The Craving Elephant

The Influence of Dopamine on Food Choice

What do a short walk to the shop to buy a packet of biscuits and an elephant shaking a tree have in common?

During a recent holiday to South Africa, on safari in the Kruger National Park, I sat for several minutes watching a lone elephant as, fully utilising the magnificence of his trunk, he devoured his way through grass and foliage. I was fortunate to see a large number of elephants during the trip, and it was truly fascinating to observe the behaviour of these elegant beasts. But this young bull piqued my curiosity in a way that resonated with my nutritionist mind. At one point he stopped grasping at the readily accessible long grasses and branches to turn his attention to the trunk of a large marula tree. As I sat mesmerised, I witnessed him wrap his trunk firmly around the considerably sized tree’s torso and shake it hard. The agitation resulted in a number of marula fruits falling to the ground ready to be easily devoured.

Juicy Fruit

Marula trees (Sclerocarya birrea) are common in southern Africa, especially in the Greater Kruger area, and the marula fruit, when ripe, is yellow in colour. A marula tastes tart with a sweet-sour palette depending on how ripe it is. Rich in vitamin C and other micronutrients, inside the tough skin is a fleshy, succulent fruit with a soft nut kernel full of protein and minerals. A delicacy enjoyed by numerous herbivores – in particular baboons, warthogs and several species of antelope – all of whom devour marulas when they find them on the ground. Trunked pachyderms, however, go one step further: they shake marula trees to cause more fruits to fall. Interestingly, the tree is colloquially known as the “elephant tree” because of the lore that elephants get drunk on their fruit. According to the myth, marula fruits fall to the floor, where, in the heat, they lie fermenting. Although humans ferment marulas to make the liqueur Amarula, it’s a fallacy that elephants consuming marula do so to get drunk. Few fallen marulas have time to ferment as they are in such high demand by other plant-eaters that it’s seldom they remain unconsumed.

Despite the elephants-getting-drunk myth, it’s apt to ponder why, amid the abundance of nutritious, easy-to-obtain foliage and grass around him, this young bull bothered to exert considerable extra effort shaking a heavy tree. A task that likely involved expending a reasonable amount of energy compared to the minimal effort needed to scoop up grass, calories that, seemingly, his inefficient metabolism could not afford to forgo. What motivated him to exert this additional effort?

Marula fruits are much more energy dense than leaves and grass. I’m confident, however, that a comparison of the nutrient value of different foods isn’t something elephants cognitively consider. I suspect it is more likely that, having spied or smelled some delicious-looking marulas in the tall tree, our bull remembered how much he’d previously enjoyed chomping on them. He wanted more of that sweet flesh. His brain told him that wrapping his trunk around the tree trunk and shaking it would provide him with a tasty reward. In essence, our young friend had a craving.

But what drove that craving?

Dopamine’s Grip

From motivation to reward, one chemical has become something of a buzzword in recent years. One of the most hyped neurotransmitters, through its association with anything from sex to food, dopamine is responsible for driving us to seek out things we previously found to be pleasurable. With over 110,000 research papers on dopamine, it is hard to not see that the celebrity status of this chemical is one of substance. And when it comes to eating, what’s the familiar word for the motivation to seek out foods we previously enjoyed? A craving. Food cravings are an adaptation for us to seek out energy-dense nutrition. Cravings motivated our ancestors to seek out hard-to-come-by sustenance. For instance, they provided the impulse to climb up a tree to acquire fruit. Similarly, a likely evolutionary advantage in elephant behaviour that compelled them to shake trees for nourishment was driven by a food craving.

Perhaps one of the best-described mechanisms linking dopamine to human behaviour has to do with its role in learning about rewards. If you’re a chocoholic, there is every chance that the agreeable feeling of a luscious square of your favourite chocolate bar melting in your mouth will stimulate an intense feeling of pleasure, which promptly translates into the release of copious amounts of dopamine in your brain’s “reward system”, the term for the part of the brain that lights up on scans when you’re enjoying something [1]. Neuroscientists believe that dopamine’s dash in this circuit is the strategy that evolution has developed to reward us for engaging in life-sustaining behaviour. Or so they thought …

A study published in 2010 indicated that roulette players display the same levels of dopamine in the nucleus accumbens when they lose money with a near-miss as when they hit the bullseye [2]. Whilst it’s unlikely that this makes gambling a life-sustaining behaviour, these results echoed previous work which showed that dopamine not only signals reward but also how close you got to the reward so as to encourage another attempt. While this may seem paradoxical, we now know that this is because dopamine is not merely a pleasure molecule, delivering a simple reward. Rather, it alerts us to unexpected rewards, spiking both when there’s a near-miss or when the reward delivered far exceeds the expected result. This ties in with the current understanding of dopamine: it drives us to seek positive experiences and avoid negative ones. It is easy to imagine how such a delicate, fine-tuned system can go awry when using drugs that cause the brain to go from a low dopamine state to a high one. Or when you’re continuously tempted by a vast array of familiar sugary treats which are merely a cupboard-walk away.

This raises some questions about the broader implication of dopamine in behaviour and perception: if dopamine can reinforce highly rewarding behaviour, does this mean that it can influence our actions? On the surface, there doesn’t seem to be much difference: if dopamine reinforces certain behaviours, surely we can conclude that it affects our actions. Since the earlier pioneering work on dopamine, other scientists have shown that the concentration of the neurotransmitter can have far-reaching effects on our tendency to gamble, feel pleasure or even just roll out of bed on a bad day. But when it comes to understanding the subtle science that drives our behaviour, things aren’t so clear-cut. Partly, this is because dopamine has been traditionally associated with what neuroscientists call “reward processing” – how good or bad an outcome is after taking a decision – but not conscious experience. In other words, neuroscientists agree that juicing up dopamine levels in the right brain areas can help you to pursue valuable rewards – the sight of molten cheese on your favourite pizza will likely spark a surge in dopamine levels, causing you to want to eat it. However, given that some aspects of reward processing require conscious decision-making – like weighing up the perceived enjoyment from munching the pizza with the regret you’ll likely feel later – it remains unclear if dopamine may also be active in regulating our conscious experience of reward pursuit.

Marula Motivation

Let’s return to the elephant-tree-shaking-nipping-to-the-shop analogy. It’s likely that you’ll recall a time when you’ve been sitting tucked up on the couch watching Netflix, feeling peckish. You’ve a cupboard laden with numerous nutritious foods a mere dozen footsteps away, yet none of these readily available items are tickling your fancy. So, you grab your credit card and take a short walk to the nearest shop to acquire tasty sustenance in the form of something that you’re more partial to, like a packet of biscuits (or cookies, for American readers). If such a scenario springs to mind, you, like our young elephant friend, experienced a craving born out of a memory of past reward. The recollection of pleasure previously derived from munching on a biscuit led to the firing of cerebral neurons and dopamine being shunted across a synapse, resulting in your brain letting you know that you’re prepared to expend energy to experience that biscuit-derived reward again.

As I sat in the vehicle admiring the bull, continuing to ponder dopamine's overwhelming grip, one final thought crossed my mind: whilst many 21st-century humans might well be motivated to trudge to the shop for a packet of their favourite biscuits, just how many would bother to make the trip for a bag of apples, let alone a few marulas?

References:
1. Arias-Carrión, O. et al. (2010) ‘Dopaminergic reward system: a short integrative review’, International Archives of Medicine, 3, 24.
2. Chase, H.W. & Clark, L. (2010) ‘Gambling Severity Predicts Midbrain Response to Near-Miss Outcomes’, Journal of Neuroscience, 30(18), 6180-7.