Survival of the fittest can be a hard nut to crack, but it appears that North American red squirrels have it all figured out.

Some squirrels mate like rabbits during seasons of abundance — bolstering the survival rates of their young, a new study from the University of Alberta suggests.

Only twice every decade, spruce trees experience what's known as a mast year, a phenomenon that sees them produce staggering numbers of cones, a staple of the red squirrel diet.

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Alberta researchers have been studying red squirrels near Kluane Lake, Yukon to find out how they are affected by changes in available resources such as food. (Ryan W. Taylor)

Mast years happen only once in a squirrel's lifetime, but some females are able to reap the benefits of an ample food supply, researchers said.

Squirrel baby boom 

"When this happens, there is enough food around to support many more squirrels than at times of low cone production," said biologist and postdoctoral fellow Anni Hämäläinen, the study's lead author.

"Any babies born on the eve of such abundance will have a much higher chance of surviving the harsh winter ahead, relying on a pantry full of cones," Hämäläinen said in a statement.

The researchers monitored the complete life cycle of red squirrels near Kluane National Park in the southwest Yukon from 1987 through 2013 via regular live captures and other reproduction tracking.

The scientific team located litters and marked pups individually within three days of birth, recording the number of pups each female produced each reproductive season.

'Enduring legacies'

North American red squirrels are spontaneous ovulators. Most females produce only one litter per year. But in some years reproduction is skipped, while in other years some females breed twice.

Mothers with the gift of foresight help ensure the survival of their young, and their genetic legacy on the landscape, the researchers found.

Squirrels able to interpret clues in the environment and anticipate food-rich years are rewarded with 30 per cent more surviving young over their lifespan, the study found. Pups born in food-rich years are much more likely to survive to maturity, said Hämäläinen.

"It is a prime example of natural selection due to variation in the environment," said Hämäläinen.

"Female squirrels that can identify a mast year and maximize their breeding efforts accordingly have enduring legacies, as more individuals in the next generation of squirrels will be carrying her genes."

The paper, "Fitness consequences of peak reproductive effort in a resource pulse system," was published in Scientific Reports.