This is merely a hypothesis. I am a layman, my undergraduate and graduate work was in economics not biology. I taught college in an economics department, not a biology department. So a speculative hypothesis is about all I am qualified to offer.
However, several University of California biology professors have said the general thesis, upon which this more specific idea on whales is based, should be submitted for publication in an academic biology journal. If you have the qualifications to publish in an academic biology journal and are interested in collaboration, please contact me. You can read more on this on my biology index page.
The most common reason given why whales reach large sizes compared to land animals is that the water holds up their body mass. This might leave us to ask, why aren't bony fish larger. Of course, according to my theory, bony fish aren't large because there are many small bony fish to give any hypothetical large bony fish diseases.
Another theory says whales are large because it is harder for a warm-blooded animal to stay warm in cold water than cold air because water is denser than air. It is easier for large animals to keep warm because they have a relatively large mass, which generates heat, compared to their surface area, which loses heat. If shape remains the same when length doubles the surface area quadruples, and the mass increases by a factor of eight. More generally the surface area is a function of the square of the length and the mass is a function of the cube. Because mass increases more rapidly than surface area it is easier for large animals to keep warm.
Note that my theory is closely related to this one, but I am looking at the opposite end of size spectrum. A small mammal, the size of a mouse, could not survive in the ocean because it would be too difficult for it to stay warm. So the smallest mammals in a whale's environment are about 50 kilograms, or 100 pounds.
Furthermore it is argued that whales are large because they live in the oceans, which are big, and have enough food to support a large animal. It is similarly argued that whales and particularly blue whales are able to grow so large because of the abundance of the krill that they eat.
Finally, a popular recent explanation is that large size facilitates lunge feeding. Blue Whales lunge into a swarm of krill, taking in a massive amount of water, squeezing the water out through their whalebone filter, and swallowing the krill. It is argued that large size facilitates this, though much smaller animals also live on krill.
Secondly, the factor I am suggesting is a very general one. It applies to the largest single stem trees, the Sequoias and coastal redwoods, and many types of large animals. Similar arguments explain the large size of many fish: sharks, ocean sunfish, sturgeon, gars, wels catfish, arapaima, arowanas, and coelacanths. It can also be used to understand elephants, well just about all large animals, past and present.
There is a rule of science that the general explanation is to be preferred over the narrow explanation. Narrow explanations are disparaged as ad hoc. Not that I want to disparage the other factors, but the explanation that is general should not be quickly rejected, in favor of the narrow, ad hoc explanations.
Birds are basically a type of dinosaur. So just as mammals are mostly immune to the current bird flu, dinosaurs would have been immune to many of the diseases carried by tiny mammals.
Another example can be found in the relationship between humans and one of our closest relatives, the orangutan. Orangutans are threatened not just by the lack of habitat but also by human diseases. It is tough being the close relative of a numerous species, and humans are very numerous. In Britain humans are almost as numerous as field voles, which are the most numerous species of mammal. I am using the British example because they have the statistics and have put them on the Internet. A rather British thing to do.
The more numerous a species is the more likely that one will have a mutation giving it the ability to live with a parasite without being harmed. If that happens the former parasite can then develop a mutualistic relationship with the organism. The parasite may kill its hosts predators, or competitors for food. Killing the predator may not help the individual who has been eaten, but it might prevent the prey's relatives and even the offspring it has already produced from being eaten. Similarly, it will help its relatives and perhaps offspring to find food if those species competing for food are rare or even extinct. It might be argued that the brain worm and the white-tailed deer have a mutualistic relationship, the worm does not seem to harm the white-tailed deer but it can largely wipe out the white-tail's competitors, the larger deer, leaving lots more green, leafy food for the white-tailed deer.
Furthermore two animals with a similar metabolism are more likely to share the same diseases. For example, we are worried about bird flu, but not lizard flu. The most recent common ancestor of mammals and lizards is exactly the same as the most recent common ancestor of mammals and birds. Nevertheless, I have tried to catch lizards in my own hands, but would not touch a wild bird. Because birds are warm-blooded we are far more likely to have diseases in common with them than lizards which are cold blooded.
