Sphenodon punctatus

Family : Sphenodontidae

 

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Text © DrSc Giuliano Russini - Biologist Zoologist

 

 

English translation by Mario Beltramini

 

 

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Tuatara (Sphenodon punctatus) lives in New Zealand and is a real fossil © Giuseppe Mazza

This reptilian, living fossil, is the only one species afferent to the order of the Rhynchocephalia.

It was discovered at the beginning of the XX century.

Probably, by that time, its population density was much higher and characterized by a geographic distribution interesting also the southern part of the Australian continent; to their decline have concurred, as we shall see later on in this text, various ecologic factors, but also the killing done by their first casual discoverers, some castaways of English schooners, not to talk about the dogs and the cats present on their vessels.

Despite the justified reputation of living fossil or derelict, the Tuatara or Sphenodon ( Sphenodon punctatus - Gray, 1842 ) does not have a particular appearance. The term of the order Rhynchocephalia, means beak-shaped head.

Zoogeography

As briefly said before, the sphenodon, or tuatara, or atteria, as it is called by the Tuatari Maori, was present all over New Zealand, including the thick group of twenty small islets off the north-eastern coasts of the main island, but also in southern Australia.

Nowadays, it is found only on some ten of islets on the eastern coast of the North Island and in the Cook Strait; some herpetological biologists tell us, after surveys performed by the end of the XX century, that the greatest population, formed by some hundreds of specimens, is located on the Stephen Islet, which has a major diameter of only 2 km.

Habitat-Ecology

The islets where they live have biotopes with poor vegetation. Mostly, it is present in open or shrubby-bushy vegetation, in a rocky habitat; such vegetation is utilized also by several species of sea birds, especially the petrels, for nidifying.

The reasons of the rarefaction of the various groups of tuatara, on the islets and in the main island, are to be deemed to be the result not only of the Maori Tuatari hunting, but also, and above all, as briefly said before, of the first Europeans who reached such regions.

If probably the Maori and their dogs, as well as the Europeans, have been a clear cause of their decline, after some herpetological biologists, however, must be called to account also a weakening of the pure blood lineage of this splendid animal.

They are reptilians which have preferentially, if not disturbed, a crepuscular-nocturne activity: they are “heliophobous”, that is they badly stand the sunlight. Usually, they spend therefore most of the day in their shelters, but it is also possible to sight them basking early in the morning. The presence of nidifying sea birds on their islets, with relevant excrements, has helped to create a particular soil, already in itself fertile due to the volcanic nature of the New Zealand archipelago, aired and rich of mineral salts (phosphates and nitrates) where an extremely abundant edaphic fauna lives: earthworms, snails, many insects, and in particular, the big endemic apteran cricket, which is a choice prey for the tuatara.

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A photosensitive eye hidden on the head looks to the sky © Giuseppe Mazza

These reptilians have available, in this way, abundant nourishment, which at times they enrich with eggs and petrels chicks.

The Sphenodon punctatus is nowadays strictly protected by the New Zealand government; and since when the integrity of its biotope is under control, especially with the regulation or the prohibition of the breeding of bovines on the islets, it seems that its future is guaranteed.

From the auto-ecologic point of view, seen that the climate of their islets is characterized by not high temperatures, remarkably stable, with strong wind currents, these reptilians, contrary to the others, which prefer to live in tropical warm or equatorial areas, have well adapted to locations with not high temperatures, to the point of badly enduring the heat.

Morpho-physiology

At first glance, the Sphenodon punctatus resembles to a big terrestrial saurian, more precisely to some herbivorous agamids and iguanids.

The head is massive, the body stocky, the legs are strong, the thick tail is flattened laterally like that of a crocodile or of a South American lizard-alligator.

On the nape and the back, a series of long soft scales from a sort of crest, which other scales, wider, prolong on the tail.

The male is rather bigger than the female, with a more massive head; they reach, as an average, the length of 60 cm.

The eyes, lateral, are black and are rather small if compared to the head. The snout is rounded.

