1- A Haast's eagle sharing a drink with a moa. Inspired by the recent depictions of tyrannosaurs drinking alongside herbivores in paleomedia.
2- A hypothetical ancestral amniote. It appears amniotes were ancestrally endothermic, so here's a hypothetic fuzzy one. Note that I don't necessarily believe they were coated in fur, just a fun hypothetical exercise, though bird scales are actually stunted feathers so what would that mean for the scales of lepidosaurs, turtles and crocodilians among others?
So, of the creatures you’ve invented for your world - are they ectothermic or endothermic?
“Cold-blooded” and “warm-blooded” are the colloquial terms for it. They’re... not exactly wrong, but incomplete. A “cold-blooded” critter simply does not have internal workings devoted to keeping it inside a specific band of temperatures No Matter What. (Though some may in fact be able to keep parts of their body much warmer or cooler than their surroundings - see the swordfish, among others, with their rete mirabile of blood vessels to keep their eyes warm in the sea to spot prey.) On the other hand a “warm-blooded” critter (birds and mammals being our key contenders) not only has the inner workings to do so, it absolutely must, or perish.
If you’re not sure what your critter should be, consider that these are two different ways of adapting to life with distinct advantages and disadvantages. Sort out which of those fit your critter best, and you should know what to pick.
First, let’s take ectothermy. As have most animals in the history of the world. It’s the most common way of life for a reason: it works. Since ectotherms rely on the outside world to keep them warm or cool, they don’t need to spend any energy on maintaining a constant internal temperature. This makes needing to eat much, much less of a worry; critical if you’re in a low-prey environment. Full-grown Nile crocodiles can pretty much live on only two wildebeest a year... and guess what? The herd’s migration usually takes them across particular rivers twice a year. Convenient!
(Herbivores prey on plants, algae, etc. Those, too, can be in short supply.)
Three distinct disadvantages, though. First, you have to make proteins that function at a wide range of temperatures; this costs extra resources. Second, your eggs are usually more temperature-sensitive than you are. Finding the right environment for them is critical. Third... if you really really need a fever, say to kill off a bacterial or fungal infection, you have to figure out how to get your environment to do the work. You have to give yourself a behavioral fever. Alligators, crocodiles, and their relatives will all do this by basking in the sun longer and more often than normal.
So. Endothermy. Apparently pounced on by both proto-mammals and some of the dinosaur lineages. This is a very resource-intensive strategy. You have to eat; you have to eat frequently. And if you get just a little too hot or cold inside, you die. A snake that drops down to 50 F inside may be perfectly fine, though it probably doesn’t want to freeze. A human whose internal temp drops to 85 F? That is nightmare territory and heading toward death. Why would any creature do this?
Look at the three disadvantages of ectothermy. An endotherm doesn’t make proteins that function across the temperature spectrum; it doesn’t have to. All its building blocks can be finely tuned to the temperature it’s supposed to be, with a little wiggle room in case of, you know, a smidge too cool or too hot. This saves a lot of resources. Sensitive eggs? Can be kept with you, either in a nest you keep warm or even - gasp - internally. (Mammals are so metal.) Fungus and bacteria? Get hit by a blazing reception of fiery fevers and fine-tuned immune systems. They can kill us, but numbers-wise, ectotherms are at far greater risk.
And evolution, very often, is a numbers game. It doesn’t take much of an advantage to pull ahead. It only takes a small, constant advantage over time.
Of course, there are exceptions. Like hibernating hummingbirds (some let their temp drop to 40 F every night) and pythons shivering to keep eggs 5-10 degrees warmer than outside air (as the Everglades snarl in frustration). And then there are paleontologist speculations that the really big sauropods and some other dinosaurs had mesothermy, which doesn’t seem to exist on the planet today because the (probably asteroid impact) disaster at the K-T boundary wiped out every critter past a certain size, and you had to be sauropod-size to make it work.
Consider all these, when you make up a critter for your characters to deal with. It matters to how they behave - and that makes a big difference when your heroes might have to run through a pack of them!
I'd like to ask a question about archosaurs and warm-blooded vs cold-blooded animals. I know that determining whether an animal is warm or cold-blooded from fossils can't be easy but I was curious about how birds and their ancestors ended up warm-blooded instead of cold-blooded like their crocodile cousins. Do we know what the first warm-blooded dinosaur was? I always thought of mammals being unique in that we are warm-blooded but that can't be true since birds are too!
so actually, there's decent evidence that archosaurs were ancestrally warm-blooded, and crocodilians lost that because they didn't need to expend that energy anymore! They have a four chambered heart and similar blood cells to birds, which are warm blooded features
as for nonavian dinosaurs, we think them and their closest cousins, the pterosaurs, were all functionally warm blooded. IE, if they didn't maintain their own body temp like living birds and mammals, then their sheer size did it for them (gigantothermy)
Mammals are animals that are (mostly) covered in hair and that nurse their young with milk. They include duck-billed platypuses, mice, elephants and human beings.
