It’s late May on the tundra of the North Slope of Alaska. It’s barely above freezing, and the wind gusting doesn’t help at all. Large patches and expanses of snow still cling to the ground, reluctant to melt, resulting in a patchwork of swampy slush extending as far as the eye can see. Tiny shoots of tundra sedges poke up gingerly from the edges of the melting snow patches.
A small group of caribou walk around the snow patches, picking bites here and there from the newly emergent vegetation. From their antlers and their distended bellies, it’s evident that these caribou are pregnant females, on their migration northward to the coast of the Arctic Ocean to give birth. They eagerly harvest the fresh forages – their first taste of fresh growth in months. For most of the winter, they’ve been subsisting on lichens and dead forages dug up from underneath the snow in pits called craters. They’ve been able to fill their stomachs with this food, but it offers little in the way of nutrients for the growing calves in their bellies.
Caribou from around the globe face a similar dilemma each year – how to produce a calf on a limited “budget”. For the most part, caribou can store limiting nutrients like energy, protein, and minerals in fat, muscle, and visceral (organ) tissue until they are needed – in which case the mother caribou can break these nutrients down in her own body and transport them to her growing calf via her bloodstream (in utero) or through milk (after birth). Although caribou are able to break down their own bodily reserves to produce a calf, most of the energy to do this comes from their diets – the energy-rich lichens and dead forages dug up from craters in the winter. These winter forages tend to be relatively low in protein content, however, so most females rely on body reserves to shuttle needed protein to growing calves.
Why is protein such a necessary food component? Protein forms the basis of all cellular “machinery,” from the contractile mechanisms that make up muscles and allow animals to move, to the intro-cellular machinery that gadgets that “run” the metabolism of the cell. Protein is also a major component of larger things like hair and hooves, which are needed to help the animal survive cold winters and dig through the snow. Proteins can even help break down other proteins in your digestive system, such as pepsin and trypsin. When fat stores fall too low, protein can also be burned for energy. Without protein, no life would be possible.
Caribou rely on brief periods during the year to resupply their nutrient stores, analogous to filling up a gas tank. The highest concentrations of protein are available for caribou in spring, with new flushes of vegetation growth. Why is protein content highest during this specific time? The answer lies in the life cycle of forage plants. Each spring, as plants begin emerging from the tundra, they invest first in creating photosynthetic machinery high in protein, which allows the plant to produce carbohydrates. These carbohydrates are used by the plants for energy and to create fiber, which lays down a “skeleton” for the plants to keep growing. As the season progresses the plant tissue becomes increasingly diluted with fiber, which slows digestion in caribou in addition to physically diluting the protein content.
Plants, including the species which caribou favor, tend to grow in accordance with environmental conditions. We are interested in how these environmental conditions change the protein content of forages for caribou across the summer range of two herds on the North Slope – the Central Arctic and Teshekpuk Lake Herds. Specifically, we are interested in measuring available protein content – not all protein is able to be used in forages, so quantifying exactly how much is digestible will help in monitoring range conditions. This information will help in judging the overall quality of the range for a caribou herd. We can also look at how much protein is available in general to each caribou – individual effects, which when multiplied up to the scale of the herd, can translate into population-level effects for each of these herds.