Whilst writing the series on “What Makes Us Human?”, I started thinking about less obvious, less traditional ideas of what traits are truly human, and human alone. One characteristic occurred to me that seemed obviously to be unique to humans: recreational drug use. It seemed implausible that animals in the wild were indulging in drug abuse purely for their own entertainment, and I wondered if this could give some perspective on what it means to be human. But, as it turns out, I was wrong.
Drug Use in Animals
Animals do take drugs. And they take rather a lot of them, actually. In fact some experts believe that the idea for taking drugs may have first occurred to humans from watching animals!
Just like humans, wild animals have learned that some plants have medicinal properties, and will seek them out when needed. In Tanzania, Chimpanzees are eating Bitter Leaf Tea (Vernonia amygdalina) for its steroid glycosides that have anti-parasite, anti-cancer and anti-bacterial properties. Of course, the Chimpanzees don’t know any of this – they just know it makes them feel better. Capuchin monkeys, too, have been seen rubbing the leaves at least four different plants (Citrus, Clematis, Piper and Sloaneagenera), which have insect-repellent properties, onto their coats.
It isn’t just primates that have discovered medicinal plants, however. Bears frequently consume the Osha root (Ligusticum porter), known for treating stomach pains and bacterial infections. Birds use wild carrot to repel mitesand horses use willow stems as a natural source of aspirin. Across the animal kingdom, creatures are curing their ailments naturally.
But just like humans, they have also discovered certain plants have a more pleasurable effect. In the wild, animals have been observed to consume alcohol, magic mushrooms and opium, among other drugs. Many animals enjoy alcohol, usually sourced from fermented fruit, grain, nectar or sap. Bears, elk, and birds have all been reported to get drunk after consuming fermented plant matter. Insects also enjoy a tipple, although for them it is usually fatal. Bees, wasps and hornets all get drunk from time to time. Waxwings (Bombycilla cedrorum) also show a lack of self control, drinking themselves silly on fermented berries of the Brazilian Pepper Tree (Schunus terebinthifolius), and flying into windows and fences, often killing themselves in the process.
Hallucinogens are also particularly popular amongst animals as well as people. Reindeer consume the highly hallucinogenic fly agaric mushroom, Porcupines and Mandrills both enjoy the hallucinogenic iboga root (Tabernanthe iboga), Bighorn sheep go out of their way to get hold of a psychoactive lichen, Jaguars eat bark of the yage vine (Banisteriopsis caapi) and horses and cattle seek out locoweed (Astralagus and Oxytropis genera), a powerful hallicinogen. Even insects occasionally enjoy a bit of wimsy, and moths are drawn to the hallucinogenic datura flower.
[A note of caution: I’ve underlined the examples for which I’ve been able to find a peer reviewed scientific journal article to corroborate the tale]
Addiction and drug abuse in the animal kingdom have, on occasion, been used for our own benefit. Some gardeners have started to take advantage of a snail’s love of beer. Leaving out jam-jar lids filled beer around the garden attracts the snails who dive in and start to get drunk. Unfortunately for the snails, they don’t know their limits, and generally die from alcohol poisoning.
Humans have been taking drugs since records began. Early cave carvings from 4000 BC depict the fermentation of beer from barley, and humans started writing down beer recipes around 1800 BC. And before humans started getting high, animals have probably been doing it for around 600 million years. Many scientists believe that the desire to consume mild-altering substances is more than just that, it is an instinct, a need which is in-built. It is as persistent a human requirement as those for shelter, food and water.
Adaptation and Addiction
But how can this be? By all accounts, taking drugs (other than the medicinal ones) is bad news. Drunken birds get eaten by predators and fly into windows and cows eat themselves to death on locoweed. Drunken humans do a lot of stupid things, too. So why would such a detrimental activity be so instinctive and fundamental across the animal kingdom? That it exists in so many species suggests that it confers some major benefit. But what?
