Animals —A Mirror into Human Health, Part II: Addiction & Obesity
HHN: In Part I of this two-part series, we discussed how animals get many of the same diseases as humans. You’ve also pointed to another animal-human similarity: Many animals become addicted to drugs. Which animals have problems with addiction?
BNH: Both wild animals and pets can become addicts. Bighorn sheep in the Canadian Rockies scrape psychoactive lichen off rocks, even if it leaves their teeth ground down to the gums. Water buffalo in Asia eat bitter poppies—in other words, opium. Cattle, horses, deer, elk, and antelope in the American West eat locoweed, which sometimes disorients the animals so much, they walk off a cliff or up to a predator. Dogs in Australia’s Northern Territory and elsewhere lick hallucinogenic toxins off the skin of cane toads.
In the laboratory as well, rats will become addicted to a variety of drugs, including cocaine and heroin. To get their next fix, some neglect their children. Some forsake sex, food, and water, and end up killing themselves.
HHN: Are there commonalities underlying both human and animal addiction?
BNH: Yes. Notably, both non-human animals and humans have specialized receptors on the outsides of certain cells that allow drug molecules to enter."Both humans and animals have receptors for opiates such as heroin and morphine."For example, both humans and animals have receptors for opiates such as heroin, morphine, and various narcotics. So do amphibians, insects, and fish. And receptors for cannabinoids, the intoxicants in marijuana, have been found in birds as well as amphibians, fish, mammals, and humans.
Fascinatingly, these opiate receptors have also been discovered in jawed vertebrates that lived 450 million years ago!
These findings suggest that the receptors for opiates and other chemicals arose in order to maximize various species' chances of survival.
HHN: Why might natural selection favor receptors for chemicals that could lead to animal addiction, self-injury, offspring neglect, and suicide?
BNH: Research shows that these same receptors also facilitate chemical responses when animals are engaging in life-preserving behaviors. For example, they aid in chemical release when animals unite with a parent, play with their siblings and other animals, find food, or advance towards mates."Receptors on the same pathway as opiates aid in chemical release when a rat mother nurtures her offspring."Opiates, for example, induce a sensation in the brains of rat mothers nurturing their offspring and are also released when dogs lick their owners' faces. Dopamine, the "foraging" chemical in simplistic terms, is released when humans and non-human animals seek out and find food. The process of pursuing and connecting with mates releases a complex cocktail of chemicals which, in general, overs implied terms, includes:
- Dopamine, which appears to motivate the mate search.
- Endorphins, natural opiates, which may serve by masking any physical pain that might impede the operation.
- Serotonin, which possibly plays a role in mate selection based upon social dominance. Research shows that animals that have higher social dominance also have higher serotonin levels. In stickleback fish, which have a linear hierarchy, literally a, b, c, d, when a particular fish’s rank goes up, its serotonin level is elevated; when a fish is demoted in the social hierarchy, its serotonin level decreases. Linear hierarchy also exists in animals and birds, to varying extents.
- Oxytocin, which has been shown to enhance feelings of connection and safety during bonding.
HHN: So we know the drug receptor pathways in animal cells aid chemical release in animals when they are engaging in behaviors that support their survival and their propagation. But we don’t really know yet why this is happening.
BNH: That’s true. Personally, I think the most likely hypothesis is the "pleasure reward" theory posed by neuroscientist David Linden of Johns Hopkins University in his book The Compass of Pleasure: A "pleasure reward," built into animals' hardwiring, favors such fitness-enhancing behaviors as finding food and mates and connecting with one’s mother and social community. In effect, according to the theory, the animal "works"— forages, socializes, protects—in order to receive that pleasure reward. I’m using the term "pleasure" very broadly here, because of course we have no idea what pleasure is in fish, but the fact is, these chemicals, which we do know can mask physical pain and support feelings of connection, are released in conjunction with vital social functions in a variety of species.
HHN: Is it possible that this same evolutionary thread is influencing human behavior?
BNH: It is possible. Like non-human animals, we humans bond with parents, play with siblings and friends, and seek out food and mates. Some of the chemicals released in animals during their life-preserving behaviors are also being released during human interactions that are vital to our sustenance. Oxytocin may be the best example. It has been shown in a variety of studies to modulate parent-infant interactions—gaze, touch, affect, and vocal matching.
And the fact that SSRI drugs, selective serotonin reuptake inhibitor drugs, are used effectively in elevating moods and decreasing compulsion in human beings speaks to the important role serotonin deficiency plays in human pathology; the flip side of that is in human health.
HHN: Are there human studies of dopamine, serotonin, and endorphins in this life-preserving context?
BNH: Not to my knowledge. But we also need to factor in a fundamental principle of evolutionary biology.
HHN: What is evolutionary biology, and what is this fundamental principle?
