Fat Mobilization and Deposition

 

Monitoring fat reserves of wildlife (especially mammals) has been done for decades to assess the physical condition of animals and, if a sufficient number are involved, make inferences regarding that of the population. Usually, the main emphasis in such examinations has been in late winter to evaluate how well the fat reserves are holding up. This has a bearing on the likelihood of the animal surviving the winter and, in the case of females, if the developing fetuses will grow large enough to survive more than a few days after birth.

Collecting fat reserve data can be very interesting even during the first year of examinations. However, the greatest value of it is in comparisons with data collected over several years time which encompass a variety of weather-related conditions (winter severity, rainfall or lack thereof, temperatures during snow-free periods, etc.).

For the evaluations to have any usefulness for monitoring physical condition that year and for comparisons among years the measurements must be taken consistently at the same places on the body and the data recorded with other pertinent information. At a minimum, this information should include date, location, sex, and age. Often, the age is listed as fawn, yearling, or adult. Preferably a more accurate age for adults should be obtained via counting annual rings of dental cementum in the teeth as sometimes there is good reason to subdivide them into age groups or even listing by each year of age.

Perhaps the white-tailed deer has been the species most often of concern as they occur across the continent and are extremely popular with the public. In 1971, Minnesota D.N.R. initiated a study of fawn production and survival which lasted for several years. Vehicle killed does were to be examined and data recorded as to number of fetuses, their sex, weight, and body measurements, age of doe, and a qualitative assessment of fat reserves.

From 1974 to 1990 I continued with the fetus information and expanded the examinations to include all deer (both male and female), age via dental cementum, monitoring several parasites, taking additional quantitative fat reserve measurements, whole and dressed weights when possible, and other miscellaneous samples desired by the leader of the wildlife research unit in Grand Rapids for studies in which he was involved. Fat reserves recorded were percent fat content of femur marrow, thickness of back and xiphoid process fat at specific locations, and a qualitative assessment of kidney fat (kidneys enveloped in fat, nearly enveloped, spotted with fat, nearly bare of fat, and bare of fat).

Another major change I made was to examine deer year around rather than just when does were pregnant. This change made it possible to assess changes in both mobilization and deposition of fat reserves throughout the year. Femur marrow fat information was obtained from 2,995 deer, kidney fat from 2,076 deer, xiphoid fat from 1,246 deer, and back fat from 1,018 deer. Vehicle impact damage resulted in the lower numbers for three of the fats. Femurs were rarely damaged.

Mobilization and deposition trends were similar for all fats throughout the year in that the lowest levels were in spring and the highest were in early winter. However, fat is mobilized and deposited in a specific order. Mobilization occurs first in subcutaneous fat (back and xiphoid fats), then abdominal (kidney) fat, and finally the bone (femur) marrow fat. Deposition occurs in the reverse order.

Back and xiphoid fat were highly correlated in rates of mobilization and deposition, but back fat was always somewhat thicker. Both can be useful indicators of good physical condition in fall when they are at their peak levels (September – December), but not in spring as they are both nearly always completely gone by then. Bucks were slightly slower to reach peak back and xiphoid fat levels and began mobilizing them sooner (November) than did does. This may be due to bucks not feeding much during rut when their attention is focuses on finding receptive does.

Femur fat can be a useful condition indicator in spring since it is the last fat reserve to be mobilized, but not in fall as it has reached its peak level while the other fats are still being deposited and remains at peak level through January when the other fat levels have already declined considerably. Mean monthly femur fat for bucks was consistently lower (about 2% to 10%) than for does. Since kidney fat reserves are intermediate in both mobilization and deposition, they are also useful as the actual quantity of fat in bone marrow is rather small.

Adults had slightly greater fat reserve levels throughout the year than did yearlings and fawn fat reserves were still lower. Fawns must put a lot of energy into growth through summer and fall which results in there being less available to be stored as fat. Also, fawns took longer to reach their peak fat reserve level and began mobilizing it sooner than did adults and yearlings.

All fat measurements were negatively correlated with winter severity. The more severe the winter, the lower the fat reserves became. The milder the winter, the higher the fat reserves remained.

Foods available in winter are low quality and do not even meet maintenance needs; therefore, the need to mobilize fat reserves during winter. Fortunately, the spring growth of grasses, forbs, and leaves of trees and shrubs (as well as fallen leaves from the previous fall) are very high in digestible energy and protein which helps the animals recover from the stress of winter. Although the nutrient content of these foods may decline slightly over summer and fall, they still provide excess nutrition to be used for growth and fat storage. Nutrient content remains higher and for a longer time during cool and wet summers than during hot and dry summers. Even then, plants growing in shade tend to have higher nutrient content than does the same species growing in full sunlight; perhaps this is due to shaded areas having somewhat more moisture.

Both quantity and quality of foods available are greater in regenerating forests after logging, burning, wind damage, etc. has removed the tree canopy. Seeds and the root systems of many species sprout vigorously after such events; especially after fires as the ashes provide readily available nutrients without having to wait for remaining vegetation and ground litter to decay.