Tuatara do things a little differently to other reptiles when it comes to sex determination – not because they have temperature-dependent sex determination (thats common to lots of reptiles), but because their pattern of temperature-dependent sex determination (or TSD) is different from most other reptiles. For tuatara, incubating eggs at higher temperatures (over 22°C) produces males, while lower temperatures (under 21°C) produce females. In other reptiles with TSD, you generally either get a pattern of females being produced at high temperatures and males at low temperatures, or females being produced at both high and low temperatures, and males produced at intermediate temperatures.
As you might expect from an animal that is so evolutionarily distant from its nearest relatives, the tuatara also has some unique parasites to call its own. One of these is the tick Amblyomma sphenodonti (sometimes also called Aponomma sphenodonti), pictured here.
Like many ticks, A. sphenodonti are host-specific, spending all three of their life stages feeding on tuatara but dropping off into the soil in between stages.
So why should you care about tuatara ticks? Well, these ticks are evolutionarily distinct in their own right, and are actually quite rare – far rarer than the tuatara themselves. Read the rest of this entry »
I haven’t had much time for writing this week, so instead I thought I’d share this photo as a reminder to my New Zealand readers that it is actually spring, even though it doesn’t feel like it!
Tuatara like it cold. Unusually so, for a reptile. While reptiles in most other countries are happiest with temperatures over 25 degrees celcius, here in New Zealand our reptiles prefer much lower temperatures. Alison Cree’s group at the University of Otago has been investigating exactly which temperatures tuatara prefer, with a view to determining whether new populations of tuatara could be established in the southern South Island.
This little guy lives on Stephens Island in the Marlborough Sounds, and is affectionately known as “tree tut” to the Victoria University researchers who frequent the island. Because he lives in a tree, of course. His tree is along the pathway between the house occupied by the DoC rangers and the house where the researchers stay, so he has plenty of passing foot traffic to keep an eye on.
Stephens Island is tuatara central, home to a staggering 30,000 – 50,000 individuals. Given that the island is only about 150 ha in size, this means that tuatara are EVERYWHERE on the island and it is sometimes difficult to avoid treading on them. Stephens Island has an interesting history, which may have partly contributed to the high densities of tuatara found there. A lighthouse was constructed on the island in 1893, and three houses were also built to accommodate lighthouse keepers and their families. During World War II a radar station was set up there, and an accommodation building known as the “Palace” was constructed. The Palace is still there and these days serves as a lab and storage shed.
The clearing of land for the construction of the lighthouse and houses, and the introduction of cattle and sheep decimated the Stephens Island forest, and photos of the island from 50 years ago or so show barren hillsides with only a few remnant patches of bush. However, for the last 20 years a revegetation program has been in full swing and the forest is returning. The lighthouse was automated in 1988, and the last lighthouse keeper left the island in January 1989. The last sheep left the island in 2005, and today the only permanent human presence on the island are the DOC rangers, who live in one of the old lighthouse keeper’s houses.
Despite the human settlement and rampant habitat destruction, the only introduced predators that made it to the island were the lighthouse keeper’s cats. These cats decimated some of the local wildlife, including the Stephens Island wren, an unusual flightless passerine which famously went extinct virtually as soon as it was discovered. However, the tuatara population escaped virtually unscathed and cats were eradicated in 1925 after only about 30 years on the island. Ironically, the clearing of forest on the island may have actually increased tuatara numbers, by increasing the availability of suitable nesting sites in open areas. The island currently appears to be above its carrying capacity, and once the forest regeneration is complete tuatara numbers may decrease somewhat.
From today I’ll be starting a semi-regular series of posts about my favourite reptile and #1 study organism, the tuatara. I want to start by clearing up a misconception that I see repeated time and time again, that tuatara are “New Zealand’s living dinosaur”.
Tuatara are an entirely different lineage of reptiles from the dinosaurs. Here’s a simplified phylogenetic tree of reptiles to illustrate:
The closest living relatives of the dinosaurs are the crocodilians (alligators and crocodiles), and birds. In fact, it is birds that are the “living dinosaurs”, as they evolved from the Theropod dinosaurs, the lineage that includes tyrannosaurs, Velociraptors and Archaeopteryx. Crocodilians, dinosaurs and birds are collectively known as Archosaurs.
Tuatara are in their own Order, Rhynchocephalia, which is entirely separate from the Archosaurs. The closest relatives of the Rhynchocephalids are the squamates (lizards and snakes), but they are not particularly close relatives at all, having diverged early in reptilian evolution, around 250 million years ago. The tuatara is the only remaining species of Rhynchocephalid living today, but back in the time of the dinosaurs (around 65-230 million years ago), Rhynchocephalids were everywhere. Numerous different species of fossil Rynchocephalid have been found across Europe, Africa and the Americas. However, Rhynchocephalids appear to have died out everywhere except New Zealand around the same time the dinosaurs went extinct 65 million years ago. Why they hung on in New Zealand is a mystery, but may have something to do with the apparent lack of competition from mammals.
Modern-day tuatara share a lot of morphological features with some of the earliest Rhynchocephalid fossils. That, combined with the fact that they are the only living species from this lineage, has earned them the title of “living fossil”. However, it is premature to say that tuatara are “unevolved” or “unchanged since the dinosaur era”. For one thing, we don’t actually have any ancestral tuatara fossils from New Zealand dating back to the dinosaur era for comparison (the earliest tuatara fossil dates to a measly 16 mya), and recent studies have shown that many aspects of their morphology and biology are derived, making them just as “evolved” as any other species (but that’s the subject for another post).
So, although tuatara are the last member of an ancient reptile lineage, and still retain some morphological characteristics of these early reptiles, they are a completely different type of reptile from dinosaurs. Their ancestors lived alongside the dinosaurs, but so did the ancestors of modern-day turtles, crocodiles, lizards, and snakes.