Zoo Humans

Abnormal behavior in zoo primates (and humans) could be related to food.

Posted Jul 04, 2013

Many of our parts wither as we age, our brains among them. Octogenarians are liable to have brains 15% smaller than those of teens. Humans are unique among primates that we are the longest-lived with the largest brains, and we are also unusual in that we seem to be susceptible to neuropathology, such as dementia, during the late stages of life.

Several types of dementia will shrink out brains, but even normal human aging is characterized by neural degeneration and cognitive impairment to some extent. Various pathologies are common—amyloid beta protein deposits, the death of neuron trees, reduced synapse numbers loss of specific receptors…these seem to occur specifically in areas that are part of the brain’s resources for learning, memory, and executive function. Executive function combines the cognitive processes that control planning, working memory, attention, problem-solving, verbal reasoning, and mental flexibility. Without proper executive functioning, we have difficulty solving problems, prioritizing, and making decisions.

Researchers did comparative brain MRIs on humans and chimpanzees spanning a wide age range, which I discussed in a previous article. The humans were aged 22-88, the chimps 10-51, representing similar maturity in the two species. Chimps begin to slow down, get worn teeth, and become more physically frail after 35 and live to about 45 in the wild. Humans are generally still quite robust in the 30s and 40s and only begin to slow down a few decades later, typically living into the 80s among hunter –gatherers if one survives infancy and escapes early trauma and disease. Among the humans, there was significant shrinkage in the older brains, particularly in human brains over 50, while among the chimps there was no appreciable change. Another study of some macaque monkeys also showed no brain degeneration with age. 

Humans are the longest-lived primates with the largest brains. This fact begs the question do we represent an evolutionary extreme? Are our high-powered brains, requiring vast amounts of energy and efficiency, so high-powered that they begin to fail after a certain amount of time? As we age, our clean-up and maintenance crew in the brain, the antioxidant system and stress protection machinery, are upregulated to keep up with a slow build-up of proteins and to help detoxify and decommission mitochondria that have become damaged over time.  Ultimately, it seems our clean up crew can’t keep up with accelerating damage in the human brain. Similar states of inflammation, energy problems, cell death and even brain shrinkage occur in all sorts of different psychiatric conditions, from schizophrenia to bipolar disorder to autistic spectrum disorders to major depressive disorder and certainly in dementia. Does this mean the human species, with the Ferrari of brains, is more vulnerable to those brains breaking down and experiencing psychopathology than say, a chimpanzee whose brain is a more reliable Honda?

All of the MRIs in the Sherwood study were done on Western, First World humans and captive primates. We don’t have MRIs, so far as I know, of a population of hunter-gatherers or horticulturalists who don’t consume modern processed food. The Kitavans of Papua New Guinea rather famously have no evidence of dementia even as many of them live into the ninth and tenth decades of life. Are the incredible shrinking brains of humans a design flaw of an evolutionary extreme, or an example of a Ferrari subsisting on kerosene or diesel when it needs special 93 octane and the particular brand of supercar motor oil? Why would humans have menopause, giving mothers a greater chance to live on as grandmothers for decades after fertility is gone, if our brains merely go senile? It doesn’t make much sense that we are evolved for longevity if we would quickly become a frail burden.

Another scary fact is that primates, with their seemingly more resilient brains, experience psychopathology in zoos. Some are even put on psychiatric medications, such as Johari, a 17 year old female gorilla at the Toledo zoo, who was put on Prozac for premenstrual agitation. Her rage and irritability led to violent outbursts by the dominant silverback gorilla, causing several injuries. These episodes stopped after Johari went on the medication. Other animals at the zoo require antipsychotics and tranquilizers from time to time to sedate them if they become too violent, particularly during certain times of year.

At the Cleveland Metroparks Zoo in 2005, a gorilla died of heart disease at the age of 21. Heart disease is the leading killer of captive gorillas, who also seem to be vulnerable to developing ulcerative colitis, and autoimmune disease of the gut. That led researchers to consider changing the gorilla diets, which at typical zoos had been filled with processed animal chows, meat, yogurt, milk, eggs, bread, and cereal grains along with large amounts of various fruits and vegetables. Free ranging wild gorillas, on the other hand, eat mostly immature leaves, stems, some bark, saplings, wild fruits, insects, and some small animals. The diet of the gorillas in the wild has an incredibly high fiber content, which is used as food by the may trillions of bacteria in the gorilla’s extremely long gut. The fiber is transformed into a short chain fatty acid called butyrate that is important for colonic health and may be the reason (along with the cereal grains and other unusual foods) the captive gorillas developed ulcerative colitis.

