Too Much or Too Little of a Good Thing: Weight Management From the Zoo Nutritionist’s Perspective
American Association of Zoo Veterinarians Conference 1999
Ann M. Ward1, MS; Barbara Lintzenich2, MS; Mike Maslanka3, MS
1Fort Worth Zoological Park, Fort Worth, TX, USA; 2Daniel F. and Ada L. Rice Conservation Biology and Research Center, Chicago Zoological Society, Brookfield, IL, USA; 3Memphis Zoological Garden and Aquarium, Memphis, TN, USA


The nutrient content of a diet can be altered to facilitate weight loss or gain. Sources of calories as well as the digestibility of these sources are important factors to consider when selecting food items. Utilizing on exhibit and off exhibit space, as well as resources available through training and enrichment programs, can help achieve consumption of the appropriate food items by the appropriate animals, as well as increase their activity or energy expenditure. A successful program includes assessing body condition on a regular schedule, which facilitates making changes in the diet or management of the animals in a timely fashion to achieve the desired weight goal in the desired time.


Managing excess weight gain or loss is a challenging task. While it may not be difficult to formulate a diet to promote weight loss or gain, it can be very difficult to ensure an individual animal receives the prescribed diet. In most situations it is not practical to separate an animal to facilitate consumption of a carefully calculated diet. A successful weight management program often must include making changes in the level of activity. More recent emphasis on training and enrichment programs assists with this goal. Often, assessing body condition and monitoring progress of weight changes can in itself make a weight management program difficult. In many situations it may not be feasible to have a scale resulting in subjective assessment that may vary from individual to individual. A coordinated effort between nutritionists, veterinarians, and animal management staff can result in a successful weight management program.

Manipulating the Diet

The goal of most weight management programs is to decrease or increase the calories available to the animal, thus evincing the desired weight loss or gain. Consequently, it is important to consider the sources of calories in the diet and their different digestibilities. Protein, fat, and carbohydrate are sources of energy in the diet. The gross energy content (measured as the heat of combustion, expressed in calories) of these sources can be measured by bomb calorimetry. More energy is available from fat than from protein or carbohydrates. In general, using bomb calorimetry, fat, protein, and carbohydrates have the energy values of 9.3 kcal/g, 5.4 kcal/g and 4.1 kcal/g, respectively.3 When considering how much of the gross energy is actually utilized by the animal, digestibility must be examined. By increasing the fiber in the diet of an animal that cannot utilize fiber as a carbohydrate source, the diet becomes less calorie dense and may assist in promoting weight loss. Alternatively, the same diet offered to an animal that can utilize the fiber results in a different amount of available energy to that animal.

To alter calories available to the animal, a change can be made in the total quantity of food offered or in the composition of foods in the diet. A percentage increase or decrease of each food item (thus not changing the composition of the diet, rather changing the total amount of the diet) can be successful if an overweight animal does not have access to additional food items or if a thin animal is currently consuming all of its diet. Disadvantages to having less food available may include (1) a reduction in energy expenditure due to decreased foraging time and (2) increased competition if housed in a group situation. Regardless of whether the total amount of diet offered or the caloric density of the diet is reduced or increased, all diet manipulations should result in nutrient levels that are still within a target range established as most appropriate for the species.

Among fat, protein, and carbohydrate, fat contributes the most energy to a diet on a kcal/g basis, and is often the source of energy altered to provide a less or more calorie dense diet. A variety of nutritionally complete biscuits, canned diets, and whole prey items are available for use in zoos (Table 1). There are several reduced-calorie (via decreased fat and/or increased fiber), nutritionally complete foods available specifically formulated for domestic dogs and cats that can be useful in omnivore and carnivore diets in a zoo setting (Table 1). Biscuits formulated specifically for primates vary in nutrient content based on their specific intended application and manufacturer. Fat (and protein) content of whole prey items can vary based on developmental state, reproductive condition, species, and season/stage of harvest.1 With this in mind, it is possible to select lower or higher fat whole prey items for specific feeding situations. In some cases, it may be possible to incorporate a reduced calorie dog or cat food in place of a higher calorie whole prey item to reduce the total caloric content of the diet.

Table 1. Nutrient composition of nutritionally complete foods, whole prey, fish, and haya


Nutrient analysis on a dry matter basis


Protein, %

Fat, %

Fiber, %

Nutritionally complete foods

Primate biscuit




High-fiber primate biscuit




Dog food




Canned cat food




Whole prey

Rat adult, 280 g




Rat pup, 5.9 g




Mouse adult, 27.6 g




Mouse pup, 1.6 g




Mouse pup, 3.9 g




Mouse pup, 5.9 g




Chick, 34.4 g




House cricket




Mealworm larva




Waxmoth larva




Common earthworm





Atlantic herring




Spanish mackerel








Great Lakes smelt








Herbivore pellets & hay

Herbivore pellets




Grass hay




Legume hay




aAnalyzed values.
bNot determined.

Overweight herbivores for which fiber is a caloric source also may benefit from lower calorie feeds or increased roughage, by providing the same or increased “bulk” while maintaining or reducing caloric density of the diet (Table 1). Lower energy pellets or lower energy hays substituted or incorporated into the diet have been successfully used to encourage weight loss in several herbivorous species.

