Groundhog Day: How did it start? When is it in 2023?

It’s almost time for a furry weather forecaster in Punxsutawney, Pennsylvania to reveal his winter forecast. But how did this tradition come about?

WASHINGTON — The Story from The Conversation is from Stam Zervanos, Professor Emeritus of Biology, Penn State

According to legend, when the groundhog sees its shadow on February 2nd, there are still six weeks of winter; if not, early spring is predicted.

Of course, groundhogs — also known as woodchucks — aren’t showing up at this time just to be furry weather forecasters. So what is the real reason? Research into the groundhog’s biology shows that in early February they have other priorities than mingling with the people of Punxsutawney, Pennsylvania.

How did Groundhog Day begin?

Groundhog Day appears to have European roots. Early February is halfway between the winter solstice and the vernal equinox, and throughout history this seasonal crossroads has been celebrated. The ancient Greeks and Romans celebrated a mid-year festival on February 5 in anticipation of spring. In Celtic tradition, this time was celebrated as the festival of imbolog to mark the beginning of spring. Early Christians in Europe embraced this tradition and celebrated Candlemas on February 2 to commemorate the purification of the Virgin Mary. Traditionally, on this day, the clergy blessed candles and distributed them to the people in anticipation of spring in the dark of winter.

In northern Europe, farmers needed an indication of when to start spring sowing. They looked for the appearance of overwintering creatures like the hedgehog or the badger to herald the coming of spring. Since their appearance occurred in early February, it was believed that when Candlemas Day was sunny and hibernation saw its shadow, more winter weather was in store. But if it rained or snowed on Candlemas, the rest of the winter would be mild.

This tradition was brought to America by the Germans who migrated to eastern Pennsylvania. They found marmots in abundance in many parts of the state and decided that this mammal was a perfect replacement for the hibernating animals they had left behind in Europe. So the tradition continued in America.

Hibernation helps with survival

In my study area of ​​southeastern Pennsylvania, the average date for groundhogs to emerge from their burrows is February 4th. This fits with the folklore and timing of Groundhog Day. However, forecasting the weather is not their goal.

The real reason has to do with Darwinian fitness – a measure of an organism’s ability to pass its genes on to the next generation. The process defines natural selection and is based on an organism’s ability to survive and successfully reproduce. High Darwinian fitness suggests that an individual will pass their genes on to many healthy offspring.

Hibernation contributes to the Darwinian fitness score. It improves survival by conserving energy during times of limited food availability. The ability to hibernate is found in several groups of mammals, including all marmots, many species of ground squirrels, chipmunks, hamsters, badgers, lemurs, bats, and even some marsupials and echidnas. Curled up in their burrows, they spend the winter months when food would be hard to come by.


Hibernation is characterized by a significant drop in body temperature and metabolic function. This process is commonly referred to as solidification. During torpor, bodily functions including heart rate, respiratory rate, and brain activity are reduced. The overall benefit to the animal is to conserve metabolic energy at a time when it is not eating.

However, for some unknown reason, hibernators regularly wake up during their hibernation period. These excitations are associated with large energy costs. Therefore, arousal must be vital in some way, otherwise the animals would not waste their energy on it. Some possibilities include maintaining cellular functions or clearing body waste.

In Pennsylvania, these bouts of torpor and excitement continue throughout the hibernation season, beginning on average in mid-November and ending in early March; a total of about 110 days. In one study, an average of 15 seizures occurred during this period, with excitement in between. Marmots awoke for about 41 hours and then reverted to the torpor state for about 128 hours in males and 153 hours in females.

In a 2010 study, we found that marmots’ hibernation periods increase with increasing latitude. The hibernation corresponds to the duration of the winter. The celebration of Groundhog Day would have to be changed depending on the latitude to perfectly coincide with the appearance of the groundhog.

One of the disadvantages of hibernation is the reduced time available for reproduction. Therefore, hibernators have evolved mating strategies to maximize reproductive success. Marmot mating strategies include transient emergence in early February, mating in early March during their last arousal, and giving birth in early April. This behavior improves reproductive success as the young are born as early as possible (but not too early) and can begin feeding in May when plenty of food is available. This gives them enough time to gain enough weight to survive their first hibernation.

But why do marmots show up in February when mating doesn’t happen until next month? The answer lies in their social structure. For most of the year, male and female marmots are solitary and antagonistic to each other. They aggressively maintain a feeding territory around their burrows and rarely have contact with each other. February serves to reestablish the bonds necessary for mating and allows mating to resume without delay in early March.

So for the animals themselves, Groundhog Day is more like Valentine’s Day. On February 2nd, groundhogs don’t show up to predict the weather, but to predict if their own mating season will be successful!

This article is from The conversation, an independent, non-profit news organization dedicated to spreading ideas from experts. Republished under a Creative Commons license