Life at the Zoo

by Phillip T. Robinson

  The science of animal nutrition is the most basic, but historically one of the least applied, disciplines in zookeeping. Feeding practices often vary widely from zoo to zoo, based on tradition and pragmatism as much as anything to do with science. In the past, most zoo diets were empirically formulated, but animal nutrition technology is now defining and improving feeding practices in zoological gardens. The Nutrition Advisory Group was formed within the American Zoo and Aquarium Association in 1994 to coordinate the documentation and development of suitable formulas for feeding captive wildlife, including neonates. This growing body of information is now Internet-accessible through www.nagonline.net.

  Zoo diets were historically determined by considering the natural feeding behavior of animals in the wild and the practices used in feeding comparable domestic species. Practical adjustments were made over time, based on the outcomes of the captive-feeding experience. Many of the historical husbandry problems were attributable to inadequate knowledge of animal nutrition. The first zoos to hire full-time animal nutritionists were the Metro Toronto Zoo (1975), the National Zoo (1978), and the Brookfield Zoo (1980).

  The dietary requirements for specific nutrients can vary significantly between species. What is a minute, essential dietary substance for one species may be produced internally by the body in another from dietary precursors. By definition, vitamins are organic substances that are required in tiny amounts and, unlike proteins, fats, and carbohydrates, do not provide energy or protein-building units. Their role is to act as coenzymes or parts of coenzymes to facilitate key metabolic reactions. Some animals, for example, can produce vitamin C (ascorbic acid) in their own tissues, but gorillas, some monkeys, and humans must obtain it from the food that they consume. It has been known for three hundred years that citrus fruits protect against scurvy, an illness that was ultimately identified as a vitamin C deficiency; Englishmen were called “limeys” because their sailors commonly consumed citrus fruit to prevent scurvy, long before they knew the chemical basis of the disease. It was not until 1932 that ascorbic acid was first chemically isolated from lemons.

  For most of the history of zoos, even the existence of common vitamins was unknown, let alone specific requirements for various species. Except for specialized feeders, many animals in nature eat a large array of foods, varying in abundance with soil, temperature, rainfall, and season. Unlike in captivity, there is a shifting interplay between animal preferences and food availability, resulting, overall, in acceptable levels of nutrition on a population basis (individuals are on their own). Whereas animals spend a good deal of their time in nature finding, manipulating, and consuming food, zoo cuisine often comes in several compact daily portions, distilling this food-acquisition activity into short periods of hyperconsumption.

  The case of the gorilla provides evidence of the detrimental effects of certain human-imposed feeding routines in zoos. Some captive gorillas have commonly regurgitated and reingested (R/R) their food, a vice that has defied most efforts at deterrence. This behavior is not observed in wild gorillas. Often, captive gorillas will vomit food on the ground in a matter-of-fact manner and then leisurely reingest it, sometimes joined in this activity by their exhibit companions. Needless to say, this behavior is not endearing to the public. A controlled study by several zoo researchers led to a significant reduction in these undesirable activities, focusing primarily on the traditional practice of feeding milk to gorillas. By eliminating milk feeding, substituting a small volume of fruit juice, and increasing the proportion of fiber-rich dietary ingredients, R/R was reduced by nearly 40 percent. This behavioral modification may be particularly important in breaking the R/R cycle from generation to generation, since such behavior in young gorillas seems to have an important learned component.

  Much of the current interest in zoo animal nutrition derives from the many years of work by Dr. Duane Ullrey, the former director of the Comparative Nutrition Laboratory at Michigan State University. His hundreds of published papers on animal nutrition, and the generations of graduate students that he has mentored, have pioneered the research in this field. Surprisingly, zoos employ more geneticists and reproduction specialists than animal nutritionists. Complete formulated rations were developed at the Philadelphia Zoo in the 1930s. H. L. Ratcliffe implemented the earlier dietary work of Dr. Corson-White, a pathologist with the zoo. It would be many decades, however, before the first trained animal nutritionists were employed in zoos.

