Winter is a threatening time for many species. We all have our own ways of getting through. But many tiny insects experience potentially damaging thermal shocks on a daily basis. Even a passing cloud could impair a chilled insect's progress. However, insects are remarkably adaptable creatures, and under some circumstances, the briefest pre-cooling period seems to protect them from perishing in the cold. But Richard Lee was curious to discover whether this phenomenon of `rapid cold-hardening' extended beyond basic low-temperature survival. Could it permit an insect to maintain a wide range of physiological functions that would normally be lost when the temperature dipped(p. 1797)?
Lee explains that when it comes to `ecologically relevant functions' the ability to mate is probably one of the most fundamental. So he reasoned that if rapid cold-hardening could reset an insect's ability to mate at low temperatures, these cold-hardened insects would have a significant adaptive advantage over unprepared insects.
Meanwhile, as Lee was pondering this question, a new undergraduate student arrived in the lab, Scott Shreve, only recently graduated from high school. Lee paired Shreve up with Jonathan Kelty, an experienced graduate student in his Miami University lab, and set them the task of discovering whether rapid cold-hardening allowed Drosophila to mate at low temperatures.
Having chosen an ecologically relevant function to test, the team decided that the rapid cold-hardening temperature drop should be relevant too. The team introduced a chill into a warm summer's day by lowering the temperature from 23°C to 16°C. After allowing individuals an hour to adjust Shreve paired the insects up and watched. The rapid cold-hardened insects courted enthusiastically, and 11 of the 20 couples mated successfully.
But what happened to fruit flies that hadn't cold-hardened; would the 7°C drop affect their performance? Introducing unprepared chilly males and females to each other, Shreve watched their sedate courtship ritual and waited for them to mate. But after an hour the tell-tale signs failed to appear. Lee admits that he's rarely seen such clear data before; rapid cold-hardening allowed fruit flies to mate at temperatures where other chilly flies lost the urge. And when the team tested the rapid cold-hardened insects back at 23°C, they were every bit as active as they had been at the cooler temperature. The rapid cold hardened insects appeared to have broadened their active thermal range.
Wondering whether rapid cold-hardening might have a metabolic basis, Shreve and Lee compared rapid cold-hardened and unprepared flies' metabolic rates,and found that they were essentially unchanged. Rapid cold-hardening hadn't reset the insect's metabolism.
Although Lee is none the wiser about the mechanisms behind this remarkable adaptive phenomenon, he now believes that rapid cold-hardening is a widely used process, where the insects `fine tune their physiological function to match small changes in environmental temperature'.