Drugs promote lateral thinking. And some scientists believe that this may be the benefit. By freeing us from our usual patterns of thought, drug use may allow us to solve problems better, and cope with our environment. Others have noted the beneficial effect of drugs on mental wellbeing. In particular, a wide variety of hallucinogens are being tested for their use in alleviating existential anxiety, particularly in terminally ill patients. In fact, in many ways, drugs, (and hallicinogens in particular) solve a lot of the problems that religion solves. Some theories suggest that hallucinogens help us to overcome our fear of death, in much the same way that many religions can. However, religion is one of the few traits that really does appear to be uniquely human. So this explanation cannot account for the alarming frequency of drug use in the rest of the animal kingdom. Surely there is a more universal answer?
While drug use itself may confer some evolutionary advantages, addiction is surely a negative side-effect? Addiction is the compulsive seeking and taking of a drug, despite adverse consequences. It involves many complex processes in the brain, some of which are extremely long-lasting. Although we can now explain some of the pathways influencing tolerance, sensitisation and dependence to drugs, we have not yet discovered the longer-term controls which influence relapse. Increasingly it is becoming apparent that learning also plays a crucial role in addiction, and this may provide the key to understanding the long-lasting impacts of addiction on the brain.
Psychologically, one characteristic that is highly influential in drug addiction is impulsive behaviour. Impulsive behaviour may seem maladaptive, but it, like drug use, is rife in the animal kingdom, and biologists believe it may play an important role in rapid decision-making. There is substantial variation between humans, and between other animals, in the levels of impulsivity exhibited. Certainly in humans, this trait is highly heritable, and has been linked to increased likelihood of drug abuse and drug addiction. Alcoholics, particularly those who start drinking early in life, score higher for a range of tests for impulsivity, and alcoholism in general has been linked to highly impulsive behaviour. Lack of impulse control has also been implicated in crack, heroin and cocaine addiction in humans. Additionally, low tolerance to delayed gratification is also a key personality trait that has been linked to the chances of relapse. Those suffering from addiction suffer deficits in processing reward information and their ability to monitor, suppress and override reward-controlled behaviour. And addiction can become a downward spiral, as drug use further impairs impulse control.
Impulsivity is highly heritable. There is a major genetic component to both propensity to take drugs, and to probability of addiction. In Vervet monkeys, only around 5% of moderate drinkers took the next step to serious drinking problems. In humans, around 40 – 60 % of addiction risk, to any drug, is genetically determined. So what genes are causing addiction?
Since the human genome project was completed, and the cost of sequencing has dropped to sensible levels, genome-wide studies have started to reveal a few insights into the biology of addiction. However, initial investigations revealed that the trait (addiction), rather unsurprisingly, is controlled by a huge number of genes scattered across the genome. What we can say is that, again rather unsurprisingly, many of the genes play a role in the specification and maintenance of neuronal connections in the brain. Many of the genes that have been identified are still no more than unexplored regions of the genome, where we really have no idea what the genes are doing. Furthermore, recent research indicates that the susceptibility genes are different for different regions, at least for heroin addiction. They identified a few genes which influence heroin addiction; cytosolic dual specificity phosphatase 27 (DUSP27), regulating synaptic membrane exocytosis protein 2 (RIMS2), and three alleles of the cardiomyopathy associated 3 gene (CMYA3). So there!
Dopamine and the Pleasure System
What are these genes doing, and where? Primarily, drugs trigger pleasure. This is achieved most commonly through interference with the mesolimbic dopamine system. Pleasure is achieved by the combination of many brain regions which control processes such as wanting and liking, as well as learning. Our brains tend to seek out pleasure and this can lead to addiction and dependency.
The areas of the brain which control addiction are evolutionarily ancient, which makes sense if the same behaviours are also present in a diverse range of non-human animals. These regions comprise several regions of the limbic system including the amygdala and the prefrontal cortex. They are normally responsible for our behaviour towards natural pleasure such as food, drink, social contact and sex. Addictions to these drugs appears also to be controlled in the same regions of the brain. Evidence from rat, monkey and human studies indicate that the orbitofrontal cortex may be crucial in controlling drug-seeking behaviour.
Dopamine receptors respond to the neurotransmitter dopamine and are at least partly responsible for creating pleasure. They are G-protein coupled receptors found predominantly in the central nervous system. Many recreational drugs have been found to interfere with dopamine, and other G-protein coupled receptors such as the imaginatively named opiod receptors, cannabinoid receptors and serotonin 5HT2A receptors. Both opiates and cannabis stimulate their respective receptors, whilst the serotonin receptor is sensitive to hallucinogens. Consistent with all this, responses to drugs and addiction behaviour have been linked to the mechanisms that control receptor sensitivity.
