In animals, seasonal changes are tracked by the photo-neuroendocrine system, a sensitive collection of glands, hormones, and neurons that are wired to adjust our internal chemistry as the length of a day changes. When the photoperiod begins to shorten in the fall, squirrels take notice.

Just as they store food underground for use during the winter, squirrels store fat on their bodies in preparation for the cold. When food is scarce, a good layer of fat offers the energy a squirrel might need to survive. It also helps them stay warm when the temperature drops.

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