"These results are pretty surprising," Stan Boutin, an author of the study and a biologist at the University of Alberta in Edmonton, told United Press International. "We found some of the first evidence that organisms are actually evolving in response to climate change."
In the study, which will appear in the journal Proceedings of the Royal Society of London next month, Boutin's team studied four generations of red squirrels -- approximately 5,000 of them -- in Canada's southwest Yukon over a 10-year period.
They found the squirrels are giving birth an average of two to three weeks earlier than they were 10 years ago. Over this period, the climate has warmed slightly, by 2 degrees Celsius, and springs have started to arrive earlier.
Using a technique called quantitative genetics, which is used by animal breeders in agriculture, the researchers determined the increased birth period was because of genetic changes in the squirrels rather than behavioral changes.
"Part of the timing of the breeding is certainly determined by genes," Boutin said. The findings indicate individuals with genes that allow them to breed earlier are being selected for, he added.
His team has not yet been able to determine the specific genes that have changed but the quantitative genetics technique they used is "pretty tried and true in captive breeding" for weeding out genetic changes from behavioral changes, he said.
The technique involves constructing pedigrees for the animals and comparing breeding changes to related individuals, such as from daughter to mother to grandmother, he said.
Boutin plans to begin studies next year to determine the specific genes that are being affected.
This is the second documented genetic change occurring in animals due to global warming, D. Liane Cochran-Stafira, an ecologist at Saint Xavier University in Chicago, told UPI.
Scientists reported in 2001 they had detected evidence that milder winters were causing a mosquito species to delay its hibernation time -- a characteristic that is genetically based.
In addition, some birds and frogs in Britain have been observed moving up the times of their egg-laying and spawning, respectively, although it was not determined if this was because of a genetic change.
The animal changes resulting from global warming could have an impact on humans, Cochran-Stafira said.
Although a small change in a species' breeding time may seem inconsequential, there is some evidence global warming might be having an "effect on us in terms of disease and disease-carrying organisms," she said.
"Some of the viral diseases being carried by insects, such as mosquitoes, are moving farther north than we're used to seeing them because the insects are moving farther north and surviving (the milder) winters," Cochran-Stafira said. "(Global warming) does (have) implications for us."
Climate changes also could affect plants. There are no specific examples of this yet, but she said, "I wouldn't be surprised that we see some changes in plants" because many of their biological processes are in response to a combination of light and temperature.
(Reported by Steve Mitchell, UPI Medical Correspondent, in Washington.)