The dietary needs of mammals
Like the rest of the animal kingdom, mammals need food for energy and the maintenance of bodily processes such as growth and reproduction. The chemical compounds used to supply the energy and building materials are obtained by eating plants or organic material. Both plant- and animal-based sources of food are made up of highly complex compounds that need to be digested and broken down into simpler forms.
Four of the most common naturally occurring elements— oxygen, carbon, hydrogen, and nitrogen—make up 96% of the total body weight of an animal. The remaining 4% is made up of the seven next most abundant elements—calcium, phosphorus, potassium, sulfur, sodium, chlorine, and magnesium, in that order. Necessary for many physiological processes, any change in their concentrations may be deleterious or fatal.
An animal's major dietary components are fat, water, protein, and minerals. The main digestion products of these compounds are amino acids (from proteins), various simple sugars that are present in the food or derived from starch digestion, short-chain fatty acids (from cellulose fermentation), and long-chain fatty acids (from fat digestion). The oxidation of these digestion products yields virtually all the chemical energy needed by animal organisms.
Despite carbohydrates' essential role in animal metabolism, their total concentration is always less than 1%. The two major animal carbohydrates are glucose and glycogen.
Body lipids act as energy reserves, as structural elements in cell and organelle membranes, and as sterol hormones. Because lipids can be stored as relatively non-hydrated adipose tissue containing 2–15% free water, eight times more calories per unit of weight can be stored as fat than as hydrated carbohydrates. This is the reason fat storage is essential for active animals, while carbohydrates are a major energy reserve for plants. Hibernating mammals deposit fat and may double their body weight at the end of the summer prior to hibernation; the white adipose tissue reserves allow them to survive the winter.
In addition, there are 15 elements making up less than 0.01% of the body of a mammal. These elements occur in such small amounts that they became known as trace elements. Still, they have vital physiological and biochemical roles. Iron, for instance, is a key constituent of hemoglobin in blood and several intercellular enzyme systems. While the amount of iron found in an adult human is only 0.14 oz (4 g)—70% in hemoglobin, 3.2% in myoglobin, 0.1% in cytochromes, 0.1% in catalase, and the remainder in storage compounds in the liver—growing animals need more iron, and adult females need to replace that which is lost in reproductive processes such as the growth of the fetus and menstruation. The dietary requirement for adult mammals is very small since iron (from the breakdown of hemoglobin) is stored in the liver and used again for hemoglobin synthesis. Other trace elements include copper, zinc, vanadium, chromium, manganese, molybdenum, silicon, tin, arsenic, selenium, fluorine, and iodine.
Animals can differ markedly in their vitamin requirements. Ascorbic acid (vitamin C), for example, can be synthesized by most mammals, but humans and a few other mammals such as non-human primates, bats, and guinea pigs, need to have it supplied in their diet.
Ruminants do not appear to need several vitamins in the B group since the microbial synthesis of vitamins in the ruminant stomach frees these animals from having to seek out additional dietary sources.
Adaptations in the digestive system
All carnivores, when fed a whole prey-based diet, consume proteins and fats from the muscle, vitamins from organs and gut contents, minerals from bones, and roughage from the hide, feathers, hooves, teeth, and gut contents. Felids are set apart from other, more omnivorous meat eaters because of their inability to effectively utilize carbohydrates as an energy source. They therefore depend on a higher concentration of fats and protein in their diet, as well as dietary sources of preformed vitamin A and D, arachadonic acid (an essential fatty acid), and taurine.
Herbivores, on the other hand, have adapted numerous methods of utilizing roughage-based diets. Plankton feeders such as the baleen whales have a filtering apparatus that consists of a series of horny plates attached to the upper jaw and then left hanging from both sides. As the whale makes its course through the ocean, water flows over and between
the plates, and plankton is caught in the plates' hair-like edges.
Ruminants and some non-ruminant herbivores (e.g., sloths, hippos, colobines, large marsupials) utilize pre-gastric microbial fermentation to break down cell wall constituents, while the odd-toed hoofed animals, or perissodactyls, rely primarily on post-gastric fermentation.
