Reindeer weight fluctuates throughout the year in accordance with the seasons. Once spring comes along in the tundra (around April) and plant life returns, herbivorous reindeer can graze to their hearts' content. As vegetarians, reindeer and caribou get their fill from the grasses and shrubs that grow in the tundra. Males reach their peak weight of around 220 pounds (100 kilograms) in August [source: University of Alaska Fairbanks]. At this point, they go "in rut," which means they begin mating. Mating season is also the cutoff point for females to chow down, since they'll have to endure a winter pregnancy with scarcer rations.
Once winter sets in and snow covers the tundra, the reindeer diet narrows down to one food: lichen. Lichen survives the cold winters because of its unique biological combination of algae and fungus. Thanks to the algae, lichen doesn't require much sunlight to produce chlorophyll; the spongy material of the fungus also holds up well against the harsh temperatures.
Reindeer sniff out lichen beneath the snow and use their curved hooves or antlers to uncover it. During this time, reindeer maintain more sedentary habits to preserve their energy [source: University of Alaska Fairbanks]. The lichen-only regimen is the reverse of the Atkins Diet. The plant contains high amounts of carbohydrates, but no protein. Consequently, the carbs provide the reindeer a source of quick-burning energy that carries them through winter. However, lichen isn't a particularly hearty food, and for that reason, reindeer will eat 4 to 11 pounds (1.8 to 4.9 kilograms) of reindeer moss each day [source: Dieterich and Morton]. That's why reindeer pack on the pounds in the warmer months when there's more to choose from. In fact, these animals gradually lose weight starting in the fall and continuing to March [source: University of Alaska Fairbanks].
Recently, the changing climate has threatened the reindeer and caribou populations. Hotter temperatures have led to more rain and snow melt in the tundra regions. That, in turn, results in sheets of ice forming on top of the snow, once the water refreezes. Reindeer and caribou cannot safely break through the ice with their hooves, leading some to starve to death [source: Mcfarling].
With their seasonally influenced diet, the survival of reindeer depends on the balance of nature. If climate change tips that balance too far, reindeer and caribou could become zoological relics of the past.
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