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Comment by fouc | Hacker News Reader

> the need to stay warm sets a floor for the body size of oceanic mammals, while the need to eat sets a ceiling. And the gap between them, Gearty found, is surprisingly narrow—and far more so than on land.

Interesting. If the range is so narrow for mammals, how did Dinosaurs get big?

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1. The snippet you're quoting is about marine mammals

2. Dinosaurs were not aquatic

3. The biggest aquatic reptiles were much smaller than modern whales (plesiosaurs and ichthyosaurs topped up around 15m long and 2 tons, which is 2x the length but 1/3rd the weight of a male Orca and nowhere near large whale league)

4. Dinosaurs weren't necessarily homeotherms and may have thus had a larger (but lower) range of efficiency

5. As the snippet notes the upper bound has to do with food limitations, and large sauropods largely evolved ways to mow down ridiculous surfaces (and thus amounts of food), which could have combined with higher plant growth/density from higher O2 levels to put a higher cap on food issues

Looks like there is a small typo: should read “higher in CO2” whouldnt that produce faster plant growth?

Your point of #3 is probably wrong. It’s harder to get fossils of aquatic creatures so there’s likely a vast number of animals that we will never know about. Saying with certainty that the biggest aquatic reptiles were smaller than whales is a logical fallacy. Only the ones we know about are smaller but we don’t know if there ever were, we might get lucky and find a fossil that is larger.

I don't think you are correct. Plenty of places that used to oceans and seas have become land. This has given us access to aquatic fossils.

Dinosaurs were not mammals. The jury is still out on whether they were warm blooded (but most people agree that not quite as warm-blooded as mammals). The upper limit on size (according to the article) is defined by food availability and energy consumption, so I would guess food was simply more plentiful, or their energy demands were lower (due to lower core temperature). Thus they grew bigger.

Also, average ambient temperatures in the Jurassic were significantly higher than today - up to 8C warmer according to some estimates, and even higher in the Cretaceous.

Warm blood was less of a requirement for dinosaurs than it would be today.

I don't understand how mammalian evolution worked through this period. Temperatures rose even further after the KT crash, so there would be no evolutionary benefit to being warm blooded. And yet - here we are.

It was probably a big advantage for nocturnal animals. And lots of mammals ancestors are believed to be nocturnal during this ancient ages.

A couple people have already hinted at it but self-regulating body temperature is a big benefit for crossing into non-native environments.

> Also, average ambient temperatures in the Jurassic were significantly higher than today - up to 8C warmer according to some estimates, and even higher in the Cretaceous.

Note that this is not necessarily an advantage for large land-bound animals, an elephant's biggest thermal issue is getting rid of heat not producing it.

Surely there degree (no pun intended) to which a species can be"warm blooded" is a gradient. Even fractional presence of temperature regulation could have allowed for species to live in colder parts of the planet.

My recollection of theory is many mammals (that we know survived to evolve) at the time of the KT crash were tiny, and burrowers and able to use both of these (less exposure to damage from initial strike and need less resources to survive/procreate) to out survive the dinosaurs.

The variable he's not talking about in that equation is body temperature. If you cut the difference between body temp and ambient temp, you proportionally cut energy needed to maintain that difference. I wonder what fundamental physiological force is keeping these mammals from evolving a lower body temperature ...

Isn't it the temperature needed for enzymes to properly work their metabolic magic (1) temperature helps reaction rates 2) enzymes need a certain temperature to maintain their shapes)

It affects peak efficiency of enzymatic reactions, but that's a tradeoff because now the species has to maintain internal temperature within a narrow band or they die. A common alternative is to allow more fluctuation of core temperature at the cost of a lower overall efficiency. That's basically mammals and birds versus reptiles and fishes.

But enzyme production mechanisms are subject to evolution too, no?

But also to basic thermodynamic considerations. As far as chemical systems go, mammals actually operates near the low end of temperatures.

Seems dinosaurs were somewhere between cold-blooded and warm-blooded, I guess they need less food for a given size compared to mammals.

Not just less food - they produced less heat. A mammal the size of a dinosaur would not have enough surface area to get rid of the internally-produced heat.

Many reasons, but one reason cited for huge dragonfly-like creatures was the richer oxygen content of the air, allowing more primitive lung-like systems to support larger bodies.

Reptiles need much less food than mammals.

But birds need more food than mammals. And birds are a group of dinosaurs. Under some classification systems dinosaurs (including birds) are in the reptile tree, but in others they aren't.

Birds need more food? Where does it say that? Do they have a less efficient metabolism or digestion maybe?

They are smaller, have higher body temperature and use a more energy intensive way of moving. They're necessarily consume more food.

It's probably reversed if you compare a small bat to ostriches, but it's true in general.

Bats aren't birds.

Rexxar's point was about mechanisms of locomotion and body size. While we generally expect birds to have a faster metabolism than mammals because they're small and because flying takes so much energy, the reverse would probably be the case when comparing ostriches and bats. The ostrich is a 250-pound non-flying bird, and bats are very light flying mammals. In that case, we expect the mammal to have the higher metabolism.

I've also wondered if non-flying birds defecate less often, and flying mammals more often. I always assumed that birds emptied out so frequently because it's important to keep your weight down if you're flying, and much less important if you're walking.

Flying takes a ton of energy. Hummingbirds are an extreme example but they have the highest metabolism of any animal and can eat multiples of their body weight in food each day

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