It is true that all mammals can produce heat from within, a talent known as endothermy. This means that most mammalian species do indeed have warm blood. They maintain a high and fairly constant body temperature, which allows them to function efficiently across a range of conditions.
This is why entry-level textbooks often refer to mammals as being "warm-blooded". This distinguishes them from "cold-blooded", ectothermic creatures whose body temperature is wholly dependent on their surroundings.
The thing is, in biology rules are made to be, at the very least, severely bent.
There are plenty of mammals that take a much more relaxed approach to body temperature. For these "heterothermic" animals, the term "warm-blooded" does not really capture what they are doing.
They are certainly not "cold-blooded" in the same way as fish, amphibians and reptiles are "cold-blooded". But they are capable of some impressive feats of cooling.
"The more we look, the more species we find that do that," says Justin Boyles, a physiological ecologist at Southern Illinois University in Carbondale.
One of the most extreme heterotherms is the Arctic ground squirrel.
In a classic paper that made the front cover of Science in 1989, physiologist Brian Barnes of the University of Alaska Fairbanks studied the squirrels during hibernation. He found that they drop their core body temperature below zero, in one instance to -2.9C, without freezing solid.
"It's hard to get much more cold-blooded than that," says Boyles.
Kudos to Graham Humphrey and Janakie Balasuriya, who both specifically mentioned the Arctic ground squirrel.
Clearly, the squirrel is a special case. Still, many mammals are capable of some kind of chilling.
For instance, newborn mammals' body temperature is entirely dependent on the temperature of the environment. The ability to produce internal heat only kicks later in development.
Similarly, when mammals sleep their body temperature usually falls by a degree or two.
Smaller mammals – including many rodents, insectivores, bats, marsupials and even some primates – have evolved a way to push this temperature reduction much further. They enter an energy-saving state known as daily torpor.
For instance, the common blossom-bat can lower its body temperature from around 36C at night to just 20C in the day. Similarly, the Brazilian gracile opossum seems able to chill at 16C for hours on end.
In a more extreme case, Madagascar's pygmy mouse lemurwill spend around 10 hours a day in torpor, its body temperature falling below 7C.
Some mammals can enter a more prolonged torpor. We call this "hibernation" if they do it in winter, and "aestivation" if it is a summertime thing. Then, their blood runs even colder, as George Uren, Mary Wyman and Indrani Ghosh noted.
What was going on in more northern colder places where cold blooded animals can't survive while reptilian dinosaurs were the dominant thing? Were there any larger mammals or...?
Dinosaurs were warm blooded, so, they lived up there too. And in the Southern cold places.
I’m feeling curious 1 : Are birds warm blooded or cold blooded
Are birds warm blooded or cold blooded?
Mammals and birds are warm-blooded, which means that they can make their own body heat even when it is cold outside. Whether it is sunny and hot outside or there is a snowstorm and it is very cold, warm-blooded animals have body temperatures that usually stay the same.
Warm-blooded animals, such as birds and mammals, and known as endotherms, generate their own heat and maintain a body temperature independent of the environment. Cold-blooded animals, known as ectotherms, include amphibians, reptiles, invertebrates and most fish.
Endothermy is the ability of some creatures to control their body temperatures through internal means such as muscle shivering or increasing their metabolism (Greek: endon = "within", thermē = "heat"). Some writers restrict the meaning of endothermy to mechanisms that directly raise the animal's metabolic rate in order to produce heat. The opposite of endothermy is ectothermy.
Homeothermy maintains a stable internal body temperature regardless of external influence and temperatures. This stable internal temperature is often, though not necessarily, higher than the immediate environment (Greek: homoios = "similar", thermē = "heat"). The opposite is poikilothermy. Mammals and birds are Homeothermic animals.
Tachymetabolism maintains a high "resting" metabolism (Greek: tachys/tachus = "fast, swift", metabolēn = "throw beyond"). In essence, tachymetabolic creatures are "on" all the time. Though their resting metabolism is still many times slower than their active metabolism, the difference is often not as large as that seen in bradymetabolic creatures. Tachymetabolic creatures have greater difficulty dealing with a scarcity of food.