Gorillas in the wild eat forty-five pounds of food daily, so you can see how it would be easier for zoos to give out smaller amounts of more processed, refined foods with starch and sugar and a dense calorie content. In Cleveland, the gorillas were switched to mostly leaves and various lettuce greens, some saplings and bark, and two multivitamins given inside a banana. These gorillas changed their behavior with the new diet, spending 60-65% of the time eating (similar to wild gorillas), and they lost an average of sixty-five pounds each. In addition, some abnormal behaviors never observed in wild gorillas disappeared in the captive ones eating a native diet. These captive gorillas used to regurgitate their food up to four times an hour, then eat it again. In addition, they would pluck their hair out obsessively. This behavior, a sign of anxiety, greatly decreased with the change to the wild diet.

The behavior of chimpanzees in zoos is also highly abnormal. In one study of forty chimps from six different zoos (all of them housed in social or family groups, which should ameliorate some of the worst psychopathology observed in chimps when they are taken from their mothers as children), every single chimp showed some sign of psychopathology and behavior not normally observed in the wild. Behaviors include rocking, eating feces, hair pulling, drinking urine, and self-mutilation (typically biting oneself) along with other stereotyped behaviors such as compulsive hand rubbing.  In the wild, chimpanzees generally forage over wide areas and have a varying social structure. They eat a large variety of flora and fauna obtained from a variety of hunting and gathering methods. In the zoo, much of these life elements are controlled by humans, and the chimpanzees are typically fed something along the lines of Purina primate chow augmented by fruits and vegetables. 

So if these primate brains are more resilient than our own (as evidenced by the fact that even captured chimps’ brains don’t tend to shrink over time, while Western humans certainly do), what does chimps’ and gorillas’ abnormal behaviors eating a processed foods diet augmented by fruits and vegetables mean for us, with our high-powered but delicate noggins?

One woman, Jessica O., suffered from a relatively common condition in humans called trichotillomania, or compulsive hair-pulling. She started the behavior at the age of nine and had tried everything, including therapy, prescription medications, supplements, and hypnosis to try to stop it. By the time she started Dallas and Melissa Hartwig’s strict thirty day paleo eating program called the “Whole30,” she was missing about half the hair on her head. Within the first week of the program, she stopped pulling her hair, and her hair started making a comeback. Jessica felt it was nothing short of amazing.

Hair-pulling, grooming, and picking is a generally soothing behavior common to many primates, including humans. In times of increased anxiety, humans will often pick or pull out hair or eyelashes more (compulsively biting the nails also fits into this group of behaviors).

How much of our anxiety and psychopathology come from being “zoo humans?” One aspect of our environment that we do have a modicum of control over is our diet. It is a worthwhile endeavor to stick to human-appropriate foods the majority of the time. A core diet of enriched processed foodstuffs augmented by plenty of fruits and vegetables didn’t keep the zoo primates from acting strangely or developing heart disease and autoimmune colitis. There is no reason to consider enriched processed food, even those labeled as containing “whole grains” and carrying the heart healthy check you see on many cereal boxes, to be any healthier for the minds and bodies of human beings. Build the diet around unprocessed foods. It is likely our modern diets are more risky for our brains than the similar standard zoo diets are for our primate cousins.


*”Zoo humans” is a phrase often used by Erwan Le Corre, inventor of MovNat. It appears it was originated by Desmond Morris in his 1969 bestseller, The Human Zoo (hat tip to the commenters). 

Thank you to John Durant for the inspiration for this article. His new book, The Paleo Manifesto: Ancient Wisdom for Lifelong Health will be available in September. His second chapter also explores quite a lot about zoo primates (including a visit to the Cleveland Zoo) if the topic interests you.


Copyright Emily Deans, MD


Image credit: gorilla

Other image is from personal collection.

More Posts