Energy Demands on the Caribou
In the winter the caribou must paw through the snow to get to the food (lichens). Fortunately the caribou can smell the lichens under the snow. If the snow happens to be deeper, it takes more pawing to find the food with less time eating it and a greater energy expenditure. However it was found that if reindeer eat lichens as opposed to other food, they do not need to drink as much water. In cold weather, drinking water or eating snow has an energy cost associated with it and in eating lichens the energy cost of taking in water is reduced. In the late winter when the nutritional levels are low, gastrointestinal nematode parasites can be at high infection rates and this can decrease the body condition and fitness of the animals.
It is not all that easy in the summer either. During the peak insect weeks, mosquitoes, biting flies, and parasitic nose bot and warble flies harass the caribou. If summers are warmer, the insects are more numerous and the caribou feed less and spend greater amounts of time twitching and running.
Body fat stores are lowest over the months of April to mid-June. Energy stores become a concern again in early summer to mid-summer (~ first week of August) when the insects are most bothersome. The caribou are the fattest in late summer to late fall (~ end of November).
The energy balance of the cows can be estimated for each season of the year and depends on which habitat is most vital. A habitat vital for the survival of the calf and cow is the one that supports her needs when she is using more energy than she takes in as food. If the cow is somehow displaced from the normal range or activities, she will not recover that energy difference and will be in an energy deficit for that period of time. The calving period is when the cows are most vulnerable to this. The cows are at their lowest energy levels, are in the poorest physical condition, they are least tolerant of human disturbances, and consequently approximately 50% of the calves die. The mid-summer is the next most risky time when the cows have high lactation demands (lactation doubles their energy demands and caribou milk is rich in fats) and intense fly harassment. Fall and winter are the least challenging but if body fat levels are not high enough the cow will not become pregnant and if snow depth or cold conditions prevail, feeding and movement will be hampered. In the winter the animals are more tolerant of human disturbance and there is a low density of animals over the expansive wintering range.
Calves can be weaned at any time but they are most likely to survive when weaned in the fall during the rut. If the calf is weaned through the summer or early fall, it will not survive howver the cow will have a good chance of getting pregnant again. Extended lactation until the following spring is not unusual in Porcupine Caribou and is associated with low fat reserves in the calf. The cow will not likely get pregnant due to lactational infertility but she will increase the chances that her calf will survive. These cows also generally have the lowest body condition scores.
Adaptations to Cold Climates
Caribou and Reindeer are well adapted to living in extremely cold environments. Their overall biochemistry and metabolism, the digestive processes, and their physique is well suited to this environment.
In late winter when the caribou are migrating and undernourished due to the terrain and time of year, different fatty acids are mobilized from fat stores. The fat stores or adipose tissues are located around the kidneys and in the abdominal cavity, around the sternum, at the top of the shoulder, along the backbone, around the heart, and over the rump (See Body Condition Scores). Fat is also stored in the marrow of the large bones. When analysed these stores generally have high proportions of oleic acid (C18 fatty acid) and palmitic acid (C16 fatty acid) in the early winter and late spring. If animals, particularly calves are undernourished, the essential fatty acids linoleic and alpha-linolenic acids (C18-polyunsaturated fatty acids) are selectively mobilized during the winter. Interestingly newborn calves have brown fat stores with the capability to produce heat but by the time the calves are around two months old the brown fat has been converted to white fat.
Thyroxine hormone levels in reindeer appear to decrease in the winter whereas growth hormone increases in the winter. This could indicate that caribou have high lipolytic activity and reduced metabolic rates in the winter.
Blood tests on captive reindeer show that total protein in the serum (including albumin and globulin) decreases in the winter with some increase in urea. This suggests that some muscle is being broken down for protein needs because the animals are not getting enough nitrogen in their food. However when undernourished there is still little or no muscle wastage in calves. It appears that the caribou have adapted to the low nitrogen nutritional levels and with little protein degradation from muscle.
Rumen fermentation is reduced in the winter as are nitrogen levels in the rumen contents. Caribou are ruminants but also use their cecum to digest their food. With change of the seasons the microflora populations change and contribute to the ability of the animal to maximize the nutrition it receives as plant types change over the year.
In the summer the heart rate of caribou increases and in the winter the heart rate decreases. This is likely because of increased food intake in the summer and decrease in the winter, which relates to blood flow to the gastrointestinal tract based on meal size.
The legs of caribou are long and thin with large surface areas and are therefore major avenues for heat loss. However heat loss is minimised through countercurrent heat exchange. Arterial blood, originating in the animal's core is warm whereas the venous blood returning from the limbs can be very cold. Arteries in the leg lie next to the veins and as the warm blood moving through the artery passes the vein, the warmth is passed to the venous blood moving in the opposite direction. By the time the arterial blood reaches the periphery, the blood is precooled to within a few degrees of the ambient temperature. Conversely the venous blood is warmed by the arterial blood so that it is nearly the same temperature as the body core.
Because the temperatures in the winter can be so extreme, the legs and feet of caribou must tolerate being nearly frozen. The cell membranes of these tissues have a higher proportion of lipids that are less saturated (and a lower melting point) than the fats of the body core. If winter conditions are such that the caribou are in poor condition, the protective lipids of the limb tissues can be selectively mobilized to use as energy and if advanced, may impair the functioning of the legs in the cold.