Hey guys so I've posted on this site before and I've finally made an account so I'd like to ask again about mine and my friend's webcomic WIP. Biology wise, I've made three main groups of animals, a four-limbed, mammal-like group, a six-limbed group that is quite mammal-like but also very insect-like, with adaptable limbs, and of course the obligatory insect/arthropod group.

The group I want to focus on is the first one however, as the main character's companion and also the sentient race fall under this group.

To elaborate, they have smooth, sometimes scaled skin (scales grow in patches and lines in certain places according to the needs of the creature), no feathers or fur and they can get quite large. behaviorally, they are also quite like mammals, deer-like and cow-like herbivores are abundant and predators most commonly rely on stalking and capturing instead of lying in wait. Their environment, or at least the one touched upon in the comic, will be a fairly stable, fairly even temperature plains and forest. Of course its an entire planet so there are deserts and tropics but the planet does not really have oceans, more like large seas. The plains and forest are the focus though, the mammal-like group will mostly be on the grass plains.

Of course that's all quite general, being an entire group of animals, but it's the most accurate mode i can make. I know it's a broad question but I need help deciding whether they are ectothermic or endothermic, maybe even in-between (so far I'm leaning towards the last two).

  • $\begingroup$ can you describe what kind of environment they live in because there is no point saying it is a cold blooded animal but everywhere is just ice ;) $\endgroup$
    – user6760
    Oct 15, 2015 at 8:07
  • $\begingroup$ edited hope that helps, it's still in early stages. $\endgroup$
    – XenoDwarf
    Oct 15, 2015 at 8:31
  • $\begingroup$ It's endothermy and ectothermy, not exothermy. If you are talking about warm and cold blooded creatures. I think you should edit it again because exothermy is totally different (chemistry related) phenomenon and it will throw the posters on a totally different track. $\endgroup$ Oct 15, 2015 at 8:53
  • $\begingroup$ sorry i thought it was ectothermy but it corrected me saying it wasnt a word so i thought i was mistaken. Thanks for the help ill edit it $\endgroup$
    – XenoDwarf
    Oct 15, 2015 at 8:54
  • $\begingroup$ I just edited it :) $\endgroup$
    – Tim B
    Oct 15, 2015 at 8:55

2 Answers 2


The choice of metabolism type (warm or cold blooded) depends on the activity type of your creatures. Endothermic (warm blooded) creatures tend to consume dozens of times more energy than ectothermic (cold blooded) creatures and are also far more active.


If you want your carnivores (meat eaters, hunters) to be active predators and need to hunt a lot, then you have to make them warm blooded. A python or boa constrictor eats only once in 3 months or so and then goes on a long sleep. Other snakes, lizards and alligators (all cold blooded reptiles) eat far less than mammals which are warm blooded and far more active in their environments.

This gets all the more important when your carnivores are large sized (10 feet or more). Such creatures would need a lot of warming up before they could get active on a cold winter morning. You don't see snakes out in the wild during winter as the temperature is too low for them to be active at all. In contrast, wolves, lions and other mammalian predators are active year round because of their warm blooded metabolism.

The summary of all this is that if you want active, agile predators that are often hungry (need to hunt at least three times per week), then you should go with a warm blooded metabolism for them.


Now comes the issue of herbivores. Here you have more choice. Some herbivores (take fore example, elephant sized ones) are so large they don't need to run to protect themselves from predators. Due to their low energy requirements (lazy lifestyle), these can afford to be cold blooded. Smaller herbivores (anything that needs to run to protect itself against predators) would definitely have to be a warm blooded creature.


For big, huge, giant animals like Sauropod Dinosaurs, things get a bit complicated. They are so huge that the fat layer(s) on their muscles protects their inner organs from cooling down even when the temperature is too less. This shielding effect provides them with a sort of semi-warmblooded metabolism which is known as gigantothermy.

Giant carnivorous dinosaurs such as T-Rex, Giganotosaurus, Spinosaurus, Carnotaurus etc are also thought to be semi-warmblooded if not completely endothermic.

Growth Concerns

Another important factor determining warm or cold blooded metabolism is how fast you want your creatures to reach adulthood. For example, for creatures such Bruhathkyosaurus, scientists estimate that if they were purely cold blooded, their hatchlings would require more than 100 years to reach adulthood. However, if their bodies incorporated any type of warmbloodedness or semi-warmbloodedness (e.g. gigantothermy), then the hatchlings could reach adulthood in ~35 years. Now that's some serious difference!

Feeding Concerns

A purely warmblooded creature requires nearly 4 times the food that a cold blooded creature of the same type and weight requires. Elephants spend most of their time in feeding. Now think about a sauropod dinosaur and imagine how much it would need to eat, if it were a purely warmblooded creature? Also consider how much vegetation is available in the region. A group of warmblooded sauropods, the size of Seismosaurus, Diplodocus or Amphicoelias would literally strip an area (of 2-3 miles) clean of all vegetation in less than a week. Also consider how much dung they would spread in the region due to their fast digestive systems. I guess you don't want to have that in your environment. Shit happens

My Recommendation

  • If you have active carnivores, they have to be warmblooded, no matter what their size.

  • If your 4-legged mammal-like creatures need to run to protect themselves, they should be warmblooded.

  • All herbivores which are just too large to be in the scope of any predator, should be placed under gigantothermy (basically cold blooded, but so large that their internal body temperature remains nearly the same because of thick fat layers).

  • You CAN have creatures which switch between cold and warm blooded metabolisms as they grow from infancy/hatchling to adulthood. If you have elephant sized coldblooded/gigantotherm creatures, I suggest you keep them warm blooded in their juvenile years and then gradually switch them to coldblooded as they reach adulthood. This would simply mean that they produce less heat from their food. Although no examples exist in our world, it is theoretically possible. This is specially important if you don't want to put your creatures in juvenile times for dozens of years (like the sauropod dinosaurs).

  • $\begingroup$ wow great answer, i knew id be putting the herbivores in warm-blooded, as you said it makes sense, and the creatures wont be getting to sauropod size in most cases. i guess i should have made the predators the focus, as they are the ones i unsure about. if i go by your guidelines then ill have to agree that warm-blooded is also the best option for my carnivores. $\endgroup$
    – XenoDwarf
    Oct 15, 2015 at 9:38
  • $\begingroup$ You cannot have primarily coldblooded predators for warmblooded prey (a few crocodiles or large snakes don't count). So yes, if your herbivores are warmblooded, then your carnivores have to be warmblooded too. A predator must be able to overtake and overpower its prey which means the predator must be physically stronger and faster. $\endgroup$ Oct 15, 2015 at 9:51

The lack of feathers or fur would suggest warm blooded as those are adaptations used by warm blooded animals to regulate their body temperature. On the other hand in a stable warm environment that may not be needed. For example elephants have neither.

You should look at the behaviour of your animals. If they are nocturnal or early morning they pretty much need to be cold blooded to explain that.

If they bask in the morning to warm up then are active during the day then that also says cold blooded.

Either would certainly be plausible, however if you want them to be sentient then I would suggest warm blooded. The variations in body temperature would effect their thinking.

  • $\begingroup$ I think you have reversed endo and ecto therms in your answer :) Warm blooded creatures are endo and cold blooded are ecto therms. You don't have cold blooded creatures active in the night or early morning. $\endgroup$ Oct 15, 2015 at 9:31
  • $\begingroup$ @YoustayIgo Typing answer in a hurry means mistakes :) Made it clear. $\endgroup$
    – Tim B
    Oct 15, 2015 at 9:34

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