By Antony Wildon and Rick Kleyn, Avi-Products (Pty) Ltd.


Gross and marginal nutritional deficiencies or excesses may lead to reduced fertility, suppression of the immune system and ultimately the death of captive wild animals. Much of the specific information required for many species is simply not available. In order to successfully feed a wide range of species, comparisons and parallels must be drawn from species where data does indeed exist. This has lead to the establishment of a branch of nutrition called comparative nutrition, and it is the comparative nutritionists who have become the zoo nutritionists of today.

The role that comparative nutritionists could/should play in the management and welfare of animal collections has been recognized in both North America and Europe. Formal codes of practice for the nutrition and feeding of captive animals have now been applied. This has not been the case in South Africa thus far.

The role of the nutritionist in designing diets for captive animals will be discussed. This would include; a comparison of the requirements of various species and individuals; the manner in which human intervention impacts on the nutrition and feeding of captive animals; a comparison between wild and captive diets; and techniques that can be used to improve the feeding of captive animals. Feeding correctly balanced diets will have a bearing on the overall cost of feeding. It will also impact on the ultimate health and reproductive success of our captive animal collections.


The importance of nutrition both for ourselves and our animals is generally recognised. It is known that even if animals are not showing signs of gross nutrient deficiencies or excesses, marginal inadequacies or surpluses of many of the nutrients may lead to reduced fertility and a suppression of the immune system. This may well have an impact on not only the survival of the animal (or species) but may also have important economic implications. This short article will attempt to put the nutrition and feeding of captive animals into perspective.

Zoo keepers, veterinarians and nutritionists are responsible for the care and feeding of many thousands of different species of animal. There is relatively little scientific information to serve as a starting point for this monumental task. Nutritionists therefore need to rely on the information published for domestic animals as the basis for much of what they do. Sadly, much of the information that has been published regarding zoo animals is factually flawed and contains irrational concepts which only serve to confuse us (Ullery, 1996). Perhaps the worst culprits in the regard are advertisers, who are often only to happy to sell an idea rather than a proven product.

Most domesticated animals have undergone intensive genetic selection for growth and reproduction. Many people believe that for this reason they are not always appropriate to use as a model for wild animals. This is probably not an entirely valid argument, as most nutritional research in animal agriculture is published in a systematic way. That is to say, it is know how much protein and energy is required to produce a gram of egg or a gram of muscle tissue. It is believed that these values are constant across species as they are all governed by the same underlying biochemical reactions. For example, the relationship between heat production and body weight for nonpasserine birds is as follows:

M = 325W0.72
Where M = heat production (KJ/24 hours) and W = body weight (kg).

This relationship is very similar to that for mammals (Sturkie, 1976). In addition, we have a good understanding of what the impact of size and insulation (feather or hair cover) has on the maintenance requirements of the animal. Borrowing information known about domestic species and applying it to other species may not be entirely correct, but it is better to use a model that is slightly flawed than it is to do nothing at all.

In the United States, the discipline of nutrition has received official recognition within the American Zoo and Aquarium Association (AZA) which has lead to the establishment of the Nutrition Advisory Group (NAG) (Dierenfeld, 1996). In Europe, a survey conducted among the members of the European Association of Zoos and Aquaria (EAZA) showed that continuing health problems in zoo animals may be caused by inadequate diets and that additional research in the field of zoo animal nutrition is most definitely required. This led to the formation of the European Zoo Nutrition Centre (EZNC) in 1999 (EZNC, 2006).

Although sound texts on the feeding and nutrition of wild animals (Robbins, 1993), birds (Klassing, 1998) and fish (Halver and Hardy, 2002) have been published, the formulation of diets for most zoo and aquarium animals still requires extensive extrapolation of data from species with known nutritional needs and major applications of faith (Ullrey, 1996). This means that comparisons are required. Zebras and Camels can be compared to horses; exotic fish to commercial species; birds to poultry; bears and wolves to domestic dogs. Primates are usually compared to humans, though some species have digestive physiology more in common with horses. Comparing the similarities and differences in digestive physiology and feeding behaviour brings the zoo nutritionist another step closer to satisfying requirements (EZNC, 2006).

