Momma moose with twins, in Yellowstone National Park. These twins are from earlier this spring, and stay by mom at all times. Twins aren't very common, but when there's plenty of nutrition in the environment and the cow is really healthy, twins can occur up to 30-40% of the time. They'll stay with her through the winter, right up until she's ready to have new calves the next year. As the largest members of the deer family, moose can stand 5-6.5 feet at the shoulder, and large bulls can weigh up to 1,800 pounds. They look like ancient animals that walked right out of the last ice age... because they did. The modern moose appeared in the late Pleistocene, about the same time humans came to North America about 150,000 years ago!