For the overweight animal, it is desirable to lose weight as fast as possible without sacrificing the overall health of the animal. As a result of a lack of measurable data on animal species, it may be appropriate to extrapolate successful approaches used for weight management in humans. Methods used by human nutritionists applied successfully to mammals achieve weight loss at a rate ranging from 2.6% of body weight per month (1 pound per week per 70 kg animal) to 4% of body wt per month. These guidelines have been used with a variety of species to achieve desired weight loss.

Manipulating the Animal

The ability to manipulate the animal is often the biggest obstacle to offering the most appropriate diet for weight management. In most situations, it is not practical or possible to house animals individually to ensure consumption of a carefully calculated diet. Many times, the dominant animal is the obese animal in the group while the animals with specialized needs, such as growing or geriatric animals or the “poor doers,” are the subordinates. As a result of this social situation, getting the desired foods to the desired animals appears almost impossible. While animals may not be separated on exhibit, this may still be possible off exhibit. Keepers can utilize off exhibit time and space to work closely with the animals. This can include separating within holding and offering the concentrated sources of calories in the diet at this time, thus allowing the greatest control over the most calorie-dense portions of the diet. When animals cannot be separated in holding, selecting favorite high or low calorie items from the diet and offering them at the cage front can be a successful method to facilitate consumption of the appropriate calories by each animal. On or off exhibit creep feeders also can be employed to make food items accessible to some but not all individuals within a group.


Changes in activity resulting in increases in energy expenditure also facilitate weight loss. Training and enrichment programs can be utilized to encourage more activity in almost every species. Even a sedentary individual can become active if it must forage for live food items. Changes in feeding schedules and food presentation, such as chopping into smaller sizes and scattering, hiding, or strategically placing food items can increase activity by increasing time spent foraging. It is important to remember any item used that is ingested (whether used for enrichment or not) is a source of energy for the animal and must be considered a part of the diet. Several nonfood items have been used successfully to increase animal activity. It is not possible within the scope of this paper to discuss all of the successful items used for this purpose. However, whenever possible, modifications should be made to exhibits and holding areas to encourage activity. Although potentially not aesthetically pleasing, exhibits or holding areas filled with climbing and swinging apparatus may promote great amounts of energy expenditure. Animals that may not want to use these structures can be encouraged to do so if the diet is offered in or on these structures. In addition to enrichment methods, target training can be utilized to encourage activity and get specific food items to specific individuals.

Assessing Body Condition/Monitoring Weight Changes

Assessing body condition and following changes in weight is crucial to a successful weight management program. Regularly weighing animals is the ideal method for following progress. Often it is difficult, if not impossible, to weigh large animals such as elephants, giraffes, and hippos. In the absence of a scale, body measurements have been shown to be helpful to assess body condition of Asian elephants,2 and may hold promise for other species. However, it may not be possible to handle an animal to allow for body measurements. Photographs taken overtime with an object or background as a common denominator may be useful. Condition scoring used to judge domestic animals may be applied in a zoo setting to score different individuals based on “species normals.”


A successful weight management program is a multifaceted project. A weight loss or weight gain diet must be formulated to achieve a weight goal and meet all nutritional needs of the animal. Animal staff must be committed to developing methods to manipulate the animals to achieve the necessary consumption of the appropriate foods. Physical activity or energy expenditure facilitate weight loss and thus are important aspects of the program as well. Training and enrichment play an additional role in getting animals the appropriate diets and increasing activity. A schedule and protocol for assessing body condition should be established at the onset of any weight management project to (1) achieve consistency in assessment, (2) facilitate the necessary adjustments in the diet in a timely fashion, and (3) become firmly established as a part of the daily management of, not only over- or underweight individuals, but all animals in the collection.


The authors would like to acknowledge all of the zoos that have contributed to the experiences related in this paper: the Brookfield Zoo (Chicago, IL), the Fort Worth Zoo (Fort Worth, TX), the North Carolina Zoo (Asheboro, NC), and the Memphis Zoo (Memphis, TN).

Literature Cited

1.  Allen, M.E., O.T. Oftedal, and D.J. Baer. 1996. The feeding and nutrition of carnivores. In: Kleiman, D.G., M.E. Allen, D.V. Thompson, and S. Lumpkin (eds.). Wild Mammals in Captivity Principles and Techniques. University of Chicago Press, Chicago, Illinois. Pp.139–147.

2.  Hile, M.E, H.F. Hintz, and H.N. Erb. 1997. Predicting body weight from body measurements in Asian elephants (Elephas maximus). Journal of Zoo and Wildlife Medicine. 28(4):424–427.

3.  Murray, R.K, D.K. Granner, P.A. Mayes, and V.W. Rodwell (eds.). 1988. Harper’s Biochemistry. 21st ed. Appleton & Lange, 25 Van Zant Street, East Norwalk, Connecticut.

4.  Ullrey, D.E. 1997. Hay quality evaluation, fact sheet 001. Nutrition Advisory Group Handbook.


Speaker Information
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Ann M. Ward, MS
Fort Worth Zoological Park
Fort Worth, TX, USA

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