Many zoo administrators seem to believe that the nutrient needs and feeding programs for captive wildlife have been thoroughly defined, obviating the need for fulltime staff trained in these subjects. Unfortunately, the demonstrated ability of untrained personnel to apply objective nutrition information in animal care is often akin to medical practice in the 18th century. (Ullrey, 2004). In an attempt to address this shortcoming, a branch of nutrition known as comparative nutrition has emerged, culminating in the establishment of the Comparative Nutrition Society (CNS) in 1996 ( Those people involved in captive and wildlife nutrition call themselves “Comparative Nutritionists”.

The AZA Nutritional Advisory Group (2001) has published a set of feeding program guidelines for AZA Institutions, intended to provide direction for the development and maintenance of an inclusive, practical feeding program for Captive Wild Animals and as a basis for accreditation of an institution. In the broadest terms any feeding strategy should:

  • Aim to provide a nutritionally balanced diet.
  • Provide a diet that reasonably stimulates natural feeding behaviours.
  • Provide a nutritionally balanced diet that the animal consumes consistently.
  • Provide a diet that is practical and economical to feed.
  • Keep detailed records of both the feeding practice and animal welfare.Balancing a diet to meet an animal’s requirement for the 40 or 50 feed components (nutrients and energy) or physical characteristics that are required is simply not possibly using hand formulation techniques. Given that hundreds of individual calculations may be involved in a single dietary evaluation nutritionist rely on computers (Oftedal and Allen, 1996). Specialized software used by experienced nutritionists is required to carry out this task effectively. When homemade diets are used, two kinds of problems are prominent: the diet and the owner. It was found that 90% of the hypoallergenic home made diets, prescribed by 116 veterinarians in the United States, were not adequate for maintenance of adult dogs and cats. The second problem is the accuracy and precision by which the owner follows the guidelines given (Hesta et al., (2002).

As far as is known, there is no nutritionist in South Africa who has specialized in the feeding of Zoo animals, so this role is fulfilled by people who have qualified in other areas. It is encouraging to note that the Chairman of the American CNS, Prof Kirk Klassing, is a poultry nutritionist. Sadly, the pool of available nutritionists within South Africa is small, with enrolment at Universities dropping.

An aspect of feeding zoo animals that can never be ignored is that of cost. The EZNC (2006) estimated that some 40 million Euros are spent of feeding zoo animals in 1000 European zoos each year. It is our experience that correct formulation of a diet, not only balances the nutrient profile correctly for the animal in question, but mostly does so at a significant reduction in cost. Moreover, further saving may result from improved food hygiene, fewer animals needed to be treated by a veterinarian for feed related problems and in the time and labour required to prepare the diets.

The remainder of this paper we will discuss the various components of designing a diet for captive animals. This would include a comparison of the requirements of various species. The manner in which human intervention impacts of nutrition and feeding of captive animals is discussed as is a comparison between wild and captive diets. Lastly, techniques that can be used to improve the feeding of captive animals will be discussed.

The Animal

Animals require a diet for only three things – water, energy and nutrients. Water is deemed to be the most critical component of any diet but largely falls outside the gambit of this paper. Energy is supplied principally in the form of carbohydrates (sugars and starch) and fat. The principal nutrients that concern us are protein and their constituents the amino acids, vitamins and minerals.

All animals have only three nutritional needs or requirements in order to survive as a species. These are their requirements for maintenance, their requirement for growth and their requirement for reproduction.

As nutritionists and animal keepers it is our role to provide a balanced diet to each animal on each day of its life cycle. An animal’s diet would include all food offered and available to the animal on a daily basis. A balanced diet provides the animal with adequate clean water, all of the required nutrients, together with sufficient energy for normal physiological function – without causing any gross excesses or deficiencies.

It would be true to say, that the similarities between the nutritional requirements of different species of animal are far greater than the exceptions. For example, the requirement for a specific ratio of calcium to phosphorus remains the same across nearly all species. It should be remembered that the differences from within the population of one species are often greater than any differences that occur between species.

