Tuatara deserve chance to flourish

ROGER HANSON
Last updated 08:02 13/11/2012

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On October 16 this year, 220 tuatara were flown from Takapourewa (Stephens Island) in the Marlborough Sounds to new homes on the East Coast, in Waikato and Otago.

About half of New Zealand's 60,000 tuatara live on Takapourewa, which is becoming overpopulated, hence the urgent need to encourage tuatara to thrive in other places.

Why all this trouble for an endemic New Zealand reptile? Because like many other native species in this beautiful country, tuatara are very special.

Like all reptiles, tuatara have eggs with a double membrane that allows the embryo to breathe on land, they have either an external bony plate or scaled skin and are cold-blooded (their body temperature is controlled, to some extent, by external means, such as basking in the sun).

The common ancestor of tuatara, lizards, snakes, crocodiles and birds was probably a small lizard-like creature that lived about 280 million years ago.

This creature evolved over millions of years to produce two evolutionary branches, one the ancestor of crocodiles and birds (archosaurs), the other the ancestor of lizards, snakes and tuatara (lepidosaurs). Archosaurs have several shared characteristics first found in their common ancestor (surprisingly, crocodiles are more closely related to birds than to lizards).

Lepidosaurs have shared characteristics not found in archosaurs. For example, lizards and snakes have overlapping scales, crocodiles have scales that form a horny layer that is shed piecemeal and not in one layer as in snakes and lizards; lizards and snakes can lose and then regrow their tails, crocodiles can't.

Tuatara share a common ancestor with snakes and lizards but their evolutionary branch split some 200 million years ago and so tuatara are not lizards. They have several unique features and are so special that biologists have had to dedicate a separate branch of the tree of life just to tuatara.

About 250 million years ago tuatara were distributed worldwide, however, coincident with the advent of mammals, they became extinct - except, that is, in New Zealand.

Male tuatara measure about 80 centimetres and weigh up to 1.3 kilograms (females are only about 40cm).

There are two species, Sphenodon punctatus and the much rarer Sphenodon guntheri, or Brothers Island tuatara.

They each have a distinctive spiny crest on their backs; in te reo, "tuatara" means "peaks on the back". Tuatara don't have teeth as such, but projections from their jaw bone. These wear away and so as tuatara age, they must switch to softer prey such as worms. Unique among living species, they have two rows of "teeth" in the upper jaw which overlap a single row on the lower jaw.

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The eyes of a tuatara focus independently and have two types of visual cells, one for day and one for night.

They also have an unusual photo-receptor, dubbed the "third eye", which is thought to use the sun's UV light to set their seasonal cycles.

This third eye has a lens, cornea and retina with degenerated nerve connections to the brain.

It used to be thought that tuatara had changed little over the last 200 million years - they were called "living fossils". However, recent studies show that at the molecular level they have changed significantly.

Tuatara have many cold- weather adaptations, allowing them to thrive in New Zealand. These adaptations may be unique because when their ancestors were living, the Earth was much warmer.

That said, tuatara are extremely primitive; their heart is the most primitive of any reptile.

They also retain basic features in their skull from their distant evolutionary ancestor, features lost or modified in snakes and lizards.

The females lay eggs only once every four years and it takes up to 15 months from copulation to hatching. The incubation temperature determines the sex: at 18 degrees Celsius it will be female, at 21C it's a 50:50 chance of male or female and at 22C or above it will be male. Their average lifespan is 60 years but there is a documented example of a tuatara living up to 200 years.

Tuatara used to be widespread in New Zealand. However, they became confined to 32 mammal- free islands. Thanks to world-class work by the Conservation Department in eradicating rats from some offshore islands and thanks to captive breeding programmes, the number of individuals in both species is increasing.

Hopefully the release of the 220 tuatara last month will usher in the establishment of increased numbers on mainland New Zealand and allow this hugely important and uniquely New Zealand reptile to survive and prosper.

- © Fairfax NZ News

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