The oddest morphological characteristic is represented by the presence of a third eye, called “pineal eye”, present on the top of the head, under the skin, which is capable of light reception.

The existence of such third eye, has not yet found a plausible explanation among the zoological biologists, also because, as we have seen, they are animals with a prevailingly nocturnal activity, for which the presence of a photosensitive eye appears to be obsolete.

Some zoologists think that this pineal eye may serve to the sphenodons when the warm up in the sun during the first lights of the day, as a regulating filter which, by means of the light, stimulates the pineal gland (epiphysis), in secreting appropriate hormones (melatonin) which may act on the biological clock of the animal as well as on organs, for instance the “adrenal glands”, stimulating them to secrete the cortisol (hydrocortisone) which, in turn, acts on the glycaemia increasing the same.

Furthermore, this third eye is also seen as an evolutive trace of what will be later the cartilagineous skull of the birds, through which the sunlight passes for directly acting on the brain.

Ethology-Reproductive Biology

Usually slow and inactive, these reptilians in some cases prove agility and vigour, in particular during their nocturnal explorations. They often utilize the underground nests of the petrels, even if they are absolutely capable to dig them by themselves. Even if the take advantage, this is sure, of the presence of these birds, it is sure that such intimate association, as at times it has been affirmed by ornithologists too hasty in their conclusions, does not exist.

As already told, the climate where the sphenodon lives is characterized, especially in the Cook Strait, by not much high temperatures and remarkably stable. It is therefore unquestionable that these animals have very scarce thermal necessities.

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Often lays eggs in petrels underground nests. Only females are born if it’s cold © Giuseppe Mazza

The temperature of 72 specimens, studied in the wild by a group of French herpetological biologists during the eighties of the last century, during the months of April and November, was, as an average, of 10,9 °C with lowest and highest extremes of 6,2 and 13,3 °C, respecti- vely.

Quite few reptilians are voluntarily active in these conditions and, above all, no one else would be able to perform the digestion at so low temperatures.

In the same geographic regions, do live well some geckos and some small skinks, but they are diurnal and take much more advantage from the insolation.

In any case, it seems that the tuatara has a very low metabolism.

The growth is slow and it appears that the sexual maturity is never reached before the age of twenty, which implies a potential longevity of the order of a century and even more.

Also the duration of the incubation of the eggs, from twelve to fifteen months, is abnormally long.

The mating, the exact mechanism of which is still ignored, as there is no copulatory apparatus and having never been observed or filmed, happens during the “austral” summer and the deposition of the eggs, from 3 to 16, in the following spring, from October to December.

Fluctuations of the ambient temperatures, like for the other reptilians, are a determining cause for the sex of the unborn. When the temperature is of 22 °C, the percentage of male births is much higher, whilst at 20 °C, the percentage of females is much higher and at 17-18 °C only females come to life.

The biologists (zoological-herpetological-evolutive) have often wondered how it goes that the Rhynchocephalians ( Rhynchocephalia ), widely spread on the surface of the planet by the end of the Secondary (Mesozoic) era have completely disappeared, whilst one of them has survived up to our days.

As it is logical, only speculations may be done, about what has happened when we were not there, but we must keep in mind that New Zealand had the characteristic of not hosting neither terrestrial mammals, not even the Australian marsupials or monotremes, nor any reptilian of a size similar to that of the sphenodon.

Only small saurians (gekkonids or scincids) may have reached, casually, these isolated lands; in fact, among them it is possible to recognize some ancient immigrant which has evolved for long time on the spot, for instance, some odd viviparous geckos, not to talk about the most recent immigrates which have not yet had the historic time to form new species.

It is therefore quite logical to think that the survival of the sphenodon in New Zealand is more the result of the absence of ecologic competitors, in particular mammals and squamates, than of a particular climate or biotope.

Actually, the rhynchocephalians as well as the small sphenodons were too slow and too little fertile for being able to withstand the competition of the mammals and that of the other lepidosaurs, saurians and serpents.

 

The photographic file of Giuseppe Mazza

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