Eighty to 90% of the digestion and assimilation of energy and nutrients takes place in the small intestine, whether the animal is herbivorous, omnivorous or carnivorous. The small intestine is thus the most important component of the digestive tract and is remarkably similar across all species. Furthermore, its related organs, the liver and the pancreas, are remarkable adaptive. In feeding an herbivorous animal flesh, the system adapts to digesting and assimilating flesh. When feeding a carnivorous animal plant material, the system adapts to assimilating plants. It is thus apparent is easily comparable across varied species (Clemens, 1997). What may be overlooked however is the prehensile mechanics of the animal, which define how the animal eats, whether it be rapidly or slowly and what the animal eats be it roughage (grass), fruits or flesh. These all have a direct bearing on the rate of passage through the digestive tract.

Despite the similarities between animals, real differences do exist. Birds for example, exhibit a wide range of physical, physiological and behavioural adaptations that equip them to acquire and then utilise their food. One only has to look at the wide range of specialised beaks that occur in the avian kingdom to appreciate this (Klassing, 1998). Perhaps of more concern is that of the few species that we have studied in detail, many nutritional peculiarities exist. For example, high iron levels in softbill diets can lead to iron storage disease (haemochromatosis) and ultimately death. How many of the species that we keep and feed have similar nutritional peculiarities is hard to know, as much research still needs to be carried out.

In order for an animal to digest and assimilate its food, it is first required to remove the water from the diet. Most nutritionists assume that plentiful clean water is supplied to the animals. The truth is, that if water supply is limited it will restrict feed intake. On the other hand, high levels of moisture in the total diet can dilute the nutrients to such an extent that the animal is not capable of consuming an adequate nutrient intake. This may be the case in diets high in fruits and vegetables. The second step in the nutritional process is for the animal to break the diet down into its basic components (digestion) – before it can be assimilated. It is important to realise that we are feeding animals digestible nutrients and not specific feed ingredients. Not all diets are equally digestible and not all forms of a specific nutrient can be assimilated by the animal. Specifics in this regard are something that a good nutritionist will always bear in mind.

The Human Element

It is people that keep animals in captivity and the way in which people behave towards their animals, their welfare and their care, are very often controlled by human emotion. We tend to behave in an anthropomorphic (giving human-like qualities to the animals) and this can be a problem in itself. People get very possessive of “their” animals and take great offence if someone tries to tell them what to do or offer corrections, particularly if they think they are doing the best that can be done.

Human beings tend to eat discrete meals and, almost without thought, have imposed a similar pattern on almost every species kept in captivity. As a result, animals are fed discrete meals at specific and predictable times, year in and year out. Diets are usually prepared according to some predetermined recipe and vary little throughout this period.

In addition, we tend to feed captive birds diets that contain ingredients that are important in human agriculture. These would include commercially available fruits and vegetables, mass produced grains, fish species that are commercially fished, by-products of the agricultural process and livestock feeds, with or without some form of modification. These ingredients differ greatly from wild grass seeds and fruits for example.

Wild vs. Captive Diets

The first issue that needs to be examined is what exactly changes when an animal is held in captivity.

In the wild, animals exhibit a wide range of physical, physiological and behavioural adaptations that equip them to acquire and then utilise their food. For example, grazing animals develop strong rumen musculature so as to prevent to stratification that develops in the rumen on grass based diets. Browsing Ruminants lack this adaptation and will avoid stratifying forages such as grass (Clauss, 2002). One only has to look at the wide range of specialised beaks that occur in the avian kingdom to appreciate the degree of adaptation found in the wild (Klassing. 1998).

The food sources themselves are often a widely distributed and their availability will differ depending on the season. As a result of this, wild animals spend much of their time looking for and consuming their daily diet. The seasonality of food supply means that animals are often confronted with food scarcity, which is precisely why the nutritional costly process of breeding is confined to times of the year where food is abundant. Captivity removes much of the complexity and seasonality of food resources.