Another barrier to infection can be temperature. Many primitive groups of mammals like monotremes, marsupials, and, armadillos have lower temperatures than most placentals. The virus that can cause leprosy in our cooler extremities tends to infect armadillos because of their lower body temperature. Rabies very rarely infects the Virgina opossum because like other marsupials it has a lower temperature.
The same rule applies for the smaller toothed whales. The largest of the toothed whales, the sperm whale, prey principally on squid, once again an invertebrate. Smaller toothed whales, for example dolphins, eat fish, a vertebrate.
We can see examples of this outside of whales, the largest two species of shark, the whale shark, and the basking shark eat plankton. The largest bony fish, the ocean sunfish, and the largest fully ocean adapted reptile the leatherback turtle eat jellyfish which are invertebrates.
Of course many large animals, elephants, hippopotamus, rhinoceros, long-necked sauropod dinosaurs, and the hornless rhinoceros, indricotherium, are or were herbivores, and plants and animals rarely, if ever, share the same diseases.
It is dangerous to eat your close relatives as your prey can infect you with deadly diseases, particularly if they are closely related and therefore share the same diseases.
That this is an issue is likely, because most whales usually do not eat other whales, yes there are occasional exceptions. But more than this most whales do not eat other mammals, or even birds, which like mammals are warm blooded. The famous exception is the orca or killer whale. Even a large portion of the killer whales limit themselves to cold-blooded flesh, principally fish.
What is more most seals follow the same restriction, they do not eat other seals, and they do not eat birds. The major exception is the leopard seal. There are other exceptions, for example Weddell seal sometimes eats penguins and seal pups, and some sea lions eat seal pups and birds, but even these species that occasionally prey on warm-blooded animals, occasionally prey on all warm-blooded animals, both mammals and birds.
It is interesting that both the killer whale and the leopard seal eat both mammals and birds. Mammals and birds are particularly dangerous to eat raw. We humans eat raw meat in the form of sushi but sushi is made from vegetables, invertebrates, and cold-blooded fish. We rarely eat mammals and birds unless we cook them, an option not open to whales or seals. It appears that the killer whale and the leopard seal specialize in having immune systems that can handle the consumption of warm-blooded animals. I believe that being able to handle the diseases of prey is an important part of the adaptation of all predators. The fact that whales and seals normally do not eat mammals or birds makes obvious good sense if it is an adaption to avoid disease, but it is hard to explain otherwise.
Only a large animal could prey on a giant whale, which limits the predators to another whale, with the obvious problem of disease, and sharks. Great whites have attacked small whales, and the extinct giant shark, megalodon, probably preyed on large whales. Since the extinction of megalodon a million and a half years ago, only the killer whale has been left.
We see a similar pattern among dinosaurs. The largest of the dinosaurs were the long-necked sauropods. The sauropods were lizard-hipped dinosaurs, saurischians, as opposed to bird-hipped, ornithischians. While the sauropods were herbivores, their fellow lizard-hipped dinosaurs, were the predatory theropods. So the predatory theropods, for example allosaurus, were more closely related to the sauropods than they were to stegosaurus and most other herbivores. So the predatory dinosaurs were more likely to be infected with a disease if they ate a sauropod as opposed to another plant-eating dinosaur. Most whales and seals do not eat warm-blooded animals, maybe most theropods did not eat sauropods for much the same reason. There seems to be a pattern. The largest animals who ever lived in the oceans are only preyed upon by their close relatives, and the largest dinosaurs were only preyed upon by their close relatives. The danger of disease to predators could easily be a factor in both phenomena.
The blue whale is a baleen whale, so it has baleen or whalebone but no teeth. Baleen is not an effective weapon. Blue whales may hit an attacker with their huge tails, but that is just using their size, and that is the point. The blue whales' only defense, beyond disease as mentioned above, is their size. So blue whales have to be very large to gain a measure of immunity from predation.