Some small herbivores, like rodents and rabbits, have relatively higher nutrient requirements compared with larger herbivores. In order to meet these requirements, they must routinely practice coprophagy to obtain the protein, water, enzymes, vitamins, and minerals provided by the microbes. Coprophagy, which comes from the Greek copros, meaning "excrement," and phagein, meaning "to eat," is of great nutritional importance. If coprophagy is prevented in rabbits, their ability to digest food decreases, as does their ability to utilize protein and retain nitrogen. This process is reversible, however. When coprophagy is allowed again, the rabbits' ability to digest cellulose is restored.
The soft feces that a rabbit re-ingests originate in the cecum, or "blind gut," a large blind pouch forming the beginning of the large intestine. Upon ingestion, these feces are not masticated and mixed with other food in the stomach. Instead, they tend to lodge separately in the base of the stomach. A membrane coats the soft feces, and they continue to ferment in the stomach for many hours. One of the fermentation products is lactic acid.
For most herbivores, the gastrointestinal microbial population is an integral component of the feeding strategies, especially since most of them live on food that make cellulose digestion essential. Some of the most important domestic meat- and milk-producers (cattle, sheep, goats) have specialized tracts that are highly adapted to symbiotic cellulose digestion; they are known as ruminants.
The stomach of a ruminant consists of several compartments, or in more precise terms, the true digestive stomach, the abomasum, is preceded by several large compartments. The abomasum corresponds to the digestive stomach of other mammals. The first and largest compartment of this system is the rumen, which serves as the main fermentation center in which the food, after it has been mixed with saliva, undergoes heavy fermentation.
Both bacteria and protozoans reside in the rumen in large numbers. These microorganisms work to break down cellulose and make it available for further digestion. The fermentation products (mostly acetic, propionic, and butyric acids)
are absorbed and utilized, while gases (carbon dioxide and methane) formed in the fermentation process are released through belching. A cow fed 11 lb (5 kg) of hay a day can give off 1 qt (191 l) of methane each day.
What is rumination?
The act of rumination, or "chewing the cud," is the regurgitation and remastication of undigested fibrous material before it is swallowed again. As the food reenters the rumen, it undergoes further fermentation. The products of fermentation—in the form of broken-down food particles—then slowly pass to the other parts of the stomach, where the usual digestive juices of the abomasum perform their work.
Ruminants secrete copious amounts of saliva that serve to buffer fermentation products in the rumen. It also serves as a fermentation medium for the microorganisms. The total secretion of saliva per day has been estimated at 6–17 qt (6–16l) in sheep and goats, and 105–200 qt (100–190 l) in cattle.
Since sheep and goats have an average weight of 88 lb (40 kg) and cattle, 1,100 lb (500 kg), the daily production of saliva may reach about one-third of the body weight.
The obligate anaerobic organisms residing in the rumen include ciliates that occur in numbers of several hundred thousand per fluid ounce (milliliter) of rumen contents. Laboratory extracts from pure cultures of rumen organisms have demonstrated cellulase activity, the enzyme that breaks down cellulose so that its byproducts become available to the host mammal.
Rumen microorganisms can also synthesize protein from inorganic nitrogen compounds such as ammonium salts. Dairy farmers have been supplementing the feed of milk cows with urea—normally, an excretory product eliminated in the urine—to increase protein synthesis, rather than through the use of more expensive high-protein feed.
In the rumen, urea is hydrolyzed to carbon dioxide and ammonia—the latter being used by the microorganisms for the resynthesis of protein. Since a camel fed a nearly protein-free diet of inferior hay and dates excretes virtually no urea in the urine, it can recycle much of the small quantity of protein nitrogen it has available this way. A similar reutilization of urea nitrogen in animals fed low-protein diets has been observed in sheep and, under certain conditions, rabbits.
Rumen microorganisms can also contribute to the quality of the protein that is synthesized. If inorganic sulfate is added to the diet of the ruminant, the microbial synthesis of protein
is improved and the sulfate is incorporated into the essential amino acids, cysteine and methionine.
Since the microbes in the ruminant stomach can synthesize all the essential amino acids, ruminants are nutritionally independent of these, and therefore the quality of the protein they receive in their feed is not of vital importance.
Some important vitamins are also synthesized by rumen microorganisms, including several of the vitamin B groups. The natural supply of B12 for ruminants, for example, is obtained entirely from microorganisms.
