When researchers try to predict animals’ decisions they often assume that animals know everything useful, such as the chances of finding food in different places. Or, they assume that animals are unfeasibly good at learning, such as having the ability to update their beliefs about food availability following probability theory. Because such learning abilities will be costly, such as needing a big brain, animals are likely to have simple rules of thumb to cope with uncertain conditions. Researchers may be overestimating the abilities of animals and so incorrectly predict their behaviour. We used computer modelling to predict how animals should behave to maximise their survival when the food supply is unpredictable and the environment contains predators.
According to the model, an animal that bases its decisions only on its current energy reserves can survive almost as long as one that uses its brain to calculate the best thing to do. We find that updating probabilities gives a strong advantage only if fluctuations in the food supply are very strong and reasonably frequent. This shows that surviving in difficult and dangerous conditions does not necessarily require high brain power. Instead, animals should be sensitive to their body condition, such as how hungry they are or fat stores. An animal’s body condition tells it how successful it has been in the past, which is a useful guide to how it should behave tomorrow. This simple, physiological form of memory may have allowed animals to avoid investing in brain tissue, which requires a large amount of energy.
The usefulness of such memory means that animals, including humans, may appear to be processing a great deal of information in the brain when in fact they are just following their gut. When researchers predict behaviour they are often accused of being unrealistic; our results suggest that animals can achieve impressive performance just by responding to their body. The findings raise the possibility that simple memories may also be encoded in other physiological states, such as emotions. This might be why it takes a long time to calm down after feeling threatened. Since the threat may come back, the emotion keeps the body ready to fight or flee. It is possible that their usefulness as a ‘memory’ is the reason humans and other animals have emotions. The research has implications for conservation too. By using their body condition as a cue, the animals in our model can still perform well when the environmental conditions change suddenly. This suggests that some species might be able to cope with the effects of climate change better than expected.