The sauropod dinosaurs were similarly without horns for defense. They had small heads on long necks. This helped them reach up into the trees like modern giraffes but was not useful for defense, so once again they had to be huge to gain a degree of immunity from predation.
Similarly the largest land mammal that ever lived indricotherium had a fairly long neck, was able to eat leaves out of reach for other ground-dwelling mammals but was protected only by its huge size. Indricotherium was a hornless rhinoceros. It weighed about three or four times as much as the African Elephant, which is the largest land mammal today. It was not as well armed with horns or tusks as today's rhinoceros, elephants, or hippopotamus.
Smaller relatives frequently make it difficult for larger relatives to survive in the range of the smaller animal because the smaller species carries diseases that they are immune to but kill the larger species. As mentioned above larger deer can not survive in the Virginia white-tailed deer's range because the white-tail carries diseases that are fatal to them, but do not kill the smaller white-tailed deer.
So if a smaller rhinoceros could gain immunity from predation by having a horn at a much smaller size than indricotherium then it might also be able to drive indricotherium into extinction by infecting the indricotherium with pathogens they had in common. So the horns of the rhinoceros may have doomed indricotherium, the giant hornless rhinoceros.
In the Mesozoic a somewhat similar pattern occurred. In the earlier Jurassic the huge long necked sauropod dinosaurs lived. In the later Cretaceous the herbivores were much smaller, about the size of an African Elephant, but the horned dinosaurs and the Ankylosauria had horns and/or armor to defend themselves. So perhaps we can say there is a pattern. In both the Mesozoic and the Cenozoic there was first the evolution of huge species without horns and other defenses beyond their huge size, which were eventually replaced by smaller species with horns, tusks, other weapons or hard coverings.
It should be mentioned that the large size of the baleen whales, the long necked sauropod dinosaurs and the indricotherium are also related to what they ate. Animals that eat leaves high off the ground like the long-necked sauropods, indricotherium, and giraffes are all relatively defenseless except for their huge size. It is hard to put a heavy horn on a small head that is high off the ground. Similarly animals that eat plankton, like baleen whales, whale sharks, basking sharks, and manta rays are relatively defenseless. Eating plankton usually does not involve teeth, so animals that feed on plankton frequently do not have teeth to defend themselves from predators. The leopard seal seems to be something of an exception to this. The leopard seals front teeth are used to prey on penguins and seals and could be used for defense. Its back teeth are able to strain krill like its relatives, the crab eater seal.
There may be other examples of large animals being relatively defenseless. The largest fully ocean adapted reptile the leatherback turtle does not have the hard shell of most other turtles, including the other sea turtles. The largest two fish, the whale shark and the basking shark seem to be relatively defenseless beyond their size, as does the largest ray, the manta ray. The giraffe is similarly relatively defenseless, the ossicones, the giraffe's small equivalent of horns, being relatively small.
So there seems to be some trend that the largest animals do not have particularly deadly horns, tusks, or other defenses. They have to rely on their immense size for defense.
If we ask an unrelated question, why is the American economy the world's largest when measured using regular exchange rates the answer might be because America is one of the most productive nations per person, and it has the third largest population of any nation. When you multiply the high productivity per person by the large population you get a big economy. So we should not be fixated on finding the right answer when there are several factors. Once again in the case of blue whales there are no doubt many factors.
As mentioned above several of the biology professors at the University of California at Davis said I should try to publish the idea that is at the center of the above. They even suggested a specific academic journal.
Currently I am a substitute teacher and staff member for the local public schools. Formerly, I was a full time economics instructor for seven semesters at St. John's University in New York City. I have a B.A., M.A. and Ph.C. in Economics from the University of California at Davis. You can read more about my qualifications on my biology index page.
If you are not a biologist, well you may be able to experience the thrill of knowing that you knew about about a major advance in biology before the biologists. Link to this page and contribute to a major break through.