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Long story short, a child mad scientist wants to scale up their pet dog - they read about Clifford, and wish to replicate that story, as a young mad scientist generally does. The billionaire mad scientist parents want to assist them in this endeavor as a birthday present.

They have solved many issues related to scaling an animal up, from food intake (hyper-densified nutrients in dog food) to bone durability (carbon nanotubes) to the ability of their lungs to extract oxygen from the atmosphere (bird-style lungs), and they've generally ensured that said dog will have a quality of life equal to or better than when it was smaller. The only problem they have left to solve is heat dispersal; at a certain point, once it's large enough, the animal simply can't dump enough waste heat to avoid frying itself.

How large can they scale this dog up without its body heat killing it? Human-sized? Horse-sized? Elephant-sized? Kaiju?

Assume the dog is a small, old-aged and relatively sedentary German Shepard - about 30 kilograms and 60 centimeters tall. The dog doesn't move around much before it was scaled up, and it's not really interested in doing so afterwards; as such, assume the dog isn't really going to exert itself once it's been enlarged. There's a very big difference between the heat the dog puts out while sedentary and the heat the dog puts out while exerting itself, so this lets them make the dog larger.

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  • $\begingroup$ The Blue Whale at 135 metric tons is thought to be the largest creature ever to live on the Earth - and the blue whale is warm blooded. However, water is your friend. A blue whale would not survive on land because its weight would crush its internal organs long before it would overheat. Note also that endothermic homeotherms regulate their heat. In other words, I don't think it's an interesting question to ask when it would overheat. If the creature can live at all at any size, it must be capable of regulating its heat. (I'd hate to be under that slobbering tongue when it tries to cool off!) $\endgroup$ Jan 17 at 5:27
  • $\begingroup$ @JoinJBHonCodidact Well, that's kind of a nonanswer; I'm asking how large you can make a dog-shaped entity before its body heat outstrips its ability to dump it. Elephants have big floppy ears to partially mitigate that heat; the dog does not. $\endgroup$
    – KEY_ABRADE
    Jan 17 at 5:30
  • $\begingroup$ How much are you willing to modify before you stop calling it a "dog"? You mention changing the lungs and bones - why not just change the whole thing to work exactly like a dinosaur? $\endgroup$ Jan 17 at 7:55
  • $\begingroup$ @IndigoFenix It's dog-shaped. That's about all I can say. $\endgroup$
    – KEY_ABRADE
    Jan 17 at 7:59
  • $\begingroup$ You can get more bulk if you make him a basset hound. The drool capacity is enhanced, and older bassets are incredibly good at laying there while doing absolutely nothing. Of course I wouldn't want to be in the area of a horse or elephant sized basset decided to bark (or howl 😱). $\endgroup$ Jan 17 at 10:45

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Make it macrothermic.

There is a prominent theory that the largest land animals that have ever lived - the sauropods - didn't actively generate heat in the manner that true warm-blooded animals do, but weren't strictly speaking cold-blooded either. Instead, they had low-energy lifestyles and kept warm simply by being huge. If you make the dog's internal chemistry function more like a reptile, you might be able to manage making it as big as a sauropod. As an added bonus, it will be able to get by eating less than a mammal of that size would need.

You should note, though, that a lot of a sauropod's "length" is in its neck and tail, which are both much thinner (and thus able to dissipate heat more easily) than any part of a dog-shaped giant would be. Something with less extreme proportions, like Shantungosaurus, might be a better model. Overall, assuming you can deal with the other issues of the square-cube law, you're probably looking at a dog around 10-13 meters long - roughly the length of a bus.

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If you already allow fiddling with the muscles and bones and lungs to sidestep the square-cube issues, then why not just do the same for the thermal problems?

Maybe the new tissues in the lungs are stable at 800C, and the Clifford's flamethrower almost-plasma breath is its way of cooling down?
(There's a reason why Godzilla has more impressive breath weapon the larger it becomes)

Or for more moderate scales, how about the simple expedient of adding sweatglands to the whole doggy's skin, not just cooling via panting?

Without any fiddling other than size, the limit would be about bear sized. Cooling a furred carnivore mammal's metabolism via breathing evaporation only is problematic.

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About as big as the largest living land animals, which is around 50 metric tons. At that point the square-cube law means that Clifford would be so heavy that his legs wouldn't be able to hold him up.

Why aren't I mentioning anything to do with thermoregulation? Because as an animal becomes bigger, it's basal metabolism becomes lower per unit mass. Put simply, it's more efficient to be big than to be small.

That's not to say that Clifford couldn't overheat, just that the challenge of dealing with overheating is little different for big Clifford than for small Clifford. Being a dog, Clifford would pant, much as any dog. Being bigger, he would lose heat faster, but would have more heat to lose.

For animals which must maintain a constant body temperature, the challenge is retaining heat in cold environments when small. Big animals like elephants and rhinos - or even the extinct sauropods - don't have a great deal of trouble with thermoregulation. Sure, they have thermoregulatory adaptations, but they don't need significantly more ability to shed heat as they get bigger.

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    $\begingroup$ "Big animals like elephants and rhinos - or even the extinct sauropods - don't have a great deal of trouble with thermoregulation" is laughably false, sorry. $\endgroup$
    – PcMan
    Jan 17 at 7:23
  • $\begingroup$ @PcMan I meant in comparison to smaller animals. I didn't mean that they have no problem with thermoregulation at all. $\endgroup$
    – Monty Wild
    Jan 17 at 8:54
  • $\begingroup$ Since dogs thermoregulate through panting, the square-cube law hits again. Even with basal metabolism reducing as size increases, it would have to do so at a rate such that heat generation increases slower than the size of the heat reduction surfaces - the tongue and throat - which increase at square rather than cube. $\endgroup$
    – Corey
    Jan 17 at 9:46

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