  For most of the history of zoos, the public was allowed to bring food for the animals as part of the entertainment experience, as well as to subsidize the zoo food budget. Most public feeding is now banned. This transition included various schemes to control feeding, such as providing vending machines with specific animal food, rather than having the public bring their own junk-food choices. The death of widespread public feeding of zoo animals was slow and painful, and it taxed the creativity of zoo staff, whose traditional approach was to post a “Don’t Feed the Animals” sign. For many visitors, the feeding of animals was their link to animal interaction—a long-presumed right of zoogoing. In the final stages of banning feeding and other interactive public behaviors at the San Diego Zoo, a more amusing approach was taken that was better received than the typical “Don’t” signs. A summer student was put to work observing negative public interactions with animals and was asked to make some suggestions. His product, a clever sign, has generated significant revenue through the sale of thousands of copies in the zoo gift shop and has helped modify public expectations and behavior.

  Many zoos, including the San Diego Zoo, used to make it a practice to withhold all food from the big cats one day a week, and it was still in fashion when I first arrived there. I received interesting comments from some of the mammal keepers when I inquired about this practice. Since appetite is one of the more reliable indicators of how an animal is feeling, I was a little perplexed about the loss of this vital information on all of the zoo’s big cats for an entire day each week. One keeper said that he thought the cats were supposed to be healthier because of it, but he wasn’t sure why, except that it was considered “more natural” since the big cats don’t eat every day in the wild. Another attributed it to cost-cutting, since tigers and lions are expensive to feed, and this had been done for more than fifty years. A third simply replied that they skipped feeding on Mondays because that was the main day when most of the regular keepers did their heavier cleaning and maintenance; skipping the feeding gave them more time for other chores. Inquiries at other zoos turned up similar, mixed responses. Instead of fasting, one zoo fed chicken necks to the big cats one day a week because they were cheaper than horsemeat, although they might have contributed to oral health by reducing dental tarter accumulation. It seems that skipping the feeding of carnivores once weekly originated primarily as a cost-saving practice and was rationalized by naturalizing it. We did away with the fasting custom, and all carnivores were fed daily after this brief survey.

  Indeed, the cost of feeding animals, and carnivores in particular, has been on the minds of San Diego Zoo managers from the start. In response to criticisms about the cost of keeping a private collection of animals, just donated to the new San Diego Zoo, the San Diego Union-Tribune published the following commentary in 1917:

  When the question of the acquisition of these animals is broached to certain conservatives they appear stricken with fear at the enormous amount of meat the lions and tigers are supposed to consume, quoting figures they must have obtained from some farmer’s almanac, and shrinking from joining the Zoological Society because of the supposed high cost of maintaining large animals. Some compromise by desiring the Zoological Society to only place on exhibit the local ground squirrels, gophers and field mice, thereby losing sight of the real function of a great zoological garden, which is to house and exhibit, along with the local fauna, a representative collection of the large exotic species. When these people think of a lion eating 15 to 20 pounds of meat a day, they think of the cost in terms o
f beefsteak at the butcher shop. As a matter of fact, the meat problem at a zoo is far more simple than at a home as the animals are fed horse meat. Old horses cost from $2 to $4 depending upon their size, and one fair-sized horse lasts all the animals in the Zoo for one week.

  Still struggling with the meat bills eight years later, an article with the following title appeared in the same newspaper: “Lions, leopards and tigers at Zoo are getting hungry—old nags wanted to feed them.”