Captive animals are obviously faced with feeding choices, or lack thereof, which are very different from those in the wild. There is little reason therefore to suppose that the choices made in captivity bear any resemblance to the choices that are made in the wild. On the contrary, nutritionists and veterinarians see countless cases where the choices made by captive animals, particularly birds, result in nutritionally imbalanced diets. Unfortunately, little scientific evidence exists in support of the notion that captive animals posses “nutritional wisdom” that enables them to select diets that are appropriate for their needs.

The way in which we keep and manage birds and animals in captivity also has a major impact in the way in which they should be fed. They are often kept in groups of different ages or physiological states (breeding versus non-breeding birds) and/or in groups of mixed species. These mixed populations confound the attempts of even the best nutritionists. Firstly, many of the individuals have completely different nutritional requirements. Secondly, very clear pecking orders become established both within a species and between species. This represents another reason why the argument that animals are able to select what they require will mostly not work in a captive situation. Simply put, those at the top of the pecking order will often consume all of the “favoured” items. Wild fruits are generally higher in fibre and not as sweet as modern fruit cultivars. As many species are know to develop a sweet tooth, those individuals at the top of the pecking order mostly eat a high sugar diet and become obese, while the less aggressive individuals often go hungry.

Until such time as we are able to reproduce the seasonal, spatial and nutritional complexity of the diets found in the wild, captive animals will be faced with choices that they have not been evolved to make. For this reason, it will remain the prerogative of the humans that keep animals to make these choices on their behalf.

Another feature that is probably unique to captive populations is that many of the animals that we keep could accurately be termed geriatric, as they survive long past the age that they would have survived in the wild.

Towards Better Captive Animal Feeding

It should be accepted that we can not expect captive animals to balance their own diet, which places them firmly in the hands of their keepers. Balancing and then preparing a diet is not as straight forward as it seems. It takes time, specialist knowledge and a wide range of ingredients. Nutritionists and veterinarians see as many animals that have been overfed or supplemented as they do animals that are showing deficiency symptoms, which attest to just how difficult it is to get the balance correct. Sadly, the first symptom many animal owners see when an imbalanced diet is fed is a failure to breed successfully.

The first step in designing a for an animal objective is to establish exactly what nutrients it is that animal require in order to meet their requirements for their current physiological status, be that a mature adult, a rapidly growing young animal or a breeding pair. If a scan of the available literature fails to supply the exact information for the species concerned, then comparisons need to be drawn. The starting point however, will almost always be an accurate assessment of the animals’ weight as this will give the nutritionist a fair idea of what its maintenance requirements are.

Secondly, the nutritionist needs to understand the animal’s natural habits. Despite the fact that we are principally concerned with ensuring that the animals consume adequate levels of nutrients, feeding veld hay to browsing animals would not be advisable as already mentioned. Equally, expecting a fruit eating birds to consume a seed based diet would be ill advised.

The third step is to asses the ingredients that are available. This is perhaps the most taxing problem. It is essential to establish the nutrient profile and the degree of variability in each ingredient. Ideally, all ingredients should be analysed before use as this is the only way of being sure of what it is that you are buying. In general a determination of the protein, fat and fibre content of the ingredient will give an idea of its nutrient content. The determination of the amino acids, vitamin and mineral levels is both expensive and time consuming.

Finally, formulation can begin. It needs to be borne in mind that formulation will only be as successful as the data used is accurate. It is possible to formulate diets by hand or even using a spreadsheet, but this is not to be recommended. Specialist software such a Zootrition (2006) may also be used. Although this program contains useful data on animal requirements, much of the data it contains on feed ingredient may not be relevant under our conditions. Also, formulations are not balanced automatically (no mathematical algorithm is used). Rather the user is required to tweak the formulation by hand until it is balanced.

It is felt that the best option for feed formulation is to use a Least Cost Feed Formulation package, of which there are many available. “Least Cost” diets sometimes have a bad reputation amongst the uninitiated. This perception is unfounded. The nutritionist remains in control of the process (in other words the program makes no decisions of its own) and the ultimate formulation is a function of the nutritionists’ skill and experience – something that does not change whichever method is used to formulate a diet. The advantage of using a feed formulation program is that it considers the nutrient balance, the available ingredients and their costs simulatiously. The final diet is thus nutritionally sound at a reasonable cost.

