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This Is Why Squirrels Have A Lot Of Babies

These tiny mammalscontribute a significant number of offspring to the gene pool, reveals a new study

Squirrels boost evolutionary fitness by gambling with litter sizes, a new research highlights.

Some have more babies even when they know food is going to be scarce – to give them “a shot at the jackpot.” 

They outperform more conservative counterparts – even if it costs them in the short term, say scientists. Natural selection favors females that have large litters in years when food is abundant. They contribute lots of babies to the gene pool, explained the University of Michigan team.

A squirrel sits in the backlight of the morning sun in Frankfurt’s Günthersburg Park, nibbling on walnut from the winter supply that it had previously dug up from the ground on Jan. 03, 2023. PHOTO BY FRANK RUMPENHORST/GETTY IMAGES

Lead author Dr. Lauren Petrullo said: “We were surprised to find some females have large litters in years when there won’t be enough food for their babies to survive the winter.

“Because it’s biologically expensive to produce offspring, we wanted to know why these females make what appears to be an error in their reproductive strategy.”

They analyzed red squirrels in the Canadian Yukon. They experience a phenomenon dubbed “mast years'” – booms in seeds from the cones of white spruce trees. They get more of their favorite snack once every four to seven years.

Squirrels forecast the large crop of food before it occurs – and increase litter sizes in the months prior. It ensures better future survival for their babies and better fitness for themselves.

Dr. Petrullo said: “There is a constant tug-of-war between the trees and the squirrels at our study site, with each player trying to deceive the other for its own fitness gain.”

The discovery is based on an analysis of data collected by a collaborative field study called the Kluane Red Squirrel Project, that has been running for more than three decades.

Co-author Professor Ben Dantzer said: “Each year, we collect data on how many babies squirrels produce and how many spruce cones the squirrels eat.”

Reproduction was calculated during food booms and busts – revealing fitness differences whether they gambled with their strategy or not. Some played it safe by keeping litter sizes small each year. 

But peers that took a “pie in the sky” approach by having large litters even when food was scarce enjoyed greater rewards if they got to experience a mast year.

Dr. Petrullo said: “In some ways, this strategy of gambling with litter sizes is like playing with fire. “The average squirrel lifespan is 3.5 years and masts only happen every four to seven.

“A female could potentially be sabotaging her fitness by having too many babies in low-food years, hoping for a mast when she may die before she ever gets to experience a mast at all. This could be pretty costly.”

Alternatively, the cost of not gambling at all can be insurmountable. Dr. Petrullo said: “It is essentially impossible for a female to recuperate the fitness costs of not ramping up reproduction in a mast year, so the stakes are extremely high.”

Females that increased litter sizes in low-food years did take a short-term hit to their fitness. But they were more likely to increase litter sizes if and when they experienced a mast, taking home the ultimate prize of greater lifetime reproductive success, she said.

IN FILE – A squirrel is seen at Lincoln Memorial in Washington D.C., United States, on Dec. 29, 2022. Squirrels increase their evolutionary fitness by playing about with their litter sizes. PHOTO BY CELAL GUNES/GETTY IMAGES

The squirrels’ best bet is to take their chances and suffer short-term fitness costs in order to avoid the unmatched cost of missing the fitness jackpot completely.

Dr. Petrullo said: “Determining the relative costs of different types of errors is key to understanding why animals make what look to us like mistakes.”

Scientists are still unsure exactly how the squirrels are able to forecast future food production. The animals may be eating parts of the spruce trees that affect their physiology and alter the number of babies they produce.

Dr. Dantzer said: “This is exciting because it suggests that squirrels are eavesdropping on the trees, but we still have much more to do to solve this puzzle.”

Many animals use food and other cues in their environment to make reproductive decisions. Reliability is declining due to global climate change. Scientists also wonder how the costs of these types of errors will alter what the best strategy is. 

Added Dr. Dantzer: “If the predictability of a food boom is reduced and squirrels can no longer forecast the future, this could impact the number of squirrels out there in the Boreal forest.

“This could be problematic given that squirrels are prey for many predators.”

The study is published in the journal Science.

Produced in association with SWNS Talker.

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