  There is still a common, but misguided, belief that animals are born with innate nutritional wisdom that empowers them to choose a balanced diet from a cafeteria of foods. This cafeteria model works in the wild, where the menu is not human-made but one with which the animals evolved. Cafeteria feeding assumes that when provided with a blinding array of choices, animals know what is best for them and will select the correct ingredients and proportions to form a nutritionally complete diet. Why we would assume anything as presumptuous as this is hard to fathom, but it seems to derive from our abiding trust in the wisdom of Mother Nature. Take the following examples: man’s closest animal friend is, indisputably, the domestic dog. Humans have spent thousands of years and millions of hours selectively breeding and teaching domestic skills to dogs, yet you can’t trust dogs to watch your food. Give a dog a bath and it won’t hesitate to roll in the first dead and decayed piece of protoplasm that it encounters. Dogs consistently prove themselves nutritionally incompetent—eating bones and garbage until they get deathly ill, and then doing it again only a few days after they recover. And one last thing about dogs—and this is why I don’t understand why people trust animals to select a balanced diet—tie a dog to a clothesline rope on the porch, and most will remain there until they starve to death, never thinking that within seconds they could chew themselves free. I love dogs just the way they are, but I have no delusions about their critical thinking skills.

  Studies have shown that animals naturally do what most people do: they have preferences for the most palatable rather than the most nutritious ingredients. In other words, they behave just like our children—and us. A feeding study with monkeys demonstrated that their zeal for junk food was as strong as in humans. Forget the tofu, broccoli, and brown rice they were offered when the equivalent of Twinkies, popcorn, and hot dogs was available. They consumed carbohydrates at the expense of protein, using palatability as the principal decision maker.

  Since many contemporary zoos have had to survive at the mercy of municipal governments, the expense of feeding zoo animals has always been regarded as a burden. Particularly with specialized feeders such as many bird species, it is common to experience significant food wastage. The cost of feeding berries and cultivated insects to birds at the San Diego Zoo accounted for at least some of the financial anguish experienced by the zoo’s comptroller.

  The public brought its surplus food to the bear pits of old European city centers to support these charitable attractions, and this tradition carried forward into the twentieth century. Leftover bakery goods, second-class fruits, and vegetables from produce vendors and fallen animals from slaughterhouses and farms became the everyday staples for many zoo animals. Even in the year 2000, zoo animals in some locales were at risk from questionable food supplies. The deaths of twelve tigers at India’s Nandankanan Zoo, for example, were caused by the consumption of decomposed and contaminated cow meat, according to an official pathology report.

  Virtually no commercial zoo diets were available for zoo animals prior to the 1960s, and the quality of dietary husbandry varied widely. In a 1923 edition of the San Diego Union-Tribune, a crisis was declared when it was announced that the monthly feeding costs for the entire San Diego Zoo had exceeded an alarming three hundred dollars. A public plea was made for donations of mice and rats to feed the reptile collection, which was expanding rapidly. Requests also went out to farmers and ranchers in the region for donations of fallen and unwanted livestock to feed the zoo’s carnivores. From these frugal, penny-pinching beginnings, the practices of feeding zoo animals took their formative steps. In fact, one need only travel to zoos in some less prosperous or enlightened countries to see the same problems today.

  Many wild animals failed to arrive alive at their zoo destinations because they lacked wholesome and balanced diets to sustain them en route. Particularly with stress-prone or nutritionally specialized species, transitions from the wild into captivity often overtaxed their abilities to adapt as they passed through a complex custodial chain in the animal trade. When vitamin reserves are depleted and protein and energy intakes are below what is needed, the fasting body literally begins to consume itself. Glycogen reserves and fat deposits are mobilized and burned up, accompanied by the utilization of muscle for energy and for the protein needs of more vital tissues such as the heart. This catabolic state marks the beginning of an accelerating process that rapidly jeopardizes survival.

  During episodes of the CBS television series Survivor, there were noticeable changes in the physical status of the competitors. What began as a general shortage of groceries eventually challenged the participants nutritionally. The physical and emotional effects of calorie deprivation and stress were apparent in all of the contestants—they got on one another’s nerves, lost focus and ambition, and in some cases became emotionally vulnerable. In zoos, the signs of nutritional stress in animals may not be so readily apparent. A dwindling state of nutrition, coupled with chronic stress and poor sanitation, causes significant mortalities in wildlife before they can adapt to captivity. Younger animals with more demanding nutritional requirements, leaner body masses, and less resistance to common pathogens are particularly unlikely to survive unless special efforts are made to nurture and rehabilitate them. The lack of essential amino acids, vitamins, and functional enzymes to catalyze normal metabolic reactions results in immunosuppressed states similar to those characteristic of HIV/AIDS, where even garden-variety bugs can be fatal.

