During the winter season grasses and dwarf shrubs are covered in snow and nearly inaccessible to northern-latitude herbivores. In order to adapt to the extreme situations that come with changing seasons, red deer have learned to "shrink" their gastrointestinal tract and extract nutrients from food more efficiently. 

In a recent feeding experiment, veterinary scientists from the Research Institute of Wildlife Ecology at the Vetmeduni Vienna and Hannover University investigated the seasonal diets of 16 female red deer. Their study took place over a period of three years, during which time they were able to observe changes in how the animals regulate their food intake and nutrient uptake, according to a news release.

It turns out red deer eat half as much food in the winter as they do in the summer,  even when food pellets were readily supplied. Furthermore, their visceral organs mass was significantly lower, suggesting these well-adapted animals "shrink their stomachs."  

Red deer (Cervus elaphus) are European natives and among the country's largest resident deer species. Historically these animals have been hunted for their venison and ornate skins and antlers, as well as by farmers that are protecting their economically-important crops. While the animals were once threatened by over hunting, conservations efforts established in the 20th century are proving to boost red deer populations across the U.K.   

"Reduced appetite in winter apparently helps the red deer to avoid time and energy expenditure for unproductive search for food," Walter Arnold, first author and head of the Research Institute for Wildlife Ecology, explained in the release. "A lower food intake leads to a smaller gastrointestinal tract, which in turn helps to save energy."

Reduced consumption, however, raises the concern that these deer are not getting sufficient nutrients but after examining the active uptake of glucose and proteins from the digested food, researchers found the deer actually enjoy a more efficient nutrient uptake in winter.

"The extraction of nutrients from food is more efficient in winter than in summer. Red deer take in less and nutrient-poor forage in winter but exploit this to the maximum. Every calorie counts, because the energy balance is notoriously negative in winter, and fat reserves are limited," Arnold added. "In the summer red deer don´t spend much time on digesting food. They maximize energy uptake by eating a lot and quickly making room for more food, since they can easily access food resources during the summer."

Their findings were recently published in the American Journal of Physiology.

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