Rumen fermentation takes place in the anterior portion of the gastrointestinal tract so that the products of fermentation can pass through the long intestine for further digestion and absorption. This way, the mechanical breakdown of the food can also be carried much further, and coarse and undigested particles can be regurgitated and masticated repeatedly.
If a comparison is made of the fecal material of cattle (ruminants) and horses (non-ruminants), it will be found that horse feces contain coarse fragments of still-intact food, while cow feces are smooth and well-ground up with few large, visible fragments.
Multi-compartment stomachs are not unique to the ruminants, or even the ungulates. Animals such as the sloth, the langur monkey, and even certain marsupials have rumen-like stomachs. The diminutive quokka, for instance, has a large stomach harboring microorganisms that participate in cellulose digestion. For an animal weighing in at 4.4–11 lb (2–5 kg), its stomach equals about 15% of its body weight, a number similar to that found in most ruminants.
The kangaroo and wallaby are ruminant-like large marsupials that utilize the same mechanism of microbial fermentation taking place anterior to the digestive stomach. At the beginning of the dry season, when the nitrogen content in the vegetation starts to decline, wallabies begin to recycle urea and continue to do so throughout the prolonged dry season. This way, they are relatively independent of the low quality of the available feed.
There are other species-specific anatomical morphologies that adapt to the animal's specific nutritional needs. The small intestine, for instance, is the primary site of enzymatic digestion and absorption. The mammalian small intestine is morphologically divided into a proximal duodenum looping around the pancreas, intermediate jejunum, and distal ileum. The length of all intestinal segments in mammals relative to body weight is longest in herbivores, intermediate in grain- and fruit-eaters, and shortest in carnivores and insectivores. Relative intestinal length, weight, and volume within each species vary with sex, age, seasonal food habits, as well as with changing nutritional requirements or food quality and the level of intake.
The large intestine, to provide another example, varies in length depending on the species and its dietary regimen. Its length relative to the small intestine averages 6% in small carnivorous mammals, 33% in omnivores, and 78% in herbivores as fiber digestion, bulk, and a reduced rate of passage increase in importance.
The ceca and the large intestine work towards the fermentation of plant fiber and soluble plant matter, as well as the absorption of water and small water-soluble nutrients such as ammonia, amino acids, and volatile fatty acids. They also function to synthesize bacterial vitamins.
Seasonal changes in nutritional requirements
Changes in diet often follow the changes in seasons. When researchers at Sea World, Durban, traced the annual food consumption of their female dusky dolphin (Lagenorhynchus obscurus) over a 13-year period, they found that her annual food intake jumped from 4,784 lb (2,170 kg) when she was five years old, to nearly 6,393 lb (2,900 kg) the year after. This increase coincided with the installation of a cooling system that was used in the summer in the years thereafter, and after her sixth year, her food consumption fluctuated between 5,290 lb and 6,173 lb (2,400–2,800 kg) per year. In general, her food intake was above average during autumn and winter, and below average during spring and summer, and the average pool water temperature fluctuated seasonally.
A similar study of California sea lions (Zalophus californianus californianus) found that the voluntary decrease in food intake during summer was associated with increased aggressive behavior in males, while the seasonal fluctuation in nonreproductive females was negligible. Seasonal fluctuations in male food intake was especially pronounced between the ages of four and eight, when sexual maturity was reached.
Territorial male California sea lions defended their territories in the breeding season, during which they do not feed, and remain in their territory for an average of 27 days. In captivity, they have been shown to lose as much as 198 lb (90 kg) during the breeding season—independent of food availability, suggesting the possibility of an endogenous rhythm. The simultaneous increase in aggression suggests testosterone involvement as well. The females, on the other hand, showed less profound fluctuations in monthly food intake than their male counterparts, possibly because females are non-territorial and do feed during the breeding season.
Seasonal variation in temperatures may also be important as male sea lions in particular ate less when air and water temperatures were high and a thick fat layer was less important for maintenance of constant body temperature.
Sheep have also been observed to alter their diets according to the shifting seasons. They are known to consume a more fibrous type of forage such as tussock grass during the winter season or seasons with a scarcity of resources. Two independent studies, in the semiarid rangelands of Argentina and Australia, respectively, reported that sheep preferred tussocks only in winter, and avoided them during the growing season. Although sheep behave generally as bulk grazers, they will also consume, when offered, a considerable amount of shrubs in the fall and winter seasons. This preference for evergreen shrubs corresponds with times of the year when grasses are less available or nutritious. Scottish sheep, in a comparable high-latitude oceanic climate, also displayed a similar feeding
pattern, i.e., they consumed a high proportion of shrubs only in winter. Shrubs are also a type of forage that maintain a relatively high-protein content during the colder seasons.
