Nutrition is an important part of any animal industry, and of course is an imperative discipline for our species. I have recently started a job for a pet food company here in New Zealand that has really encouraged me to go back to the technical nutrition papers I studied at University that I perhaps did not need to utilise as much with my zoo and vet nursing work. As I said, nutrition is an extremely important part of every organisation dealing with the care of animals, but specific energy requirement calculations and formula are seldom used by nurses or zoo keepers. Mathematics was never my strongest point, so I tend to get a little cross-eyed when working with scientific calculations – but I must say, I’m getting the hang of it again! Who knows, perhaps this post may be useful to someone.
Energy is of course required for survival. Not only this, an individual’s energy requirements vary depending on different factors. For example, energy for survival maintenance is needed for respiration, circulation, temperature regulation etc. but surplus energy is then required to properly sustain different life stages such as growth and reproduction. Not only do energy requirements vary, but nutritional requirements differ over time as well.
Wolf pups are weaned from their mother’s milk at 5-9 weeks of age, but may continue to nurse several weeks thereafter. In general, wolf milk is similar to that of the domestic dog but is high in protein and lower in fat. Naturally produced wolf milk contains more arginine than some commercial puppy milk replacers, so wolf pups raised by humans and fed a milk formula commonly develop cataracts without an arginine or lactose supplement (lactose assists in the absorption of arginine). Gelatine is a common arginine source that can be mixed with puppy milk replacer.
Prolactin from the pituitary gland is required for lactation maintenance (and may also play a role in pup development). Prolatin has a definite circannual rhythm (it works in repetitive yearly cycles) in both sexes, rising in spring with the onset of long days and decreasing during the short days of winter. Prolactin can induce or maintain parental behaviour in mammals. It may also be associated with coat shedding; as Prolactin increases in spring, wolves shed their thick, insulating undercoat. In autumn as day length and Prolactin levels decrease, the summer coat is replace by a thick winter pelage. Such associations Prolactin may have with reproduction and shedding, etc., will most likely be the outcome of complex reactions with other hormones.
There are three main growth periods for new pups (based on well-fed captive wolf pups):
1) 0-14 weeks – this period sees maximum growth rate; approximately 1.2kg per week for females and 1.5kg per week for males,
2) 15-27 weeks – rapid growth rate of approximately 0.6kg per week for both sexes,
3) 28-51 weeks – slow growth rate of roughly 0.03kg per week for females and 0.02kg per week for males.
Growth ends at about 12-14 months when the growing points of the radius and ulna (the two longest bones in the forelimbs) fuse together. Rates of physical development (such as tooth replacement) generally mirror those of weight gain. Adult dentition is in place at about 6-7 months of age.
BASAL METABOLIC RATE
The eventual full-grown size of a particular individual wolf depends on many different factors such as genetics and parasites. Underlying all factors is the wolf’s ability to transform its food into useable energy. If a wolf is unable to sufficiently absorb enough nutrients from food, its body then utilises its own tissues – i.e. the body literally consumes itself for the purpose of survival. This effect, of course, is far from ideal and can ultimately result in death.
Metabolism is the sum of all chemical changes occurring in tissue, consisting of anabolism (the synthesis of smaller, simpler molecules into the complex ones of living tissue) and catabolism (breakdown of complex molecules into simpler ones, usually resulting in a release of energy). The Joule is the standard unit of energy used in metabolic studies.
Basal Metabolic Rate (BMR) represents the metabolism of an animal at rest in a thermally neutral environment. It is generally calculated with the following equation:
BMR(kcal/day) = Body Weight (kg)0.75.
Body size can affect BMR – for example, mice produce less total heat than elephants, however mice produce significantly more heat on a per-weight basis because they have a much higher BMR. Animals expend energy in proportion to their weight raised to the ¾ power – thus, they require energy on the same basis (i.e. smaller animals need more calories per kilogram of weight than large animals). If wolves had the Metabolic Rate and energy requirements of mice, they could not possibly catch enough to survive. Heart rates and body temperatures reflect also BMR. Small animals have higher heart rates and body temperatures than large animals. Wolf heart rates are similar to those of large dogs.
Feeding on protein increases BMR and heat production and elevates body temperature. In wolves, BMR increases after feeding to an average of 233.8J/hr and remains at that level for approximately 15 hours after feeding. Muscle tone contributes about 20% of total heat production; during strenuous exercise, oxygen consumption may increase twentyfold while energy expenditure may increase one-hundredfold. The different between these two measures represents an ‘oxygen debt’ which is ‘repaid’ by a continued elevated rate of oxygen consumption after exercise ends. When BMR is combines with maximum oxygen consumption, a metabolic index (volume of oxygen/BMR) can be calculated.
Wolf BMR averages 158.8 +/- 17.9J/hr. Adult males have a larger BMR than females, and pups. The energy consumption of running wolves is 5,070kJ/hr which, when combined with average wolf BMR (5,070/158) gives a metabolic index of 32 – this is about three times higher than the average for mammals as a whole. The high metabolic index in running wolves may reflect the wolf’s adaptation for high running speeds and endurance while chasing prey, and for saving energy when no prospective prey is available.
Temperature extremes also affect Basal Metabolic Rate. The average body temperature of the wolf is 39.6C. This they attempt to maintain through a combination of physical and chemical processes. Domestic dogs are unable to regulate their temperature at birth but are able to do so by four weeks of age – it is thought that wolves may be able to thermoregulate even earlier than this.
