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Squirrels in midwestern, eastern US are fatter this season due to El Nino warmth

Squirrels are extra plump this year thanks to a mild fall across the eastern and midwestern United States.

Temperatures have been 2-8 degrees Fahrenheit above average in these regions this fall. The warmth has yielded a greater food supply for squirrels to feast on.

The abnormal warmth is due to a strong El Niño pattern, and warmth will persist for the central and eastern U.S. into January.

El Niño occurs when tropical Pacific waters are warmer than normal, and the pattern can last several months to a couple of years. The warm waters can impact the weather patterns around much of the globe.

"Tree squirrels will continue to eat while foods are available, [to] increase their fat reserves and also [to] bury seeds for later use," John L. Koprowski, professor and associate director at the University of Arizona, said.

"Body mass tends to be higher in years with good food supplies and mild weather," Koprowski said.

Steve Sullivan, senior curator of Urban Ecology and director of Project Squirrel, said that the increase in fat storage during fall is fundamental to a squirrel's biology.

"These tree squirrels are active 365 days a year, so they have to find a way to stay warm," Sullivan added.

The extra fat acts as an added layer of insulation to help squirrels survive during the winter months.

"Whether they survive or do not survive is based on the amount of fat they put on during the autumn, the amount of fat they can maintain in the face of cold weather and the amount of nuts they can continue to find when nothing is being produced [during the winter]," Sullivan said.

The fat reserves also play a role in squirrel breeding activity.

"Squirrel mating activity typically begins in December and carries on through winter. So, being in great shape this time of year likely means greater reproductive success for the individual squirrel," Koprowski said.