BNH: Evolutionary biology is the study of the evolutionary processes that produced the diversity of life on Earth. One of its fundamental principles is: Behavior follows physiology. In other words, physiology, how the living organism functions, drives what it does."Most behavior in both humans and animals is driven by the bodily systems that regulate chemical release."Well, we now know that most behavior in both human and non-human animals is not emanating from the brain’s prefrontal cortex, which regulates complex cognitive, emotional, and behavioral functioning. In actuality it’s driven by the endocrine and the autonomic nervous systems, which, among other things, regulate these chemical releases.
So when we see similar behaviors in human and non-human animals, and they are being driven by similar physiology, although we cannot say for certain, the likelihood is that in both humans and non-human animals, it’s being motivated in a similar way.
HHN: If this is true, what then goes awry when animals and humans become addicted to external intoxicants that are on the same receptors as the internal ones?
BNH: Taking external intoxicants seems to disrupt animals' natural, internal instincts. This removes an animal’s drive to "work" first before enjoying its reward; instead, the animal takes the direct path to reward. And psychoactive lichen, poppies, and locoweed offer a much higher dosage of pleasure reward than would the chemicals naturally released when "working." Afterwards, animals tend to lose interest in doing their "work."
Similarly, in humans, taking drugs can impede life-enhancing behaviors. Think about the pleasurable sensations we have when being hugged or admired or when we feel close and connected. Taking a narcotic or alcohol can simulate components of those sensations—feeling relaxed, thinking everything’s going to be fine—and preempt our making the effort to engage with other people face-to-face.
Together, all of these insights—that animals also suffer from addiction; that receptors for intoxicants exist in all kinds of creatures, some dating back millions of years;"What some see as a personal failing is not unique, not uniquely human, and not uniquely applicable to our times."that these receptors facilitate many life-affirming behaviors in animals and at least some in humans; and that the endocrine and central nervous systems are regulating many of our behaviors—can help us reconceive of human addiction in a more expansive and destigmatizing way. What some see as a personal failing is not unique, not uniquely human, and not uniquely applicable to our times.
HHN: Is there a way to apply the knowledge we have to reduce human and animal dependence on drugs?
BNH: Fortunately, for many animals, intoxicant foods are typically not available year-round; they’re usually seasonal. Sometimes, too, there’s competition for the drugs. And often after the animal’s access is disrupted, it returns to "work."
Of course, when it comes to human addiction there’s typically no such external moderator.
Yet, the evolutionary perspective we’ve been theorizing about does present a potential pathway to overcome addiction:"We can try to replace the dependence on external drugs with new or increased behaviors that appear to release internal pleasure chemicals."We can try to replace the dependence on external drugs with new or increased behaviors that appear to release internal pleasure chemicals. We know that endorphins are released after exercise, adrenaline is released after taking risks, planning a meal releases dopamine, and connecting with other people releases opiates.
This may be why some addicts do so well in rehab programs. These programs emphasize planning and companionship,—behaviors which have the added benefit of releasing chemical rewards.
HHN: You also write that a great many animals will overconsume food, just as we humans do. What do we know for certain?
BNH: We know that birds, reptiles, fish, and insects go through a cycle of gaining and losing weight. When food is abundant, many mammals, birds, fish, and reptiles eat so extensively, their digestive tracts cannot absorb more nourishment. Lions eat so much after a hunt, they can barely move."Lions eat so much after a hunt, they can barely move."In the wild, we also know that beefing up betters survival. Males of many species get fatter before the mating season. Female animals store fat to help provide milk or other food for their young. Seals and snakes store fat to have extra energy before molting. Hibernating animals fatten considerably before they start to fast.
HHN: So, back to theory here: Natural selection might reward overeating in animals as a necessary means of survival in less bountiful periods?
BNH: Yes. And this is another instance where an animal’s drive to "work" results in a reward—in this case, a sustenance reward. Wild animals have always worked for their food. In earlier times, humans did as well, hunting and raising animals and farming crops—essentially organized foraging.
Yet today, our ways of seeking and finding food are largely disconnected from our ancestors' organized foraging."Approximately 25% - 30% of pet dogs and cats in America are overweight."At the same time, an increasing number of human beings and their pets are obese. Studies show that approximately 25% - 30% of pet dogs and cats in America are overweight.
HHN: How might we apply these wild animal insights to address obesity in humans and their pets?
BNH: This is a fundamental challenge.
Veterinarians sometimes simulate a wild environment for domestic animals in the sense of arranging situations that challenge them to work for their food. For instance, the vets will stuff feed into big balls; then the animals have to find ways to stabilize the balls and get their paws into them simultaneously.
As for human obesity, there is a growing movement to view it as a disease of the environment, which is how I see it. What if, instead of reprimanding people for their lack of willpower, we could reconceive of them as individual human animals in an entire herd becoming obese? Then, perhaps, we would find ways of changing the obesegenic environment—candy dish on everyone’s desk, 7-Eleven stores open 24/7—our human herd is living in.
HHN: Fundamentally, what do you hope readers will take away from this species-spanning approach to medicine?
BNH: One of the best ways of approaching today’s human medical challenges is to pay attention to these same or similar challenges in animals. Both animals in the wild and our pets can be mirrors to the human condition.by