In practice, captive animals are often fed a wide range of feed components. This would include complete products made by the various speciality feed companies, fruit, vegetables, precooked grains and roughage. Many people incorporate pet food into their feeding programs as the “complete” part of the diet. Dog and cat food is designed to meet the nutrient requirements of carnivores, and they often contain high levels of calcium, phosphorus and most importantly for birds, iron. Another mistake that is often made is that people feed birds breakfast cereals. These products not only contain lower than expected levels of protein and amino acids, but the all-important Calcium to Phosphorus balance is incorrect for most species.

Prof. Nancy Irlbeck from the University of Colorado believes that a balanced diet can only be achieved by feeding a high proportion of the total feed intake as a complete feed. She believes that if possible 80% of a captive animal’s daily intake should comprise a balanced diet. The remainder of the diet can be in the form of nuts, fruit and vegetables or insects.

The simplest way in which to manage our collections is to measure. By weighing and recording both the animals and the feed that they eat, it is possible to manage many of the situations described above. Should a small animal loose weight, it is simply not eating enough, and some method needs to be devised to ensure that either its diet is changed or it is fed in a different manner. On the other hand, if an animal is gaining weight, the only way in which to prevent it is to reduce its energy intake, or to increase its level of physical activity.


It is true that we do not know exactly what the nutrient requirements of many of the species that we keep are. However, through the application of sound nutritional principles, based on the similarities between species rather than the differences, it is possible to achieve a far higher health status and level of breeding success in a collection.

Ullrey (2004) poses the question as how best to deal with the prevention of metabolic problems that arise in zoos. Do we deal with them when they become medical emergencies? Should we rather appoint competent, trained comparative nutritionists in order to prevent the problems from arising in the first place? Many good international zoos now employ qualified nutritionists to ensure that their animals are being correctly fed.

As nutritionists, it is sad to see that much of the science and knowledge we have to offer is given scant regard. Remember that all of the animals that we feed are valuable, either in terms of their monetary value, their endangered conservation status or simply because we love them. This is precisely the reason why we often kill them with kindness by feeding them too much of the wrong foods.

While it may be difficult to match those conditions that are found in the wild, it is not as difficult to do a reasonable job of meeting the nutrient requirements of our mixed collections, if we simply apply the basic principles of sound nutrition. Avi-Products has 3 qualified and experienced nutritionists in its employ, and we are always available to help people when it comes to the correct feeding of captive species. In addition, we have both a comprehensive nutritional database and library to draw on.


Clauss, M, Lechner-Doll, M. and Streich, W.J., (2002). Ruminants: Why browsers are non-grazers. Symposia of the CNS 4: 122.

Clemens, E.T., (1997). Comparative Gastroenterology/Nutrition. Comparative Nutrition Society (CNS).

Dierenfeld, E.S., (1996). Nutritional Wisdom: Adding the Science to the Art. Zoo Biology 15: 447- 448.

European Zoo Nutrition Centre (EZNC).

Halver, J.E., and Hardy, R.W. (2002). Fish Nutrition. Blackwell Science.

Klassing, K.C., (1998). Comparative avian nutrition. CAB International.

Irbeck, N. (2004). Zoo animal nutrition. University of Colorado.

Nutrition Advisory Group (NAG).

Oftedal, O.T., and M. E. Allen, (1996). Nutrition and Dietary Evaluation in Zoos. In Wild Mammals in Captivity, The University of Chicago Press.

Sturkie, P.D., (1976). Avian Physiology. Third Edition. Springer-Verlag, New York.

Ullrey, D.E., (1996). Scepticism and Science: Responsibilities of the Comparative Nutritionist. Zoo Biology 15:449 – 453

Ullrey, D.E., (2004). Who need comparative nutritionists anyway? Symposia of the CNS 5: 181-183

Zootrition (2006).

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