Web edition: July 3, 2008
scene for unusual reptiles on certain South Pacific islands could get pretty
stressful in coming decades. Guys already outnumber gals in some areas. With
global warming, that tilted ratio threatens not only to become universal but
also far more exaggerated. Or so concludes a new study.
facet of global warming that has largely escaped attention. But upon reflection,
it’s clear that the reptiles studied should be far from the only ones that can
develop perilously skewed sex ratios if our atmosphere’s growing fever
continues its rise.
paper focuses on tuatara, somewhat-bizarre lizardlike creatures that exist as
remnant populations and only on several uninhabited
many oddities, these critters have one of the longest reproductive cycles
known, grow slowly, and may live four score years or more. Like many reptiles,
their gender is determined by the temperature at which eggs incubate. And they
incubate in earthen nests a long time — typically 11 to 14 months.
The warmer that
nest, the more likely a hatchling will be a he, not a she. So, if the climate leads
to substantial nest warming, there will likely come a time at which every
tuatara born will be male. That time could be within this century, hints a
paper published today in Proceedings of
the Royal Society B, although Nicola J. Mitchell of the
Part of the
reason: Theirs is a modeling paper. So the authors ran a range of scenarios
through their computer program, graphing the projections it spit out. Gauging
the most likely projection requires making a lot of assumptions — and hoping
that the scientists or their computer model were sophisticated enough to
account for the most likely factors that would affect gender. These include: how
much the local climate will warm in the near future, whether nests will warm
proportionately, whether tuatara will plant their eggs at lower depths to keep
them cooler in the future, and whether moms will lay their clutch at the same open
sites as the climate changes or instead begin to nest at shaded locales
elsewhere on their island.
I ran the modeling
assessments by Louis Guillette, a University of Florida herpetologist who has
studied tuatara, including those on North Brother — the island whose population
was modeled in the new paper.
finds the conclusions spot on.
have a few quibbles about interpretations that gloss over factors that he
worries may prove important. Such as genetic variability.
also notes that eggs are not the only temperature-sensitive members of a
population. “Tuatara have one of the longest reproductive cycles: It’s almost
four years,” he notes, with a female “taking almost three years to yolk up her
eggs.” Afterwards, she’ll release them into a nest. Because her body judges when
to breed based on temperature and other environmental cues, mom’s
egg-production period might become seriously perturbed by a warming world, he
problem in assessing some reptiles’ climate vulnerability: lifespan. Tuatara
and crocodilians (including Guillette’s beloved
“They probably take 15 to 20 years to reach sexual maturity and don’t reach
full size until they’re about 35 to 40,” Guillette reports. They not only
appear capable of reproducing for a span of four to five decades, he says, “but
probably live more than 100 years.”
Which points to another concern: In such slow-maturing, long-breeding species, impacts of perturbed gender ratios in the young may be masked for decades —by which time it may be too late to do anything about it.
Mitchell, N.J., et al. 2008. Predicting the Fate of a Living Fossil: How Will Global Warming Affect Sex Determination and Hatching Phenology in Tuatara? Proceedings of the Royal Society B. (July 3). doi:10.1098/rspb.2008.0438
Cree, A., M.B. Thompson, C.H. Daugherty. 1995. Tuatara Sex Determination. Nature 375(June 15):543.
Mlot, C. 1997. Return of the Tuatara: A Relict from the Age of Dinosaurs Gets a Human Assist. Science News 152(Nov. 8):300.