Most marine mammals like Whales, Orcas, seals, dolphins, etc are fairly large animals.
Whereas on land mammals come in all sizes like mice to elephants, why are marine mammals mostly large ?
Because water conducts heat much better than air. A warm-blooded animal immersed in water loses heat much more rapidly than it does in air. Larger animals have a lower surface to volume area and lose heat slower. Even so, they need a thick layer of blubber to insulate themselves. A small mammal simply can’t have thick enough fat to keep itself warm. The smallest mammal that spends much of its time immersed in cold water is the Sea Otter, which has the thickest and best-insulating pelt of any mammal. (There are smaller aquatic mammals in fresh water where its often not as cold as sea water.)
My guess is because the cetaceans (the most “largish” animals you’re talking about) are in the same clade (Cetartiodactyla) as even-toed ungulates (pigs, hippopotamuses, camels, sheep, goats, etc…). Pinnipeds were similar- their nearest land relatives are the bears, and they’re cousins to mustelids (raccoons, weasels, etc…)
So they were largish to begin with, and probably just evolved from there to fit into evolutionary niches left open by large fish species and large marine reptiles like mosasaurs and plesiosaurs.
Thanks – that makes sense. As I read your post, I thought of mink, which are a lot smaller than sea otters and spend most of their time in the water. But I bet they leave the water when it’s really cold, and they have extremely warm pelts, too.
And, hmm, platypus (what’s the plural?) are tiny, but they live where it’s very warm.
One factor (though certainly not the entire answer) is the physics of buoyancy.
On land, the larger an animal is, the more of its mass that is above ground level and must be supported by strong bones and strong muscles. This leads to evolution favouring smaller animals, though with many obvious exceptions.
In water, an animal can evolve to be significantly larger because the water itself can support the massive weight – that’s one fewer benefit of staying small, as compared to land mammals.
A second factor, perhaps:
As mammals are warm-blooded, it is important to keep the internal organs warm. The smaller an animal is, the more quickly it loses its heat.
Water is often fairly cold, so small aquatic mammals would struggle to keep their organs warm enough to function.
Platypodes.
In addition to the factors already mentioned, larger body size allows larger lung capacity which allows these mammals to spend more time underwater.
Thank you - that sounds reasonable. Follow up related question : Since sea turtles are diverse in size, how do they overcome this surface to volume disadvantage ?
Wow thanks. Also related question - why don’t whales get the Bends ? Or do they ?
Because they hold their breath for the dive. You get the bends when you breath in air that is compressed (and it has to be to allow you to breathe it in under the pressure of water) and then surface without allowing the gasses dissolved in your blood to come out of solution before surfacing. They “evaporate” into gas bubbles in your blood.
You don’t get the bends if you hold your breath and dive deeply either. You just can’t go as low or as long under water as a whale.
It doesn’t matter to them in terms of retaining heat because they’re cold-blooded. They don’t need insulation since their body temperature is that of the surrounding water. Unlike marine mammals, sea turtles aren’t very active and don’t need to swim fast because they eat either plants or slow-moving animals. The largest species, the Leatherback, eats mostly jellyfish.
If size is a factor in diving endurance, I don’t see why it would be for this reason. If proportions stay the same, wouldn’t lung capacity and oxygen consumption both vary in proportion to body volume?
It’s not large lung volume that lets whales hold their breaths for long periods; in fact, whale lungs are proportionally smaller than most land mammals. Some whales even exhale before diving. By about 200 meters down, the water pressure is such that air-filled lungs collapse anyway. Their smaller (proportionally) lungs are more effective at moving air, though - they exchange a greater percentage of gas volume with each breath than we do. Imagine that your every exhale was a paroxysmal sneeze that wrung all the air out of your alveoli; you’d exchange gas much faster because alveolar CO2 would be lower on average and alveolar O2 would be higher.
Whales store oxygen in their blood and muscles rather than as gas in inflated lungs. They have more hemoglobin in their blood, and their muscles contain vastly more myoglobin, which also binds oxygen. They restrict blood flow to internal organs while diving to minimize O2 consumption.
Whale lungs collapse during a deep dive? How do they deal with that?
They also have a much lower body temperature than placentals…
Actually Leatherback turtles appear to be able to keep a significantly higher[Pdf] body temperature than the surrounding water, at least the adults.
They sometimes show up surprisingly far North, where other turtles would be too cold to keep active, including round the UK on occasion. It’s probably not co-incidence that they are the largest turtle species.
There are various ‘plurals’ but most people would say ‘platypuses’ just as octopuses is generally used. No one really cares about ancient Greek rules on plurals.
Whales can and do sometimes get the bends. To the extent that they are immune, it is not because they are surface-breathers; it’s related to other aspects of their physiology.
Decompression sickness happens when nitrogen that was dissolved in your blood from the air in your lungs while you were in a high-pressure comes out of solution when you return to a low-pressure environment. It doesn’t matter whether you put the air in your lungs under low pressure and then move to a high-pressure environment (as whales do), or move to a high-pressure environment and then put the air in your lungs (as SCUBA divers do).
The rule for the size factor is called the square-cube law. Imagine a simple cube. Double the dimensions. the squares that make up the cubed have four times the area - (x^2) but the volume is the cube of the sides, 8 - (X^3); three times the size, 9 times the area, 27 times the volume. And so on. So if whale A is twice as big - twice the length, twice the diameter, etc. - it has twice the surface area, and 8 times the body mass volume. Since the body generates heat, the larger whale generates 8 times as much heat using the same body processes, and only sheds 4 times as much heat by being immersed in the ocean… and it’s insulating blubber (under-skin fat) is twice as thick. You can see why big whales can wander around the polar regions, while tiny porpoises generally stick to more tropical waters. For smaller mammals like otters, I imagine being able to get out of the water is a major advantage for heat conservation.
The same square-cube law applies on land. Weight goes up by cube, but legs to support it by area (cross-section of bones) so a land animal can only be so big. Plus, the smaller a mammal gets, the less body mass (volume) to generate heat vs. losing it through surface area. Cold-blooded insects can get incredibly tiny, but tiny mice are about as small as a mammal can get and still stay warm.
Yet another reason is simply that fat floats. Animals that would be too heavy for land life are buoyant, graceful, and fast in the water. Look at the sea lion. It staggers around on land, but once it enters the water, the ugly duckling becomes a swan.