Survival Secret, Tuatara's are Immune to Lizard Specific Diseases
Richard Bruce PhC Economics University of California
I am not an expert on tuatara's, lizards, reptiles, or even biology, but I can say with certainty that tuatara's are immune to diseases that are specific to lizards. I do not need data, the statement is true by definition. If a disease only infects lizards, then it can not infect animals that are not lizards, and tuataras are not lizards.
More to the point, one of a number of reasons that the tuataras, the only remaining species of the order Rhynchocephalia, have survived in New Zealand is that they are not skinks, geckos, or more generally members of the order Squamata. The order Squamata includes lizards, snakes, and another related group of reptiles. The tuataras are the only reptiles in New Zealand that are not members of the order Squamata. As the only other terrestrial reptiles native to New Zealand are lizards, more specifically skinks and geckos, the tuatara has no close relatives in its environment, or actually anywhere in the world. Rhynchocephalia probably split from Squamata in the Permian, more than 252 million years ago. So there are no close relatives of the tuatara that can give the tuatara diseases.
Furthermore, the tuatara's competition, New Zealands lizards, do have closely related species that can give them diseases. There are many species of skinks and geckos in New Zealand, all of them unique to New Zealand. So perhaps all the skinks evolved from an original New Zealand skink species, and all the New Zealand geckos evolved from a origanal New Zealand gecko species. This would make them closely related.
But even if that is not true, we can be sure that New Zealand geckos pass around gecko diseases and New Zealand skinks pass around skink diseases.
Close Relatives Share Communicable DiseasesOrganisms tend to share diseases with their close relatives. For example, humans and chimps share all of their communicable diseases. Every disease that infects humans, infects chimps, and vice versa. The more distantly related two organisms are the fewer diseases they tend to have in common. There is no animal that shares a common ancestor with the tuatara from the last quarter of a billion years.
Tuatara Specific Diseases Might Not Be FatalIf a disease is specific to the tuatara and can not survive and spread in any other population then it must avoid wiping out the tuatara as the tuatara is its only food source. A deadly plague might kill much of the tuatara population, but then it is likely to go extinct, leaving some tuataras to repopulate the now plague free environment. So there is a huge advantage to being the last of your kind. This is a major reason why we have so many living fossils that continue to live long after all the other members of their group have died.
White-Tailed DeerThis is particularly important for larger animals as is illustrated by the deer of North America. The Virgina white-tailed deer carries several diseases, most famously a brain worm, which kill larger deer but not the white-tail. The larger deer, moose and elk, do not survive in the white-tailed deer's range as a result of these diseases. The white-tail is the smallest deer in North America. Note to Europeans, what Americans call a moose you call an elk, and what Americas call an elk is closely related to the red deer.
Smaller animals tend to be more numerous and to have a shorter life cycle. This helps them more rapidly evolve immunities to the diseases that afflict them and their larger relatives. Once they have evolved those immunities they can drive their larger relatives into extinction within their range, as the white-tail has done in America.
Small Organisms Win the Genetic Arms RaceThere is a natural evolutionary arms race between closely related species. Just as zebras have to outrun lions to survive and reproduce, so a species often has to evolve immunities faster than its close relatives. The result is that smaller species tend to win these races and a declining clad that is being replaced takes the niches where the numbers of individuals supported by the environment is small, but the size of each organism is large.
There are many examples of this. The largest trees, the redwoods, are among the last of once thriving clad with many more species. The dawn redwood lives in China, the coastal redwood lives along the coast in California, the Sequoia live up in the Sierra Mountains in California. So none of them has close relatives in their environment that can carry a disease that can kill them.
Similarly, the largest birds are part of Palaeognathae, a primitive branch off of the bird family tree. Living members of Palaeoganthae include the ostrich, cassowary, emu, kiwi, and the extinct moa birds of New Zealand and extinct elephant birds of Madagascar.
The largest spiders are tarantulas part of the second earliest branch off of the spider family tree.
The largest fish are sharks, a primitive group of fish. Other living fossils among the fish include the arapaimas, the largest exclusively freshwater fish in South America, the alligator gar, the largest exclusively freshwater fish in North America, sturgeon, and the coelacanth.
It is a long list that could be made much longer, but the point here is that the tuatara belongs on the list. The tuatara looks like a lizard and fills what would normally be a lizard niche. Furthermore, it is larger than any of the fifty to a hundred species of lizard endemic to New Zealand. New Zealand has more lizard species than any other similar sized temperate environment. So the tuatara is the last of an ancient evolutionary line that has not only survived, but has survived inspite of having lots of competion.
My arguement is that it has done this because it is not as subject to disease as its competitors, and it is larger. The lizards could not evolve to sizes as large as the tuatara because the diseases carried by the smaller lizards would have wiped them out. The tuatara could not evolve smaller species because the lizards were actually superior. Without the advantage of greater size the tuatara coiuld not compete.
Other FactorsWhile I believe the above was an important factor in the tuatara's amazing survivial, there are usually several factors involved. Let me mention a few.
As mentioned above the lizards of New Zealand are all skinks and geckos. Geckos are famous for clinging to walls, which they would not be able to do if they weighed too much. This may keep geckos small.
The world's largest skink, which is from the Solomon Islands, is smaller than a tuatara. So it could be argued that skinks are naturally smaller for some reason, even though I do not know what that reason is.
Many lizards that live elsewhere, for example, many types of monitor lizard or iguana, do grow larger than the tuatara. So there is nothing about lizards in general that makes them unable to grow larger than tuataras, at least outside of New Zealand.
It could be argued that tiny lizards are more likely to successfully raft on floating vegetation to isolated islands like New Zealand. So the New Zealand lizards might be small because they evolved from small lizards. But as they evolved so many species in New Zealand one would think they could have evolved larger species too.
We can tell several different stories and there are likely to be several factors, as there usually are several factors.
Disease ImportantNevertheless, it is likely that avoiding communicable disease is an important factor. If the survival of the tuatara was just a result of its special adaptations, for example, its well-known ability to thrive at lower temperatures than most reptiles, I would expect the tuatara would have diversified into several species filling several niches. After all, both the skinks and geckos have a little over forty species in New Zealand. The fact that there is only one species of tuatara and it is the largest terrestrial reptile in New Zealand strongly suggests that avoiding communicable diseases is an important part of the reason it has survived.