The mechanisms controlling drug abuse and drug addiction seem to be quite consistent, supporting the idea of a long evolutionary history of drug abuse in the animal kingdom. To what end? Clearly medicinal plants confer many benefits, but the selection favouring recreational drug use is less obvious. Perhaps drugs improve problem solving through lateral thinking. Perhaps, in humans at least, they provide an alternative to the comforts of religion. Perhaps it was the traits that predispose us to drug use, such as impulsivity, which were actually favoured by selection. Our tendancy to get addicted to poisonous substances just an unfortunate side-effect?
Did we inherit our taste for the intoxicating from our ancestors, or did later humans copy animals, as so many legends and fables suggest? There are a variety of tales detailing how humans learnt from goats to take caffeine, or from llamas to chew coca leaves, or from jaguars to chew hallucinogenic yage root. All are just stories, and to date have little scientific evidence to support (or refute) them. One thing that we almost certainly have the other animals to blame for is santa claus. As I mentioned earlier, reindeer eat the extremely hallucinogenic (and also poisononous to humans) fly agaric mushrooms, and rather enjoy the experience that results. Local people have been known to eat snow soaked in reindeer urine or eat reindeer poo in order to have a, comparatively, safe high. It was also in this same region that the story of Santa Claus and flying reindeer was originally told, and where gift giving became an integral part of the Muscaria harvest festival. Coincidence…?
Articles in this Series:
- Intro: What Makes Us Human?
- Part One: A Brief History
- Part Two: Intelligence and Language
- Part Three: Anatomical Adaptations
- Part Four: Culture and Faith
Want to know more?
- Animal Addicts: Non-Human Substance Abuse
- The Biggest Drug Users in the Animal Kingdom
- 7 Species that get high more than we do
- Haynes (2010) The animal world has its junkies too. The Pharmaceutical Journal 285
- Siegel (1989) Intoxication: The Universal Drive for Mind-Altering Substances
- Kolter (2010) Animals on Psychedelics: Survivalof the Trippiest
- Grant and Grant (1983) Behaviour of Hawkmoths on flowers of Datura meteloides. Botanical Gazette 144 (2) 280 – 284
- Koshimizu, Ohigashi and Huffman (1994) Use of Vernonia amygdalina by wild chimpanzee: Possible roles of its bitter and relatedconstitutents. Physiological and Behaviour 56 (6) 1209 – 1216
- Baker (1999) Fur rubbing: use of medicinal plantsby capuchin monkeys (Cebus capucinus)
- Biser (1998) Really Wild Remedies – MedicinalPlant Use By Animals. Smithsonian Natural Zoological Park
- Lonzano (1998) Parasite stress andself-medication in wild animals. Advances in the Study of Behaviour 27, 291 – 317
- Huffman (2001) Self-medicative behaviour in the African great apes. Bioscience 51 (8) 651 – 661
- Dudley (2004) Ethanol, Fruit Ripening and theHistorical origins of Human Alcoholism in Primate Frugivory. Integrative and Comparative Biology 44 (4) 315 – 323
- Kinde et al Strong circumstantial evidence for ethanol toxicosis in Cedar Waxwings(Bombycilla cedrorum). Journal of Ornithology 153 (3) 995 – 998
- Ralphs and James (1999) Locoweed grazing. Journal of Natural Toxins 8(1) 47 – 51
- Winstanley et al (2010) Insight into the relationship between impulsivity and substance abuse from studies using animal models. Alcoholism: Clinical and Experimental Research 34, 1 – 13
- Nestler and Landsman (2001) Learning about addiction from the genome. Nature 409, 834 – 835
- Nielsen et al (2010) Genome-wide association study identifies genes that may contribute to risk for developing heroin addiction. Psychiatric Genetics 20, 207 – 214
- Uhl et al (2008) “Higher order” addiction molecular genetics: Convergent data from genome-wide association in humans and mice. Biochemical Pharmacology 75, 98 – 111