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When a species can’t stand the heat

Global warming could leave New Zealand’s tuatara dangerously short of females

The tuatara is the only surviving member of its order of reptiles. Global warming threatens the long-term survival of this New Zealand native. Shown here is a male tuatara with its crest erect — a typical display of aggression.

Susan Keall, Victoria University of Wellington

Earth’s warming threatens to tilt populations of an unusual reptile so dramatically that the species’ long-term survival could be put in peril. The change could leave the species, a survivor from the age of the dinosaurs, without enough females to avoid extinction.

The tuatara (TOO ah TAAR ah) is about the size of a squirrel. A crest of floppy white spikes runs down its back. Although it resembles a lizard, the grey-green species (Sphenodon punctatus) actually belongs to a separate and distinct reptile order. (An order is that place on the tree of life directly above species, genus and family).

There are four orders of reptiles. Three have many distinct species. Not so the Rhynchocephalia (RIN ko suh FAY lee uh). This order holds on with just a single member: the tuatara.

That wasn’t always true. More than 200 million years ago, different rhynchocephalians could be found across much of the globe. Alas, most of these ancient reptiles died out around 60 million years ago, along with the last of the dinosaurs. Today, their descendants inhabit several dozen islands and fenced nature reserves, all in New Zealand.

And these animals are unique. For instance, unlike other reptiles, which have one row of teeth in its upper jaw, the tuatara have two parallel rows. As the animal chews, its bottom single row of teeth slots neatly in between the top two rows. The tuatara also have extra, rib-like bones, called gastralia (or “belly-ribs”).

Humans introduced rats and other mammals to New Zealand, in the South Pacific. For centuries, these animals have threatened the survival of the island nation's unusual reptiles (see Explainer). Although tuatara have survived that catastrophe, they now face a new threat: too few females. One reason: With global warming, their island homes are becoming way too hot!

Temperature sensitive

For all of its oddities, in one important way the tuatara resemble many of their reptile cousins: Whether an individual hatches from its egg as a male or a female depends on the temperature at which that egg had incubated.

Mom doesn’t sit on her eggs. She just digs a nest in the ground and then leaves her eggs to develop. Cooler temperatures produce more girls; warmer temperatures, more boys. But with global warming, average temperatures across New Zealand have been increasing. And more male tuatara will hatch.

Adding to the problem, females don’t seem to do well when males greatly outnumber them. Already on at least one island, the local population of tuatara risks dying out. There, guys outnumber gals by more than 2-to-1, according to a study published April 8 in the scientific journal PLOS ONE.

For a long time, scientists didn't realize the impact that temperatures can have on these reptiles. Then, in 1992, Alison Cree discovered something odd. Cree is a zoologist at New Zealand’s University of Otago. She and her students needed to know the sex of some tuatara that had been born in captivity. And that required surgery.

Outwardly, young tuatara males look just like females. To tell them apart, scientists must cut a tiny slit through the animal’s skin. Only then can experts peer inside to see whether the reptile has ovaries or testes. A female’s ovaries make eggs. A male’s testes produce the sperm needed to fertilize those eggs.

All of the eggs deposited by a mom into one nest are a clutch. And Cree noticed that one clutch of seven tuatara from a New Zealand zoo were all boys. That made her suspicious.

She knew scientists had incubated the eggs in a cupboard that sometimes got warm. Could the all-male clutch reflect the influence of temperature? That certainly happens in some other reptiles, including crocodiles, alligators and most turtles. Yet extra warmth wouldn’t necessarily mean more males. In many of those species, eggs incubated at the highest temperatures produce mostly females.

So Cree’s team incubated tuatara eggs at different temperatures. And these experts confirmed that eggs kept at warmer temperatures hatched more males.

This is completely different from the way sex is decided in mammals, including people. In them, chromosomes determine a baby’s sex. A human embryo always inherits an X-chromosome from its mother. Its dad — as all men — have an X- and a Y-chromosome. If the baby inherits an X-chromosome from dad, she will be a girl. If the baby instead gets one of dad’s Y-chromosomes, he will be a boy.

