Aquatic lizard: Is being warm blooded essential?

Question

Now, I know that most likely this 50 foot long aquatic lizard I mentioned in a previous question of mine would give live birth because reptile eggs and in fact any eggs really can't keep saltwater out. Saltwater could damage the embryos. But reptile eggs would inflate further once saltwater got in there and possibly burst because they are leathery and because of osmosis.

Of course, burying the eggs would minimize this effect, but then comes the question of can the lizard hatchlings dig through before they die from lack of oxygen? And besides, if the animal is so big anyway, why not have it give live birth? It may be uncommon amongst today's reptiles but it is likely that a lot of big marine reptiles in the past gave live birth.

But on to my question. Would this lizard have to be warm blooded?

I think so because it is big and aquatic (more than twice as long as the biggest crocodile species that still exists). But there are a few advantages to being cold blooded, like all of today's reptiles.

For one thing, at adult size, this lizard would probably be taking on mature sharks and immature whales. That is a lot of food. Based on the weight of a great white shark and the amount of calories in 4 ounces of raw shark I estimate the number of calories in a single shark to be 27,636,000 calories.

That is a lot. I'm probably way off but it would still be millions of calories in a single shark. Now, this could be metabolized more quickly if the lizard was warm blooded. But who knows how far the lizard will have to travel to find another food source? Being warm blooded might actually be a disadvantage in terms of metabolism if the lizard has to travel miles and miles to another food source.

Disadvantages of being cold blooded are many. If it goes after a whale, it might get so cold that it brumates underwater. Plus there is so much heat being radiated away that, even in a temperate climate, it still might get cold to the point of brumation. None of this would happen if it was warm blooded.

So is being warm blooded essential to keep it from losing too much heat or would being cold blooded be viable, even in a cold climate, like where it might go after a whale?

Answer 1

First of all I am going to say that being warm blooded helps greatly with cold climates. However whales are native to virtually all regions of ocean being the highly adaptable creatures they are. A creature very similar to the one you describe is the mosasaurus; which was a species of aquatic carnivorous lizard that lived in the Late Cretaceous period.

They were on average 15 meters (49.2 ft) long and weighed 14,000 kg (30865 lbs),they were also (believed to be by evidence) warm blooded. So you actually have a real existing creature as a handy model to base yours off of. Being warm blooded definitely helps in the ocean,as the higher metabolism allows greater speed and strength. Like how Orca can easily kill great white sharks. Additionally it tends to lend towards greater intelligence, although that is not concrete and open to interpretation.

So in summary, yes warm blood will help these creatures. I recommend researching mosasaurus and other ancient marine reptiles for reference. Also in regards to worries about food,they (like other marine creatures) will have layers of blubber to store nutrients and insulate them from the cold of the ocean. Check how sperm whales hunt (they are much heavier than these creatures and need significantly more calories) to figure out how your lizards will hunt. The ocean is rich with life,there will be plenty to eat. (unless you change that fact)

Answer 2

Why not both? For example the Tuna is cold blooded like all fish but they have a special adaptation in their organs that means they are as close to warm blooded as they can be.

Great white sharks have a similar thing, which is why they thrive in areas like South Africa where the water is cold but the seals are tasty.

Answer 3

No, they do not need to be warm blooded.

Whale sharks average 9.8m long and 9000kg. They are cold blooded, and regulate their temperature by swimming close to the surface. This is demonstration that it is not at all necessary to be warm blooded to attain great size in an ocean environment.

Different exotherms have different tolerances for cold weather (and fully exothermic fish in particular can be found in very cold environments). This indicates that there are adaptations available to reduce the effects and/or likelihood of brumation occurring. I expect your sea-going reptile takes advantage of some of these if it is exothermic.

However, there will be advantages and disadvantages to being warm or cold blooded that may make it more or less likely your reptiles are cold blooded.

Pros and cons of exothermy

The one really big positive for exothermy is that it is very efficient in its use of resources. By being exothermic, your lizard would be able to go very long periods between feeding. This would be very useful if it is hunting comparatively rare big targets like whales and sharks. Kills would be few and far between, so it would need to be able to tolerate long periods of starvation.

The cons for your reptile would probably be reduced endurance and/or speed, and reduced ability to function in cold environments (arctic seas, or deep diving). These make chasing high-endurance, cold-water adapted and deep-diving whales somewhat tricky. They would need to ambush whales in order to make a successful kill.

On the diving front, being very large would be a benefit to your reptiles. If they are sufficiently insulated, they could maintain their bodies at surface temperature for quite some time while diving as the bigger they get the more mass they have compared to surface area.

Pros and cons of endothermy

Being able to regulate one's temperature flips these over. You can sustain activity for far longer and can survive in the coldest of climates with sufficient insulation. However, it comes at a big cost in energy requirements.

Again, being large helps with heat retention, but they will still be using quite a bit of their energy intake to regulate their temperature. This would make them better at hunting whales and sharks, but they would have to catch a lot more of them and more regularly than otherwise. This is an issue as whales and sharks tend to be at the top of their respective food chains so are not typically as numerous as the smaller fish and crustaceans they prey on.

The middle-ground: Mesothermy

Exotherms are animals that have no internal method of maintaining their temperature. Endotherms are animals that can regulate their temperature using internal metabolic processes. Mesotherms are animals that are able to somewhat regulate their temperature through other methods.

Great White Sharks utilise the heat generated by their muscles to maintain a temperature higher than the surrounding water. Your cold-blooded reptiles would benefit greatly from a similar adaptation as it would allow them to better pursue whales and sharks while retaining the efficiency of exothermy.

So what's more likely?

You have a few likely options here.

The first is an exothermic oceanic roamer (or localised species) that hunts by ambush. It will go a long time between kills, and may well be a scavenger as well as actively hunting. Catching big prey will have a high failure rate, but it is able to sustain prolonged periods of starvation. It will also have a restricted range to warmer tropical waters, or the surface of warmer temperate seas.

The second is a mesothermic oceanic roamer that is able to hunt better by endurance, and able to survive in much colder seas. In reality, it's not too far different from an air-breathing great white shark.

The third is an endothermic localised species. It is more efficient than the others at hunting whales and sharks, but is unable to sustain itself in the vast, empty expanses of the open ocean. It will be found where there are high concentrations of its prey species. It may also migrate to follow different populations of its prey species, or remain in one place and prey on a number of different species (say, if there is an area that whales use to give birth and nurse their young).

tl;dr They can viably be endothermic, exothermic or mesothermic, but these will change its behaviour and ecology. Of these, the mesothermic option has the closest real-world analogue so is probably the most likely.

Answer 4

First of all let me calculate your lizard's caloric requirements. Mosasaurus hoffmani also a 50-foot aquatic lizard, is estimated to have weighed 5,000 kilograms. Assuming your lizard is of similar mass, we can calculate its metabolic rate.

The metabolic rate for a 5000 kg reptile, rounded to the nearest whole number, is 5,946 kcal/d. Let's assume your estimate is correct - the lizard has to eat one mature shark roughly every 5 days. Anyway, 5,946 kilocalories a day is a high metabolic rate - twice as high as that of an adult male human.

Therefore, I can conclude that sheerly because of its size, your lizard will have to be warm-blooded.