A woman visiting Yellowstone National Park has shared a very special video showing a herd of bison including several calves stampeding across a bridge. The youngsters, also known as 'red dogs' due to their ginger coats, are born between April and May, and stay with the herd for two to three months.
In a Facebook reel, Vanessa Lynn-Byerly explained that the animals "literally shook the bridge", and made the road bounce so hard that it was hard to hold her phone steady. She said the experience was amazing to witness in person.
According to the National Park Service, the reason bison calves are born within such a short timeframe is 'birth synchrony' – an evolutionary adaptation that may be a response to food availability. Female bison (cows) need extra calories to provide their calves with milk, so the calving season may well be timed to happen as green grass becomes readily available, giving the newborns the best start in life.
Birth synchrony may also help increase the survival rate of bison calves. A predator may easily pick off a lone calf if they were born throughout the year, but when many are part of the herd at once, more of them are likely to survive to maturity.
Safety in calving season
Seeing the 'red dogs' can be an incredible experience, but remember that bison are fiercely protective of their young, and are likely to be particularly aggressive and territorial during the calving season.
Make sure you always stay at least 25 yards (23 meters) from bison, and never approach or distract them. If you're not sure how much space to leave, close one eye, hold out your arm, and give the animal a thumbs-up. If you can completely obscure it with your thumb, you're OK.
For more advice, see our guides how to avoid being gored by a bison and wildlife safety: eight tips for unexpected encounters.
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Cat is the editor of Advnture, She’s been a journalist for 15 years, and was fitness and wellbeing editor on TechRadar before joining the Advnture team in 2022. She’s a UK Athletics qualified run leader, and in her spare time enjoys nothing more than lacing up her shoes and hitting the roads and trails (the muddier, the better), usually wearing at least two sports watches.