Variation in the diet of a species over seasons may also be the result of habitation in different landscapes of the same region, thus linking prey use with availability. The swift fox (Vulpes velox), for example, occupies two distinct landscapes in western Kansas that are dominated by either cropland or rangeland. In spring and the fall, plants such as sunflower seeds, and birds were consumed more frequently in cropland than in rangeland. However, birds were more common in the swift fox diet in cropland during the fall.
Nutrition and the reproductive cycle
Energy requirements and food intake of pregnant females are about 17–32% higher than non-reproducing females, and yet only 10–20% of this additional energy is retained as new tissue by the developing uterus. The rest of the energy is lost as heat, slowing down the growth rate and thus lengthening the gestation period. A slower fetal growth rate may be advantageous in an environment with limited dietary protein or minerals, especially in the case of such animals as the fruit-eating or leaf-eating primate.
In females, the fetus represents 80% of the energy retained by the uterus. Most of the increase in mass in the mother occurs after 50–60% of the gestation period has elapsed. The water content of the developing fetus also decreases while the fat, protein, and mineral content increase during gestation. The mammalian newborn, or neonate, averages 12.5 ± 2.3% protein and 2.7 ± 0.8% ash at birth. Neonatal fat, water, and energy content vary between different species. For instance, neonatal seals, guinea pigs, and humans contain 4–8 times more fat than other mammals, whose content averages 2.1 ±1.0%. The fat reserves of the guinea pig are broken down just a few days after its birth, while those of the seal are retained because of the cold environment and the short milk production period.
Because of the very low fat content of neonatal mammals, their high metabolism, and frequently, poor insulation, the chances of survival are only a few hours to days without care from the mother.
After giving birth, the production of milk by the mammalian female bridges the dietary gap between the passive, completely dependent fetus to the weaned and more or less nutritionally independent juvenile. Milk production, or lactation, enables the young mammal to continue its growth in an almost embryonic manner without having to remain
anatomically attached to its mother. The female, in this way, is freed from the locomotory, nutritional, and anatomical constraints of carrying the fetus.
According to Lopez and Robinson in 1994, nutrient requirements for pregnancy are moderate in comparison with the estimated nutrient requirements for maintenance. In the case of captive Atlantic bottlenose dolphin, food consumption in the females showed little increase during gestation, but was 58–97% higher during lactation than during similar periods in non-reproductive years.
For most mammals, milk production closely follows the nutrient requirements of the newborn animals. In the first few days, the requirements of the newborn may be substantially lower than the mother's potential to produce milk. As the needs of the growing animal increase, so does its requirements for milk and milk production.
Lactation itself can put an enormous nutritional burden on the female. In terms of energy expenditure, lactation is two to three times more costly than gestation, and the female's nutrient requirements may increase considerably. The total energy expenditure, including milk produced, of the lactating female is about 215% higher than her non-lactating counterpart.
The production of milk generally rises during early lactation and hits a maximal peak. This is the point where weaning takes place since the neonate has to increase its nutrient intake further by relying on nutritional sources other than milk.
The decline in milk production once the peak has been reached can last for as little as five days in mice to many months in large ungulates. While it seems to make evolutionary sense for larger species to have longer lactation periods, there are many exceptions to the evolutionary constraints. The hooded seal, for instance, has one of the shortest lactation periods despite a maternal weight averaging 395 lb (179 kg). The pup grows from 47.4 to 96.3 lb (21.5 to 43.7 kg), with 70% of the gain being fat, in just four days.
A basic rule of thumb, however, is that poorly nourished females and those nursing larger litters often reduce milk production faster than do well-nourished females.
According to Elsie Widdowson in 1984, of the 4,300 species of mammals, only the milks of 176 have been analyzed for protein, fat, and carbohydrate. Of these analyses, she said, only the figures for 48 of those species are considered to be reliable. The difficulty in the analyses lies with the fact that milk composition changes rather markedly during a lactation cycle.
