In a country where "warm" periods don't last too long, one amphibian has devised a neat trick to take full advantage of high temperatures while they last.
New research revealed the pool frog (Pelophylax lessonae) is able to speed up its growth rate when the weather is warmest during breeding season in Sweden, thus ensuring its tadpoles develop properly.
As a species dependent on relatively warm environments not only for successful tadpole growth, but also for reproduction, plasticity is thought to be key to the frog's survival in cold climates.
Two scientists from Sweden's Uppsala University tested the growth rate of pool frogs by collecting frog spawn from ten different breeding females in Poland, Latvia and Sweden.
The frog spawn were then placed in different environments to breed: one set to a low temperature for the species (19 degrees Celsius) or one to a higher temperature (26 degrees Celsius).
While tadpoles from all three regions grew at the same rate under the lower temperature, the Swedish tadpoles grew faster than the central European (Latvian and Polish) variety in higher temperatures.
"Since Sweden has briefer periods of high temperatures than Poland and Latvia do, this increased growth capacity under warm conditions allows this frog to take full advantage of the short periods of high temperatures," said Germán Orizaola, one of the study's co-authors. "As a result, it is able to complete its life cycle - which relies heavily on warm temperatures - at high latitudes such as in Scandinavia."
In Sweden, the pool frog doesn't even consider breeding until middle or late May, when pond water temperatures are at least 16 degrees Celsius. In contrast, some frogs begin reproduction almost two months earlier, as soon as ponds start to thaw.
"Considering that pond temperatures drop once autumn arrives to levels that prevent further tadpole development, the period of time that these frog larvae have for development at northern latitudes is very limited," Orizaola added.
The pond frog's ability to use different strategies to compensate for changes in their environment is known as plasticity. Researchers noted that a greater difference in growth and development values among tadpole siblings bred at different temperatures indicates greater plasticity.
"The fact that tadpoles bred in Sweden can maximize their growth during the brief periods of high temperatures that characterize these latitudes is indicative of the Swedish pool frog's increased plasticity," Orizaola said.
Ultimately, plasticity is what allows the heat-dependent amphibians to maintain viable populations in such unfavorable northern latitudes.
Their study was recently published in the journal Evolutionary Applications.
