How Do Non-Human Animals Experience Hunger?

How Do Non-Human Animals Experience Hunger?

Welcome back to Giz Asks, a series where we ask experts hard questions about science, technology and humanity’s future. Today, we’re on a quest to find “the hungriest” non-human animal.

Illustration: Sam Woolley/Gizmodo

Whether they’re hibernating, migrating thousands of kilometres to reproduce, or living in a bone-dry desert, many animals go weeks or even months without any food. For some, to go without food for long periods is business as usual, built into their instincts and DNA. A few animals take this long-term fast to an even further extreme, such as the creepy blind salamanders called olms, who are able to go for up to 10 years without eating. Even in less excessive cases, such as bears who hibernate for winter, animals’ ability to go for months without food is an amazing feat, and one that evokes our human sympathy. But do animals experience hunger as we do? Are animals that fast always hungry, or is hunger something they experience rarely? Are they constantly suffering, the way it seems we would in their situation?

We asked biologists, psychologists and neuroscientists to explain how it feels to be a starving animal.

Ivan de Araujo

Associate Professor of Psychiatry, Yale School of Medicine

These are interesting questions. To be frank, I don’t think one could possibly know how animals of these particular species “feel” when food deprived because their physiology evolved to become resilient to the lack of nutrients. Some species spend months without eating via mechanisms such as drastic reductions in cardiovascular function (for example, some reptiles turn down their heart rate to about one beat/min while “waiting” for the next hunting season). Thus, given such a different array of internally generated signals, I would say that what they experience is very different from less adapted species like ours. But as I said, this is hard question to tackle — in mammals, it does appear that hunger produces avoidance behaviours, so it would be interesting to know whether in these animals avoidance emerges after prolonged food deprivation.

One other point is that animals don’t necessarily need to be hungry to seek food. In nature detecting prey is a random event that should be tackled opportunistically and many hunters will pursue and kill prey irrespective of whether they are very hungry.

Chris Braun

Professor and Chair, Department of Psychology, Hunter College

A species’ physiology is totally adapted to its typical food intake. If its normal feeding cycle has long gaps (hibernation, mating period, migration, et cetera), then the physiological adaptations include big fat stores, lowered metabolism, breakdown and use of muscle, stuff like that. That’s all about calorie intake and energy use. Any species that has long gaps in feeding must have these kinds of physiological tricks.

But hunger is a different thing. From a behaviourist point of view, hunger is an abstract variable that expresses the likelihood that an animal will work for food. It can be shown to be changed by past experience, including time since last feeding and memory (we always get hungry just before the dinner bell). It is also influenced by internal metabolic processes, but only indirectly. This is probably based on sensors of blood chemistry and fat metabolism within the body and nervous system.

Our own experience of hunger is a feeling of discomfort that’s proportionate to that abstract measurable hunger, but includes an empty feeling in the stomach and a motivation to find food and eat it, plus a feeling of satisfaction (or more, “yay, chocolate!”) when you get the food. It’s lunch time, so I guess you know what that feels like. But you also know from experience that [hunger] isn’t strictly tied to food intake (unfortunately).

Natural selection should fine-tune the internal sense of hunger to the feeding rhythm of the species. I think the same way a human can become engrossed in a task to the point they skip a meal without noticing, a migrating bird doesn’t “think” about food. If you grab it mid migration season and set up an experiment where it could work for food (push a lever), I guess it would never try. From the outside, it doesn’t look hungry. From what we can guess or analogise for it’s internal perspective, I guess it’s just not part of their consciousness, they would not feel hunger pangs, anxiety over where is the next meal or any kind of craving for food, the same way you don’t even notice lunchtime passing when you’re deeply engrossed in some other motivation.

Marshall McCue

Associate Professor, Department of Biological Sciences, St Mary’s University

These are difficult questions to put into objective terms. I will note that some animals will simply not consume a meal if they have consumed a meal within the previous month (examples include many large-bodied snakes). Please note that humans who fast beyond ~five days will gradually lose their overwhelming feelings of hunger (this response is potentially adaptive). Surely, animals (including humans) exhibit numerous adaptive physiological strategies for coping with natural periods of food limitation.

Brian Palestis

Professor, Department of Biological Sciences, Wagner College

I don’t know the answer to the specific question of how hunger feels [to animals], but you should consider differences between warm-blooded and cold-blooded animals (endotherms and ectotherms). A lot of the energy we get from food is used to keep up metabolism and produce heat. Small endotherms like songbirds need to eat very frequently. Animals like snakes that can go long periods without eating are ectotherms, so don’t need to eat nearly as often.

Deniz Atasoy

Assistant Professor, Istanbul Medipol University

These are great questions that I would die to know answer as well. I am not sure about prolonged starvation and how it feels for animals, but in recent years we have gained some information about the neurobiology of appetite. Specifically, scientists have identified the key neurons that makes animals look for and consume food. Artificial activation of these neurons in sated mice is sufficient to drive voracious feeding. A couple of interesting papers have investigated psychological aspects of this using high tech approaches. According to them, hunger is a negative valence teaching signal and animals try to avoid it if possible by consuming food and this state can be recapitulated simply by activating a small population of neurons in hypothalamus called AgRP-neurons, on the other hand, if you have food around when these neurons are active you get a completely different situation, now hunger and resulting food consumption becomes rewarding and animals prefer that state.