The first milk, or "colostrum," contains a high concentration of maternal antibodies, or immunoglobins, active phagocytic cells, and bacteriocidal enzymes. While neonatal primates, guinea pigs, and rabbits acquire their circulating maternal immunoglobins in utero, other mammals such as ungulates, marsupials, and mink depend on the colostrum as their sole source of a passive immune system. Yet another group, intermediate to these two, acquire maternal immunoglobins, both in utero and from colostrum. Among these animals are rats, cats, and dogs. The differences in in-utero transfer of immunoglobins are determined by the number of cellular layers in the placenta that separates fetal and maternal circulation.
In order for the secretion of colostral immunoglobins to be effective, the neonatal gut needs to remain permeable to their absorption and minimize any upper-tract digestion of these proteins. The time the mammalian intestine remains permeable to the intact immunoglobins varies between species: 24–36 hours after birth in the case of ungulates; 16–20 days in mice and rats; eight days in mink; and 100–200 days in large marsupials. The marsupial's prolonged absorption capabilities relate to the time the young reside in the mother's pouch.
Other types of immunoglobins that occur in milk, after absorption of the first wave of intact molecules has ceased, protect the neonatal gut from infection.
The major constituents of milk are water, minerals, proteins (such as casein), fat, and carbohydrates. Protein concentration ranges from under 3.5 oz/qt (10 g/l) in some primates to more than 3.5 oz/qt (100 g/l) in hares, rabbits, and some carnivores. Fat concentrations vary from small amounts in the milk of rhinoceroses and horses to more than 17.6 oz/qt (500 g/l) in some seals and whales. The main carbohydrate of placental mammal milk is the disaccharide lactose, a polymer of glucose and galactose. Lactose content also ranges from trace amounts in the milk of some marine mammals and marsupials to more than 3.5 oz/qt (100 g/l) in some primates. Marsupial milk, nevertheless, can be very high in carbohydrates, but the sugars are primarily oligo- and polysaccharides-rich in galactose.
Compromises have to be made between the physiological constraints to milk synthesis and selective pressures to maximize offspring survival. The variation in milk composition between species is one of them. Most aquatic mammals produce highly concentrated milks. The reduction in milk water content in aquatic mammals provides a high-energy, low-bulk diet that is useful in offsetting neonatal heat loss in cold environments. It also conserves water in the mothers of species (such as the northern elephant seal) that abstain entirely from eating or drinking during a relatively short, but intense, lactation. Similarly, seals that give birth on pack ice and have a short lactation period (e.g., hooded seals—four-day lactation) or those that leave the neonate for feeding trips lasting several days produce more concentrated, higher fat milks than do other seals.
For terrestrial mammals, the largest changes in milk composition over time occur in marsupials. Some marsupials can produce several kinds of milk simultaneously since they may have young of different ages. For the embryonic marsupial confined to the pouch, dilute, high-sugar milk provides nourishment similar to that occurring in the uterus of a placental mammal during its longer gestation. Once the young leave the pouch, the milk becomes more concentrated with more fat and protein and less sugar. Most terrestrial placentals produce milks that are intermediate in concentration to the nutrient-rich milk of aquatic mammals and the very dilute milks of primates and perissodactyls. The milks of domestic cattle, goats, and camels contain about one-half the protein and energy per unit volume that occurs in the milks of wild even toed hoofed mammals (also known as artiodactyls). The dilute milks from domestic artiodactyls are more similar to that produced by humans than they are to wild artiodactyls. Because sugars, particularly lactose, and some minerals such as sodium and potassium are important regulators of the osmotic potential or water content of milk in the mammary gland, concentrated milks have either a low sugar (such as marine mammals) or mineral content, while dilute milks have a higher sugar (such as primates and perissodactyls) or mineral content.
The actual composition of the milk fat, protein, and sugar also differs between animals. For example, the fatty acids of most milk are dominated by palmitic and oleic acids. However, the main fatty acid of lagomorph and elephant milk is capric acid, which is synthesized in the mammary gland. Seal milk is composed of long-chain unsaturated fatty acids that are probably derived directly from the diet.
In carnivores, the amino acid, taurine, appears to be essential for the diets of most of their neonates. The taurine content of colostrum is usually higher than in mature milk. However, carnivores have a much higher concentration of taurine in the mature milk than do herbivores.
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Jasmin Chua, MS