  I can testify first hand to the insidious effects that malnutrition has on physical and psychological well-being. While working in West Africa before attending veterinary school, I experienced vitamin A deficiency after months of field research in the rainforest regions of Liberia and Sierra Leone. I had lived on a limited diet of rice, cassava, chicken, and other readily available village staples. The problem began as a loss of peripheral vision at night—a peculiar sensation of tunnel vision. A skin rash and psychological malaise followed, and I was at a loss to figure out what was happening to me. I languished for days before finally visiting a rural missionary clinic to see an English doctor. Shuffling through the queue of pregnant mothers, crying children, and old men with festering leg sores, I was immediately diagnosed with a vitamin A deficiency. The lack of dietary green vegetables containing beta carotene (the precursor of vitamin A) had depleted my liver reserves of this nutrient, but the problem began to resolve within days after I began taking vitamin A supplements. In many ways, zoo animal nutrition is also a largely unexplored wilderness.

  When animals are collected in the wild by hunters, who bring them back to their villages and inexpertly attempt to keep them alive until they are sold, the mortality rate can be staggering. In the case of chimpanzees, it has been estimated that, even in the second half of the twentieth century, ten chimp deaths resulted from each import. This starts with the shooting of the mother and capture of the young, the typical manner of collecting chimps from the wild.

  Animals that are malnourished have abnormal behaviors. Low-protein feeding studies have demonstrated significant changes in social interaction, as quantified by alterations in eye contact and other indicators of anxiety. To the casual observer they may appear no different than animals fed normal protein diets, but there are measurable changes that, in highly social creatures, place their well-being at risk.

  Some wild animals simply do not recognize domestic foodstuffs as edible, and they may reject everything that is unfamiliar. The confinement of captivity, separation from companions, and the abrupt alteration in their n
ormal constellation of smells, sounds, and space can prove overwhelming. After a protracted period of anorexia and malnutrition, some seem unable to recover their former vitality under any circumstances. The stress of captivity may cause intractable indigestion, alteration of normal gastrointestinal flora, and inability to digest and assimilate nutrients. Microscopic changes to the cells of their damaged intestinal linings may unalterably compromise their ability to absorb nutrients and regulate water balance.

  Notable among the difficult species to adapt to captivity are some of the specialized leaf-eating mammals, such as the colobus and langur monkeys, many of which have exhibited varying states of malnutrition upon their arrival at zoos. Their digestive systems have evolved to process plant fiber through the aid of symbiotic bacteria in their complex stomach and hindgut, just as gut flora are thought to help termites digest woody materials. Surveys of diets fed to captive leaf-eating primates have revealed that zoo diets are generally lower in fiber and higher in protein than wild diets, demonstrating that further efforts will be required to approximate natural diets more closely. Other factors requiring study include the presence of toxic secondary-chemical compounds in plant materials fed in captivity and their variations in concentration with season. It was learned with koalas (obligate folivores), for example, that eucalyptus browse can vary greatly in its content of cyanide compounds, a potential concern for captive koala feeding. As a precaution, when selecting eucalyptus species for cultivation in San Diego for koala browse, testing was done to eliminate those species with higher cyanide content. It is presumed that secondary compounds serve to protect plants by deterring excess browsing. Livestock intoxications by cyanide are well known in the Australian veterinary literature, particularly when eucalyptus trees have been damaged by fire and generate sucker growth that is readily accessible at ground level. There also is evidence that free-ranging primates avoid certain plants because of the presence of deleterious chemical compounds.