Species like the brown anole
lizard maintain their body temperature by moving between sunny and shady areas,
in a process called behavioral thermoregulation. Credit: Ken
Thomas via Wikimedia Commons
The brown anole lizard in the
Bahamas is raising questions about whether some cold-blooded species may be
able to adapt to global warming.
Scientists consider cold-blooded
species particularly vulnerable to climate change because of their sensitivity
to even small temperature shifts. Species like the brown anole lizard maintain
their body temperature by moving between sunny and shady areas, in a process
called behavioral thermoregulation.
Because these species move
between "microhabitats" to avoid overheating, some researchers
believe that they don't face adaptive pressures that would favor the survival
of individuals with greater heat tolerance. This is bad news for cold-blooded
species. If they are unable to adapt, then rising temperatures will lead to
their extinction.
"The potential for the
organisms to evolve in response to a rapidly changing environment is often
downplayed, even though very few studies have examined the extent to which
species might adapt," said Michael Logan, who recently received a Ph.D.
from Dartmouth College and is now a postdoctoral fellow at the National Science
Foundation.
Logan and two other scientists
from Dartmouth and the University of Virginia set out to test the adaptive
pressures of simulated climate change conditions on brown anole lizards (Anolis sagrei).
The researchers captured and
transplanted about 100 male lizards from their forest habitat to a hotter area
with greater temperature variability about half a mile away. Then the
researchers tested how fast the lizards could sprint at a range of
temperatures. The researchers published their
results in the Proceedings of the National Academy of Sciences, with Logan
as lead author.
Running quickly comes naturally
to the brown anole lizards. "They are the Usain Bolts of the lizard
world," said study co-author Ryan Calsbeek, an associate professor at
Dartmouth, referring to the record-breaking Jamaican sprinter. The lizards need
to move quickly to hunt for food and to avoid predators, and previous research
has shown that their sprint speed is strongly correlated with survival and
reproductive success.
Before each sprint, the
researchers placed the lizards in an incubator to adjust their body
temperature, then had them run as fast as they could along a short track. Each
lizard did five sprint trials for each temperature, with a two-hour break
between each sprint.
After the initial tests, the
lizards were released, and the surviving lizards were recaptured at the end of
the breeding season. The researchers then compared the results to another 100
male lizards they used as controls.
"We weren't at all sure
whether we would see selection when we started this study. If I had to put
money on it, I would have bet against it," Logan said.
Just 22 percent of the
transplanted lizards and 45 percent of the control lizards survived the breeding
season.
When the researchers analyzed
their data, the results showed clear selection pressure in favor of the most
heat-tolerant lizards in the transplanted lizards, but no trend among the
controls.
"The individuals that ran
the fastest at the warmest body temperature and those that ran at the broadest
range of temperatures did the best," Logan said of the transplanted group.
Their research is the first time
that thermal performance has been used as part of the framework of measuring
selection and heritability in cold-blooded species, according to the
researchers.
"I was surprised that the
selection was as strong as it was. Often when you measure selection in nature,
it's at a very low magnitude," said co-author Robert Cox, an assistant
professor at the University of Virginia in Charlottesville. "The fact that
the expectation and the results match so well gives us assurance that it's
actually happening.”
The study did not show whether
these traits could be passed on to future generations. If the traits are
heritable, that could mean the lizards may be able to rapidly evolve to climate
change pressures, which would be good news for the species.
Cox's laboratory in
Charlottesville is now investigating whether the lizards are able to pass on
the ability to run fast at higher temperatures and at a broader range of
temperatures to their offspring. If they can, then it would appear the lizards
could rapidly evolve in response to climate change, according to the
researchers.
Testing heritability is easier in
laboratory conditions because all the animals are raised in the same
environment and the researchers know the relatedness of all the lizards, Cox
said.
Calsbeek said he did not want the
study's results to be misinterpreted. "This is not us saying that climate
change is not a problem," he said. It's also unclear whether other
cold-blooded species may also be able to adapt.
"Because there have been no
other studies to look at these particular traits, it's hard to tell how broadly
applicable it would be," Calsbeek said.
Luke Mahler, a researcher at the
University of California, Davis, who studies evolution patterns
in Anolislizards, is also skeptical about how broadly the researchers'
results could apply even among the nearly 400 other lizard species within
the Anolis genus.
"I think it's a stretch to
suggest more generally that tropical animal populations may be capable of rapid
adaptation to anthropogenic warming," said Mahler, who is also co-chairman
of the Anoline Lizard Specialist Group, which studies which anole lizard
species are at risk of extinction at the International Union for Conservation
of Nature.
"The reason I worry about
this suggestion is because Anolis sagrei is hardy, abundant,
widespread and tolerant of lots of temperatures. It occurs naturally over a
wide range of thermal environments, and has experienced such variety over
millions of years. What this means is that the species probably has loads of
heritable variation in thermal tolerance for natural selection to work
on," Mahler said.
By comparison, phenacosaur anoles
living in cloud forests have had very little exposure to temperature
variability for over 10 million years and are very much at risk from climate
change, he said.
Still, the study does provide
convincing evidence that there is natural selection for thermal performance
traits.
"It's great to see an answer
to the important question of whether behavior always prevents tropical animals
from adapting to climate change," he said. "I think in this case, we
have satisfying evidence that the answer is ‘no.’"
Reprinted from Climatewire with
permission from Environment & Energy Publishing, LLC.
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