammals of the same size, so maybe that's it. I mean, even a lowly parrot lives 80 years.
4AnonymousOct 7, 2026 21:21
What's the basis for saying "mammals are short-lived" in the first place?
10AnonymousOct 7, 2026 22:03
Re: #4
You know there's a correlation between body size and lifespan, right? It's just Class Mammalia that comes up short.
5AnonymousOct 7, 2026 21:26
Besides turtles, the famous long-lived reptiles, it looks like reptiles in general outlive mammals of the same size too.
6AnonymousOct 7, 2026 21:35
Wait, is the Blue Backs series (a long-running Japanese popular-science paperback line) still putting out new titles?
27AnonymousOct 8, 2026 16:26
Re: #6
There's a new one coming out about a Japanese Nobel Chemistry laureate too. For sure.
7AnonymousOct 7, 2026 21:43
Parrots live a long time, but oddly enough lifespan doesn't scale up much even for bigger birds.
– Ostriches live about 50-60 years in the wild
– Cassowaries typically live about 30-50 years
8AnonymousOct 7, 2026 21:52
The longest-lived vertebrate is the Greenland shark, which lives in the cold deep waters of the Arctic Ocean and North Atlantic.
Lifespan: about 400 years
17AnonymousOct 7, 2026 23:03
Re: #8
Low-metabolism creatures do tend to be long-lived — cave-dwelling animals, for instance.
9AnonymousOct 7, 2026 21:53
The longest-lived mammal is the bowhead whale, with a lifespan of over 200 years.
11AnonymousOct 7, 2026 22:13
Is this Professor Tatsuo Motokawa, the "Elephant Time, Mouse Time" guy?
12AnonymousOct 7, 2026 22:29
Talk about blaming everyone else for your own problems.
13AnonymousOct 7, 2026 22:41
Don't sharks basically not have a lifespan or something?
14AnonymousOct 7, 2026 22:49
The problem is their teeth gradually fall out.
15AnonymousOct 7, 2026 22:52
Humans just started wanting to live longer because we've got more to do now. Mammals already have long child-rearing periods, and once the young mature and become independent, there's no real need to live any longer than that, is there?
16AnonymousOct 7, 2026 22:53
The Labord's chameleon has an insanely short lifespan… just 5 months!?
That one got me a little choked up.
The parent generation dies off with the rainy season, and life just keeps going on instinct alone.
18AnonymousOct 8, 2026 00:00
Maybe mammals just run out of hormones faster?
19AnonymousOct 8, 2026 01:00
So mammals mature early so they can reproduce, and since getting old and slow makes you easy prey, they end up specializing in staying young?
20AnonymousOct 8, 2026 03:45
That's kind of sad… (>_<)
21AnonymousOct 8, 2026 04:14
So longevity genes should be able to win out over short-lived genes since they mean more chances to reproduce — but predation clips lifespans short enough that the difference stops mattering. And longevity genes died out anyway because they came with some kind of high cost?
22AnonymousOct 8, 2026 05:45
When it comes to speed of evolution and quick adaptation to the environment, being short-lived is actually the advantage.
23AnonymousOct 8, 2026 05:59
Re: #22
But aren't you the one getting left behind here?
24AnonymousOct 8, 2026 11:43
But being long-lived means evolving slower, right? If we'd stayed long-lived, we might never have become an intelligent species.
25AnonymousOct 8, 2026 12:19
Repeating the birth-to-growth cycle over and over is probably energy-inefficient, so in a stable environment with no predators, no competition, and no environmental change, long-lived, efficient species win out. But that approach can't handle big changes, so weaker species or ones in unstable environments cycle through generations fast, which lets them keep adapting through evolution — that's the theory.
26AnonymousOct 8, 2026 12:39
Think of it like a car.
It's a machine, so in theory you could keep driving it forever if you just kept maintaining it.
But in practice, people trade it in.
That's because there's a point where building new is faster and cheaper than fixing the old one.
Lifespan comes down to how easily the environment or your physical traits push you toward that point.
28AnonymousOct 8, 2026 17:17
The most successful land-animal model is insects.
Vertebrates just live too long.
29AnonymousOct 8, 2026 17:23
Even among mammals, some species have generation cycles comparable to insects.
30AnonymousOct 8, 2026 17:36
For example, if women get a hysterectomy and men get a vasectomy, their lifespan goes up.
And they get faster on their feet too.
31AnonymousOct 8, 2026 22:15
I think mammals trade a shorter lifespan for keeping their body temperature high.
When it's hotter, reactions like sugar-driven protein degradation probably speed up, right?
33AnonymousOct 8, 2026 22:40
Re: #31
But then why are birds long-lived?
32AnonymousOct 8, 2026 22:23
Ah, I see.
So that's basically the difference between an engine that idles nonstop and a reptile's engine that only runs when it's moving.
34AnonymousOct 8, 2026 23:00
They probably traded for faster evolution by speeding up the life cycle.
35AnonymousOct 9, 2026 01:24
If you ask which is more successful, birds or mammals, it's probably birds.
Even bats, the most successful mammal, don't beat birds.
40AnonymousOct 9, 2026 07:46
Re: #35
We're holding back on the mist nets for you, you know! (angry)
36AnonymousOct 9, 2026 03:07
This is a comparison among the warm-blooded "winners" lineage that stayed on top all the way from dinosaurs to birds — so maybe cold-blooded animals' longevity is a separate issue entirely?
37AnonymousOct 9, 2026 06:23
Maybe the reason mammals survived the mass extinction 66 million years ago, bounced back quickly, and became the stars of the Cenozoic is precisely because they were short-lived and evolved fast. Birds survived too, but they never became the dinosaurs' true successor as the dominant group.
38AnonymousOct 9, 2026 06:35
The so-called "terror birds" were thriving, so birds kept going strong for a good while even after the dinosaurs went extinct. Mammals only really became the more successful group later in the post-extinction era.
39AnonymousOct 9, 2026 07:38
Maybe each species has a size that suits it, with a switch that halts growth so it doesn't get too big, and lifespan is just a side effect of that. Dinosaurs probably had that switch set to flip later, while small, preyed-upon mammals had it set to flip early.
Background and Key Points of This Discussion
It’s worth noting that the premise “mammals are short-lived” isn’t about absolute lifespan, but about lifespan adjusted for body size. The correlation between body size and lifespan is itself a basic principle of biology, known from works like Tatsuo Motokawa’s “Elephant Time, Mouse Time” — a book commenter #11 in the thread also brought up. What’s new about this theory is the claim that, within that correlation, mammals alone live shorter than birds and reptiles of the same size, and that this can be explained by over 100 million years of predation pressure from dinosaurs. While birds, despite being dinosaur descendants, have remained long-lived, mammals kept to a strategy of short lifespans, high reproduction, and rapid generational turnover even after the dinosaurs went extinct. In the thread, several explanations ran in parallel — body temperature and metabolism (the cost of being warm-blooded), selective pressure from predation (the evolutionary-biology idea that when external mortality is high, longevity genes gain no advantage), and the timing of growth arrest — without converging on any single answer. The question of “which is more successful, birds or mammals” is also one where the verdict can shift depending on how much time has passed since the extinction or how the categories are drawn — a point neither the original article nor the thread spells out clearly, and one readers can easily jump to conclusions about.
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