The arctic tundra has several harsh conditions such as very low temperatures and a long night.
However, life still exists there, some animals and their adaptations are:
• Caribou - Some populations migrate during the coldest months of the year; also, the coat of the caribou has two layers, which provides insulation against the cold, and the blood of their legs gets heated before reentering the torso thanks to a special array of the blood vessels. Their hooves also change with the seasons to give them a better grip.
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• Musk Ox - They are noted to have a thick coat of fur to protect them from the cold.
• Arctic fox - They have thick multilayered white fur that also serves them as camouflage in the snow, which changes color during summer. The Arctic fox has a low surface area to volume ratio, which means that less of his body is exposed, and that way it doesn't lose much heat.
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• Willow ptarmigans - These birds have seasonal color changes, which serve as camouflage. They also migrate.
We can see some recurrent adaptations here: Thick fur, changes in color depending on the season, a small surface to prevent heat loss, and migration. These characteristics allow artic tundra animals to live in the tundra ecosystem.