But tuatara don’t have X- or Y-chromosomes. When a tuatara mother first lays a fertilized egg, the embryo inside is neither male nor female. In this species, temperature tends to determine how many hatchlings emerge as guys or gals. And just a small difference in nest temperature can make a difference. For example, 95 percent of eggs kept at a constant temperature of 21.2°Celsius (70.2°Fahrenheit) will develop into females. The ratio flips for eggs incubated a little more than one degree warmer — at 22.3 °C (72.1 °F). Now, 95 percent emerge as males.

That sensitivity to such minor swings in temperature has set off alarms among scientists working to ensure the tuatara’s survival. They know that climate scientists have calculated that temperatures in New Zealand could rise by as much as 4 °C (7.2 °F) by 2080. According to the new PLOS ONE study, on at least one island where the reptiles now live — North Brother Island such a large temperature increase would mean no more female tuatara. And, eventually, that would result in no more tuatara. Period.

Bad times on North Brother

This wind-battered island is a mere 4 hectares (roughly 10 acres) in size. It’s home to an old lighthouse and several hundred tuatara. And here, roughly seven out of every 10 of the reptiles are males.

Nicola Mitchell is a biologist at the University of Western Australia and co-author of the new study. She and her colleagues now estimate that at today’s temperatures, 56 percent of tuatara eggs on North Brother Island should become males. That’s far fewer than the real number. So Mitchell suspects the tiny island’s shortage of females must be due to more than just climate change. Something else must be helping tilt the ratio in favor of males.

And it may be the males’ behavior.

Her team has noticed that tuatara on North Brother have been getting skinnier over the past few decades. But females have slimming faster than males. One reason could be that males chase and harass females they try to get to mate with them. (With few females, each gal may find herself getting far more attention than she wants.) The males also are generally larger and more aggressive than the females. So the guys may be better than females at staking a claim to prime territory and food.

The end result is that the North Brother females have become slow to reproduce. Healthy females normally lay eggs every two to five years. But North Brother’s gals only lay eggs once every nine years or so. Observes Mitchell, “We’ve got higher mortality in females and lower reproductive rates.” Project this trend out into the future and within 150 years “there would only be males,” she says.

Indeed, all signs suggest the North Brother population is slowly collapsing. “You can see this spiraling pattern and it’s all heading in the wrong direction,” says Nicola Nelson. Another member of the tuatara research team, she works at Victoria University of Wellington, New Zealand.

Nelson says that it’s possible the island is just too small and barren for tuatara to survive there forever. Maybe its colony is destined to die out. But many other tuatara populations also live on tiny islands. By monitoring the struggling group on North Brother, researchers are now learning what can happen when males start to greatly outnumber females.

Seeking shade

One question that scientists still haven’t answered is whether tuatara mothers could change their behavior to match a new climate. After all, they’ve survived other swings in temperature over the species’ long history. It is certainly possible the reptiles could shift where they lay their eggs or when. That would help them to avoid soil that is too warm.

This seems to be true for at least some other reptiles that have their sex set by egg temperature. Among them is the painted turtle, notes Jeanine Refsnider. She is an ecologist at the University of California, Berkeley.

Painted turtles are a common sight in rivers and lakes across the United States. Among these colorful creatures, more females hatch when temperatures are higher. However, they sometimes adjust to change, Refsnider notes.

“Normally they nest out in sunny, open habitats,” she says. “I found that if you expose turtles to warmer temperatures than they are used to, they choose shadier spots to nest.”

But shade is not always available. One group she studied lived in the desert. For those turtles, there just weren’t any shade in which to nest.

Such a limit could endanger other reptiles living in small areas where there is little choice about where to lay eggs, Refsnider says. After all, she notes, “Reptiles don’t migrate like birds.”  

Other reptiles indeed could end up with either too many males or too many females in a warming world, points out Fredric Janzen. He is an ecologist at Iowa State University in Ames. While unfortunate, he notes, such changes could warn of potential threats facing other species.

