Fewer Male Reptiles Due to Warming -- And That's Good?

Matt Kaplan
for National Geographic News

October 2, 2008

A trend toward more females and fewer males in a type of Australian reptile may actually benefit the species in the short-term, a discovery that's contrary to previous research, a new study says.

As temperatures rise due to global warming, so does the proportion of female spotted skinks, reptiles found only on Australia's island state of Tasmania.

In recent years researchers have shown concern that climate change will push the reptiles into extinction by causing their young to be born of one gender, thus limiting future reproduction. Temperature-driven gender also occurs in other reptiles, such as crocodiles and turtles.

(Related: "Warming May Drive Gender-Bending Reptiles Extinct, Scientists Say" [November 10, 2006].)

But an increase in female spotted skinks could lead to larger populations of the reptiles, experts say.

The research is described online this week in the Journal of Animal Ecology.

Female Explosion

Erik Wapstra at the University of Tasmania in Hobart wondered to what extent temperature-driven gender occurred in nature, since most of the studies have been done in the laboratory.

From 2000 to 2007, Wapstra and colleagues observed a wild population of spotted skinks, recording the gender of their offspring and monitoring the air temperature.

Temperatures that averaged around 62.6 degrees Fahrenheit (17 degrees Celsius) when the creatures were pregnant led to mostly male offspring, while in warmer years the offspring were more likely to be female.

No one knows why the phenomenon occurs, although a longstanding theory holds that the reptiles' genes have been selected to produce offspring that will best survive their environment.

For instance, if warm temperatures are better for male survival, than males will tend to be born in warm conditions, and vice versa.

But such fluctuations do not suggest that the skink population is in imminent danger.

"Whether there will end up being so many daughters that it causes damage to the species is extremely hard to tell, because even with warming there could be the occasional cold year that yields sons," Wapstra said.

Among reptiles, it is common for one male to breed with multiple females. This makes the number of available females the factor that ultimately limits the overall size of the population. So it's possible that more females could mean more skinks in general—at least for the immediate future.

However, there is a considerable caveat that warming might also cause the food web the reptiles depend upon to collapse, Wapstra warned.

Evolution at Work

Evolution could also disrupt the trend of temperature-driven gender.

A mutant mother that produces sons at higher temperatures or gives birth to offspring that aren't influenced by temperature might appear, experts say. The offspring of such a mutant would quickly come to dominate the population.

The only question that remains is whether evolution would work fast enough to keep pace with rapid climate change.

(Related: "'Virgin Hatching'? Komodos Born Despite No Male" [February 8, 2008].)

Allison Alberts is director of conservation and research at the Zoological Society of San Diego.

"It has been suggested that species for which the sex of the offspring is determined by the environmental temperature could be seriously affected by global climate change," Alberts said.

"This groundbreaking study is the first to demonstrate that such fluctuations actually occur in nature over many years," she said.

"The impact is likely to be most severe for short-lived species and those that are unable to compensate for rising temperatures by changing their … behavior," she added.

"In extreme cases, the extinction of local populations could be the unfortunate result."