Reproductive biology

Each tuatara uses a burrow, or series of burrows; on islands densely inhabited by seabirds, these burrows may be less than 3 ft (0.9 m) apart. Males display before combat and

I Sphenodon guntheri I Sphenodon punctatus

Tuatara internal organs. (Illustration by Brian Cressman)

mating. Combat is particularly common in summer and autumn, when males establish territories to include several females. Displays during combat and courtship include stiffening and inflation of the skin and spines down the neck and back.

Mating is in late summer or early autumn every two to five years, depending on form and location. Receptive females are attracted by an exaggerated walk, called "proud walk" or stolzer Gang, during which males with crest erect and throat inflated stiffly and slowly circle females. After about 20 minutes, the females either leave or remain to mate. During mating, a male mounts a female, raising the base of her tail using his hind legs, to align his vent with hers. As in birds, sperm transfer is between the vents of each sex.

The females lay their eggs the spring after mating. Cook Strait populations of S. punctatus lay eight to 15 eggs, whereas the northern populations and S. guntheri lay four to 13 eggs. The eggs are laid in rookeries, where females may gather from distances of at least 200 yards (180 m). The rookeries include unforested areas or gaps in forest canopy exposed to the sun. Each nest is a shallow depression or short burrow up to 20 in (50 cm) deep, with the eggs covered by loose soil. Incubation takes 12-15 months. Temperature in the nest is critical for egg survival and influences the sex of the hatchlings. Laboratory studies show that the highest hatching success is achieved when soil temperatures vary between 64°F and 72°F (18-22°C). Nests at temperatures of about 68°F (20°C) produce a strong female bias among juvenile S. punctatus. Similar studies of S. guntheri found that eggs that hatched at lower temperatures of around 64°F (18°C) produced more females, but those in temperatures that fluctuated between 64°F and 74°F (18-23°C) produced more males. Tuatara take up to 13 years to reach sexual maturity. Unlike the females, males have an annual reproductive cycle. The average life span of adults is at least 60 years.

A tuatara skull. (Illustration by Brian Cressman)
1. Sphenodon guntheri; 2. Sphenodon punctatus. (Illustration by Brian Cressman)

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