Some time ago I got into a scholarly debate with one of my mad science colleagues, who happens to also be my sibling, on the topic of dinosaur feathers.

Specifically, she rode into my secret lair on the back of one of those chickens that were genetically engineered to look like a dinosaur. Also, it was a giant monster as you can see in Fig. 1. But she left the feathers on it, which I took exception to.

Honestly, the only reason we aren’t mortal enemies is that our overlord (Mom) won’t allow it. But she’s still my nemesis, Nemesister if you will, so I switched my zapinator from kill to stun.

Yes, Dinosaur Feathers were a thing

There’s plenty of evidence that smaller dinosaurs had feather coats, absolutely.  I’m sure there were plenty of small theropods that looked like fat chickens.¹Technically, chickens and all other modern birds are theropod dinosaurs. We’ve found fossil impressions of dinosaur feathers on their bones. We’ve even found actual dinosaur feathers stuck in amber.

By the way, for no reason in particular, if anyone is selling one of those I’d love to take it off your hands. By stealing it, of course. I have no money after I bought that last private volcano island.

Younger versions of large dinosaurs may have had feather coats. If you’ve seen Prehistoric Planet, which you absolutely should, there is an example of this. In the first episode, we see an adult T-Rex and their children. The baby T-Rexes are portrayed as having feathers even though the adult doesn’t.

Big dinosaurs may have had decorative feathers for attracting mates or intimidation. There’s no reason to believe that even adult tyrannosauruses didn’t have feathered crests on their heads, arms, or tail. Tyrannosaurus did have bony crests above their eyes which seemingly serve no functional purpose beyond decoration. If bone crests were so common, why wouldn’t feather crests be just as if not more so?

Square Cube Law

But, there’s no evidence that larger dinosaurs like Tyrannosaurus rex had feather coats, at least not as adults. There are plenty of physiological reasons why large dinos wouldn’t have had big fluffy coats and didn’t look like giant fat birds.

Why not? It’s the same reason why elephants are naked.²yes, mammoths had fur, but they also lived in an icebox so I don’t think that’s a valid comparison. The cretaceous was pretty warm. The square cube law. If something gets twice as big, its surface area will be multiplied by the square of 2 while its volume is multiplied by the cube of 2. In layman’s terms, as you get bigger your volume gets bigger faster than your skin. 

If you’re the kind of person who needs me to prove that fact to you using high school geometry, then that’s great. Otherwise, you can skip this next paragraph. 

Imagine we have a magical shape whose surface area is numerically equal to its volume. This is known as an equable solid. Our example of which is a sphere with radius 3; both its surface area and volume are equal to 36π. If we double that sphere’s size, multiplying its radius by 2, we find that its surface area is now 144π but its volume is 288π. A larger number.

Thermoregulation

This fact has lots of consequences for animals, even cold blooded ones. The more guts (volume) you have the more body heat you produce. You have more cells doing metabolic things that produce waste heat. But how quickly you can cool off is based partially on how much skin (surface area) you have. Therefore, larger animals are at greater risk of overheating while smaller ones are at greater risk of freezing. That’s why mice have very high metabolisms and elephants have very slow ones. 

If you shrunk an elephant to the size of a mouse it would die of hypothermia. If you grew a mouse to the size of an elephant it would burst like a hot pocket in a microwave. And before you ask, no, I have not exploded any elephant sized mice. I only use guinea pigs.

This is even something we notice on the scale of humans. If you’ve ever lived with someone of a different build you would have fought over the thermostat with them. Men are always complaining it’s too warm while women always think it’s too cold. Skinny people are always bundling up while blubbery people can walk around outside during winter in a tank top and shorts. I know a guy who does this.

Now imagine you’re a T-Rex living in the cretaceous period when annual temperatures were always between 21 and 23 °C (70 and 73 °F) (Nordt). However, you weigh several tons. That would feel really hot. Do you really need to be bundling up in a fluffy coat of feathers? Would doing that probably kill you? And that’s just around where T-Rex would have lived. Big dinos in tropical regions had to deal with an average temperature of 37 °C (99 °F). 

Dinosaurs weren’t warm or cold blooded

You may be thinking that dinosaurs weren’t warm blooded, so this wouldn’t apply to them. Well, if anything cold blooded animals would want feathers less. Insulating dinosaur feathers doesn’t just keep heat in, it also keeps it out. Cold blooded animals need to get heat from the environment. There’s probably a reason why none of the cold blooded animals we have today have fur or feathers.

Also, dinosaurs probably weren’t cold or warm blooded. We don’t really know since there’s conflicting evidence either way. Some dino fossils have growth rings in their bones, kind of like the rings of a tree, and these show periodic times of growth. Warm blooded animals tend to grow at the same rate all the time because they’re always at the same temperature. Cold blooded animals grow faster during the summer, when they have the energy, and stop during the winter. This would suggest the body temperature of dinosaurs doesn’t stay the same, so cold blooded.

Studying the fossil oxygen isotopes can also tell us the body temperature of animals which showed them to be higher than the environment at the time. This would suggest warm blooded. But their body temperatures weren’t that hot though. Still pretty low for warm blooded animals. 

What this says to me is that they were somewhere in between, known as mesothermy. Dinosaurs may have only warmed some parts of their body, like vital organs and muscles, but not other areas. This would give them some extra endurance but not be as costly. They also could produce some body heat to keep their temperature somewhat stable, but not enough to maintain consistent body heat.

Addendum:

Were you convinced of my argument? Answer in the comments. I explained it to my Nemesister but she just started shouting “FAT BIRB TREX” repeatedly. Then her Chickenosaurus crushed my laboratory and ate several dozen of my henchmen. 

For a behind the scenes and concept art of Nemesister, check out my Twitter thread.

Further Reading:

Nordt, Lee; Atchley, Stacy; Dworkin, Steve (December 2003). “Terrestrial Evidence for Two Greenhouse Events in the Latest Cretaceous”. GSA Today. Vol. 13, no. 12. p. 4. doi:10.1130/1052-5173(2003)013<4:TEFTGE>2.0.CO;2.

“Warmer than a Hot Tub: Atlantic Ocean Temperatures Much Higher in the Past” PhysOrg.com. Retrieved 12/3/06.

Grady; et al. (2015). “Response to Comments on “Evidence for mesothermy in dinosaurs””. Science. 348 (6238): 982. doi:10.1126/science.1260299. PMID 26023132.

 Eagle; et al. (2015). “Isotopic ordering in eggshells reflects body temperatures and suggests differing thermophysiology in two Cretaceous dinosaurs”. Nature Communications. 6: 8296. doi:10.1038/ncomms9296. PMID 26462135.