During times of falling or colder ambient temperatures, wolves maintain their body temperatures by curling up to reduce surfaces exposed to cold or by increasing insulation effectiveness of their fur through piloerection (the involuntary bristling of hairs). Peripheral blood vessels may constrict to reduce the thermal gradient between skin and the environment due to heat loss. Wolves have also evolved to utilise a process known as countercurrent exchange to reduce heat loss from limbs to the environment (which is significant). Within the limbs, deep arteries and veins run close together. Cooled blood returning from the limb surface picks up heat from the warm arterial blood. The arterial blood thus conserves heat by rewarming the venous blood before it returns to the body core. Arterial blood meanwhile loses less heat when it perfuses the skin. This process may be maladaptive for arctic animals exposed to extreme cold, however; when ambient temperatures are tissue-freezing, a countercurrent system might exclude so much heat from limbs that they might actually freeze. Again, wolves have evolved protective mechanisms to prevent this problem; they have unbranched arteries that carry blood directly through the footpad to a cutaneous vessel network in the pad surface. This plexus keeps foot temperature just above the tissue-freezing point (about -1C). Maximum energetic efficiency is achieved because the unit of heat exchange is located in the pad surface that contacts the cold substrate, rather than throughout the pad, where tissue damage could occur if the cooled blood fell below freezing.
External temperatures at which these heat-retaining mechanisms are insufficient to maintain a constant body temperature and at which heat production must be increased is known as the lower critical temperature. Below this temperature wolves can generate internal heat through exercise and shivering. During prolonged exposure to cold, however, the endocrine system increases metabolic heat production. Although this effect is primarily mediated by the thyroid hormones, others, such as growth hormone, insulin and adrenal hormones exert a regulatory effect.
When ambient temperatures rise, wolves may seek cooler environments (shade or a den, etc.) in which to cool down. If this strategy is unavailable or insufficient, cutaneous vasodilation (the dilation of blood vessels) increases and raises the skin temperature to foster convective heat loss to the environment. Heat is also dissipated by sweating – domestic dogs do this via glands in the footpads, but wolves actually have 80% fewer sweat glands in their pads. Panting coupled with increased salivation expels body heat through evaporative cooling.
Wolves in deserts have a higher Basal Metabolic Rate in summer than in winter. To dissipate excess metablolic heat and maintain a constant body temperature, these wolves must evaporate a substantial quantity of water. Fortunately desert wolves are extremely mobile, allowing them to find water that other species may not have such an ability to do. Wolves are, nonetheless, certainly better adapted to cold or temperate climates.
FIELD METABOLIC RATE
Basal Metabolic Rate reflects energy expenditure in a uniform state (e.g. sleeping), whereas Field Metabolic Rate is an estimate of total energy expenditure including basal metabolism, maintenance, thermoregulation and activity. After these essential energy requirements are met, extra energy may then be allocated to growth, fat deposition or reproduction. Thus, the daily food requirements of a mammal such as the wolf are determine by its Field Metabolic Rate.
According to Wolves: Behaviour, Ecology and Conservation (see end of post), a 37kg wolf has a Field Metabolic Rate of 17,700kJ per day. The average daily Basal Metabolic Rate is 3,811kJ per day (158.8 x 24 hours). The ratio of FMR to BMR is 4.6 (17,700 / 3,811). This FMR/BMR ratio is higher than for mammals as a whole.
We can also compare the FMR or wolves to that of other eutherians (placental mammals). The wolf’s FMR is about 26% higher than that of a typical eutherian of similar body mass, suggesting that wolves work harder than the average mammal to survive.
When well fed, wolves store fat under the skin, around the heart, intestines, kidneys and in bone marrow. They tend to increase their fat stores during autumn and winter. Wolves kill prey sporadically, often going for days between feeds. During fasting periods, wolves must rely on catabolism – which results in weight loss. As fasting progresses, fat from all subcutaneous and internal deposits are used simultaneously, but subcutaneous fat is depleted first, then visceral fat, then kidney fat. Marrow fat is then rapidly metabolized. Foreleg bones are depleted before hind leg bones, and within those bones proximal fat stores are depleted before distal stores. If food is finally found after this period, wolves will gorge and eat up to 19% of their weight in a single feeding. This means wolves are able to make up for weight loss as long as nutritious prey is available.
Environmental temperatures affect the energy sources that wolves use. Mammals increase metabolic heat production when environmental temperatures fall below their lower critical temperature. The principal type of fuel used under these conditions is fat. Animals in cold environments that mainly use fat for fuel should have more Free Fatty Acids (FFA) in their blood than more temperate-dwelling species, and this is indeed the case among wolves. Plasma Free Fatty Acid concentrations in arctic wolves that are fed fish and caribou are 203 times higher than those of temperate zone domestic mixed-breed dogs.
Wolves appear to be able to utilize saturated FFA at a higher rate than unsaturated FFA, causing a deficit of saturated fats in the plasma. This may be a life-saving strategy that has evolved in wolves. Saturated FFA in domestic dogs at low concentrations can cause sudden death by massive thrombosis (clotting of the blood). Unsaturated FFAs do not seem to form clots readily, so are better tolerated. Because saturated fat converts more rapidly to unsaturated fat in wolves than in dogs, unsaturated fats accumulate in the blood, while harmful saturated FFAs are rapidly removed and metabolised. Well-fed wolves will possess increased concentrations of FFAs, cholesterol and insulin – reflecting an increased intake of nutrients and the physiological responses needed to process them. Fasting wolves show a decrease in serum urea nitrogen and triiodothyronine in the blood. The nutritional status of wolves is also reflected in the urine.
For this and more information see:
Behavior, Ecology and Conservation
Edited by L. David Mech and Luigi Boitani