Biology Index
By Richard Bruce BA, MA, and PhC in Economics

Whales Are Big Because Infectious Rodents Do Not Live In the Ocean

The smallest mammals in the ocean, the smallest dolphins and seals, are about 50 kg, 100 pounds, full grown. This is more than ten thousand times the size of the smallest land mammals, the smallest shrews and bats. There are no rat, mouse, or shrew-sized mammals in the ocean to give whales diseases that are specific to mammals. This allows whales, particularly the blue whale, to grow to huge size, far larger than land mammals like the elephant that have to contend with tiny, disease carrying mammals.

A hypothesis, High Level Professors Interested

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.

Alternative Theories - Other Factors

Before I proceed let me mention some "alternative" theories. Note, I put the word "alternative" in quotes. This is because the other theories are not really alternatives. This is not because they are wrong, they are probably right. It is because there are no doubt a number of reasons whales are so big. Humans want a contest. They ask what is the reason. Realistically there are almost always several reasons or factors in the equation.

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.

The Disease Factor is Original and applies to Other Large Organisms

While there are many factors, the point I am making here about diseases is to the best of my knowledge original to me. You can read many articles, or watch many videos that will repeat the standard arguments. You will only get this argument here.

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.

Dinosaurs Were Big Because Mammals Held Rodent Niches

On land, the largest animals were the dinosaurs, and they had much the same advantage as whales. The mammals held the smaller warm blooded niches. The mammals held the niches that are now held by shrews, mice, rats, squirrels, etc. The dinosaurs may have been the foxes, the small warm-blooded predators, but today foxes are rare compared to shrews, mice, rats, and squirrels. For example, in Great Britain it is estimated that there are about 300 field voles for every fox. Check table 14 for the numbers.

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.

Disease and Deer

To find an example of this phenomena in our own time we need look no further than what is probably the most common wild mammal larger than man, the white-tailed deer. The white-tailed deer carries several diseases, particularly a brain worm, that do not kill white-tailed deer but are frequently deadly for larger deer: elk, and moose. This can make it difficult, if not impossible, for the larger deer to survive in the white-tailed deer's range. Note in Europe elk are the large deer Americans call a moose, and the American elk is closely related to the European red deer.

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.

Revenge of the Little Guys

Smaller species tend to be more numerous and have short life cycles. If you count the individuals in a three-dimensional snap shot at one point in time you will usually find that the smaller animal is more numerous. If you did a four-dimensional census, for example, how many individuals lived over a century, the difference is likely to be much larger. Because the life cycle of the small animal is usually much shorter there are far more of them over time. Of course these are only tendencies. There are so many counter examples they are not worth mentioning.

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.

Watch Out For Your Relatives

The more closely related two species are the more diseases they tend to have in common. For example, one of our two closest relatives, the chimpanzee, shares all of our communicable diseases.

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 Bigger Picture

As you will note these are very general principles which apply to a lot more than whales. This is one of the crucial principles of biology, and crucial to understanding the natural history of our planet. At the end of many popular biology articles, particularly those in Scientific American and Discover magazine the author will list some questions that still need to be answered. In a large portion of all cases the argument given above provides the answer.

Other Factors in Whale Size

Returning to the topic of why Whales are so huge, there are several other factors related to this issue of disease that further increase the size of whales.

Bigger Whales Eat Invertebrates

First, there is the whales' food. The blue whale lives largely on krill which is an invertebrate. As krill is an invertebrate it no doubt shares even fewer diseases and other pathogens with the blue whale than fish, which like the blue whale are vertebrates. The larger baleen whales tend to depend more on krill and invertebrates for food, the smaller ones consume more fish.

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.

Closely Related Predators

The blue whale, a warm-blooded vertebrate, principally eats krill, cold-blooded invertebrates. But the predator that feeds on blue whales, the orca, or killer whale, is warm-blooded vertebrate, more specifically a mammal, more specifically still a whale, a member of the blue whales order, cetacea. This means that if the blue whale is carrying a whale-specific disease it might infect, and perhaps kill the killer whales. The blue whale may achieve some immunity to predation because potential predators large enough to attack it are frequently its relatives.

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.

No Defense But Size

Another pattern we see in the very large, baleen whales, long-necked sauropod dinosaurs, and the largest land mammals, indricotherium, is that they lack horns, tusks, and large sharp teeth to defend themselves with.

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.

No Saber Toothed Predators

Another characteristic whales have in common with the long necked sauropod dinosaurs is that neither faced saber toothed predators. Both killer whales and predatory theropod dinosaurs have teeth of relatively similar size compared to various saber toothed predators of the land dwelling therapsid/mammal line. The two huge canine teeth of smilodon, the sabre toothed cat that lived up until almost eleven thousand years ago, could be eleven inches long. Longer than the teeth of t-Rex the predatory dinosaur that was more than ten times as heavy or megalodon the giant predatory shark that might have been two hundred times as heavy. It is usually thought that these large canines and similar teeth on other mammal and non mammal therapsid predators were used to kill large prey. The saber toothed predators may have been a major factor limiting the size of the therapsid/mammal herbivores.

Five New Explanations

In addition to my major explanation based on the fact that living things get communicable diseases from their relatives, I have added another four explanations to the several explanations that science has already produced. All of these explanations may be true. The Blue Whale is very big, there are no doubt a number of reasons for its great size.

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.

Wanted Biologist

I am looking for a biologist who can help me publish the idea in an academic journal. You can see the application to whales, but a large portion of all biologists are probably in a field where the principles can be applied.

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.


Here are some other biology pages. In many of these pages I use the same principles I have used concerning whales. For example, I use similar reasoning to explain why quetzalcoatlus, the giant pterosaur from the late cretaceous was the last of the pterosaurs.
Biology Index