The reptiles “may serve as the ‘canaries in the coal mine’ for all species with key parts of their biology affected by temperature,” says Janzen. Coal miners used to take caged canaries into the mines. When levels of toxic gases began to rise, the birds would have trouble breathing — or die. This would signal to the miners that they must flee to safety or risk a similar fate. Today, scientists liken many environmental warning signs to those old mine canaries.

Moving south

The tuatara could migrate to cooler climes — but only with help from people.

Part of New Zealand’s long-term plan for looking after tuatara is to return them to places they lived before humans arrived. Old tuatara bones have been found up and down the two larger islands that make up New Zealand’s mainland, from the warm tip of the North Island right down to the cool far end of the South Island.

Right now, tuatara live mostly on small islands off of the North Island. Cree says that moving some tuatara back into different types of habitat, including cooler areas, should ensure the species can survive.

With that in mind, scientists released 87 tuatara into the South Island’s Orokonui Ecosanctuary in early 2012. More than 8 kilometers (5 miles) of steel fence surrounds the sanctuary. The high fence keeps out any mammals that might view the reptiles as lunch. Temperatures too are milder there — around 3 °C (5.4 °F) cooler on average than on the islands where tuatara now live.

In fact, many potential nesting sites at Orokonui appear too cool to produce boys. Still, climate scientists predict that before the end of the century, even Orokonui will be as warm as Stephens Island, where tuatara now flourish. “That’s within the lifespan of a tuatara,” Cree says. These reptiles can live for at least 80 years and likely more than 100 years.

So moving tuatara into lots of new habitats is like an insurance policy. “We were down to 32 populations,” says Nelson. “now we’re up to 45 populations of tuatara in lots of different locations. We’ve certainly got our eggs in more baskets.”

That’s a good thing, since the tuatara face other future challenges as well. Drought likely will increase in some areas of its range. That can destroy eggs and kill hatchlings. And sea level rise will shrink the island territory available for this reptile to inhabit. “It’s climate that’s changing, not just temperature,” explains Cree.

For now, wherever tuatara live under protection, the reptiles are thriving. Scientists already have found two tuatara nests at Orokonui. Their eggs should hatch this year. Those babies will be relatively safe in their sanctuary, but likely see many changes over the course of their very long lives.

Power Words

behavior  The way a person or animal acts towards others, or conducts itself.

chromosome  A single threadlike piece of coiled DNA found in a cell’s nucleus. A chromosome is generally X-shaped in animals and plants. Some segments of DNA in a chromosome are genes. Other segments of DNA in a chromosome are landing pads for proteins. The function of other segments of DNA in chromosomes is still not fully understood by scientists.

clutch    (in biology) The eggs in a nest or the hatchlings from that collective group of eggs.

ecology  A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

embryo  A vertebrate, or animal with a backbone, in its early stages of development.

gastralia Bones nicknamed “belly ribs” that are only found in tuatara, crocodiles and alligators. They support the abdomen but are not attached to the spine.

hatchling    A young animal that recently emerged from its egg.

mammal  A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding the young, and (typically) the bearing of live young.

New Zealand    An island nation in the southwest Pacific Ocean, roughly 1,500 kilometers (some 900 miles) east of Australia. Its “mainland” — consisting of a North and South Island — is quite volcanically active. In addition, the country includes many far smaller offshore islands.

order    (in biology) It is that place on the tree of life directly above species, genus and family.

reptile  Cold-blooded vertebrate animals, whose skin is covered with scales or horny plates. Snakes, turtles, lizards and alligators are all reptiles.

sperm  In animals, the male reproductive cell that can fuse with an egg of its species to create a new organism.

testis   (plural: testes) The organ in the males of many species that makes sperm, the reproductive cells that fertilize eggs. This organ also is the primary site that makes testosterone, the primary male sex hormone.

tuatara    A reptile native to New Zealand. The tuatara are the sole remaining species of one of the four orders of reptiles.

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