STARVATION AND MALNUTRITION OF MAMMALS
The terms malnutrition and starvation are used interchangeably, when in reality, there are specific definitions for each. Malnutrition is the inadequate intake of any of the required nutrients. This can even occur in an animal receiving large amounts of food, but is not able to ingest, digest, absorb, or utilize this food. Causes for this inability are injuries, poor teeth, parasitism, disease, foreign bodies in the digestive tract, tumors, or an increased motility of the digestive tract.
Malnutrition can also occur if the food is inadequate in one or more of the required nutrients. If an animal is not able to obtain food for an extended period of time either for the above reasons or due to an unavailability of food or insufficient energy intake, this is defined as starvation. Malnutrition and starvation can be caused by diseases, injuries, the range the animal lives on, or the environmental conditions it must live in. Starvation and malnutrition occur in several wildlife species and routinely eliminates the young, old, weak, and sick animals. Winter is when mortality usually occurs due to the negative energy balance brought about by the cold weather, deep snow, increased energy demands, snow covered food, and human and predator induced stress. Historically, in Michigan the number of species diagnosed at the Laboratory as dying from malnutrition and starvation are second only to those dying from traumatic injuries.
Numerous bird and mammal species annually (depending on the severity of the winter) die from insufficient nutrition. Currently we have 3 primary species that die from malnutrition or starvation: white-tailed deer, mute swan, and wild turkey. The majority of the animals have come from Regions I and II with mortality occurring almost exclusively during the winter when food availability is at its lowest. Susceptibility to starvation and malnutrition usually occurs in the winter and early spring months for wildlife in Michigan. Animals cope with the severe weather and shortage of food in 1 of 3 ways: hibernate (amphibians, reptiles, and several mammals), migrate (most avian species), or remain active and attempt to survive. Juvenile, yearling, and old animals are the age groups most susceptible to starvation and malnutrition because they enter the winter with the smallest fat reserves, the highest nutritional demands, the greatest heat loss, and the lowest position in the social hierarchy.
Of the winter starvation deaths observed, 60 to 70% may consist of animals less than 1 year of age. Adult males and females and juveniles of both sexes of various species may have smaller reserves of fat due to breeding activities, rearing of the previous year’s offspring, and their growth requirements, respectively. Most wild animals in colder climates undergo an annual fat cycle whereby fat is deposited and then utilized as the physical condition declines. During a severe winter, adult deer may lose as much as 25 to 30% of their body weight and still survive. The loss of weight occurs during the winter because of snow depth, ambient temperatures, and the quality and quantity of the available forage. Because of these factors, the physical condition of animals at the onset of winter is critical to their survival. The adequacy of summer and fall ranges is thereby very important.
The duration and severity of the winter is critical to the animal’s chances for survival, as it determines the length of time the animal must depend on its body fat reserves and on poorer forage for survival. Deep snow and cold temperatures, especially during the latter part of the winter can result in very high numbers of deaths attributed to malnutrition and starvation. The cover available to the animal is also of utmost importance as this allows the animal to escape from the low temperatures and the wind. Offspring of weakened animals that survive the winter are also susceptible to the effects of starvation as they may be resorbed or aborted as fetuses, or if born, may be improperly cared for. Avian species are highly mobile and usually migrate, thereby lessening the chances of mass starvation.
Extremes of weather (sudden snow or ice storms) may result in birds becoming trapped in inhospitable areas and not having food available. Some species experience mortality of the females due to nesting activities in the early spring, when they are unable to leave the nest to feed. Deaths attributed to malnutrition and starvation are seen in young birds during the hatching season due to parent neglect, or once they are fledged from the nest, the inability to acquire their own food. Physiology If an animal is forced into an inadequate plane of nutrition, there are many physiological changes as the animal attempts to satisfy its energy requirements.
At the cellular level, catabolism (the breaking down in the body of complex chemical compounds into simpler ones) continues to supply the substances required for anabolism (the usage of nutritive matter and its conversion into living substance) and to continue vital functions. Reserve stores of nutrients contained in the individual are utilized to compensate for the lack of nutritional intake.