Dreams have intrigued humanity for millennia. Early civilisations thought they were direct communications between humans and the gods, while more recent paradigms have tied their meaning to our subconscious needs (1). With more information coming to light about the evolutionary origins, mechanisms and purposes of dreams, it has brought another question to the fore: do other animals dream? Are the intricate worlds our brains create during slumber something that makes us distinctly human, or do we share it with our animalian brethren?
The answer seems to be, yes, other animals do dream. Cats enter a state of atonia, much like humans do, to prevent them from acting out their dreams; rats can remember mazes from their waking life while they are asleep; and zebra finches practice their songs. This all suggests that dreaming may serve a distinct evolutionary benefit, as it occurs across multiple animal lineages. It may even have evolved over 450 million years ago, before land and aquatic animals diverged (2). What is even more amazing is that recent work suggests that some invertebrates, namely octopuses, may dream as well (3).
Octopuses are well known for their ability to rapidly change the colour of their skin, seamlessly blending in with their surroundings. This ability is thanks to neurally controlled elements in their skin: iridophores, which reflect light in a spectrally selective manner; chromatophores, which absorb light; and leucaphores, which allow them to reflect light uniformly, mirroring the colour of the environment (4,5). Additionally, octopuses can control the texture of their skin, creating anything from small bumps to tall spikes (4). These elements work together to make octopuses masters of disguise. While octopuses normally use these abilities to seamlessly blend into their surroundings as they hunt for prey or flee from predators, they have also been observed changing colours while they sleep. Costello, a Brazilian reef octopus (Octopus insularis) studied at New York City’s Rockefeller University, was observed changing colours and acting out defensive behaviours while asleep (3).
Humans undergo two stages of sleep: passive sleep and active sleep. Active sleep is where most dream activity occurs, and is characterised by high neural activity and rapid eye movements, hinting at the complex world playing out inside our heads (6). Until recently, it was thought that this two-phase sleep behaviour was a distinctly vertebrate phenomenon (7). However, studies of octopuses sleeping suggest that they too undergo biphasal sleep, with distinct ‘quiet’ and ‘active’ phases (8). When in the active stage of sleep, octopuses show heightened neural activity, much like humans do. Additionally, during this active sleep phase, their skin changes colour. Further than this, they produce patterns which can be associated with certain activities they do while they are awake. This suggests that they may be undergoing a reactivation of the waking experience, similar to when we have dreams about things we do in our waking lives (7).
That invertebrates such as octopuses share these similarities in sleep behaviours with humans raises questions about the evolutionary origin of dreams. Around 590 million years ago, animals with bilateral symmetry split into two major groups; protosomes and deuterosomes. The latter group comprises all vertebrates, including us humans, while the former group includes many invertebrates, including octopuses (9). If dreaming in octopuses and dreaming in humans share the same evolutionary roots, it would mean that this fairly complex behaviour evolved at the very dawn of complex life, before even the advent of the Cambrian explosion. While stranger things have happened, this would be a massive evolutionary leap, with animals evolving complex dreaming behaviours before they even evolved eyes. Rather, it is thought that the evolution of dreaming in both vertebrates and invertebrates is an example of convergent evolution (10).
Recent studies of dreams in humans have led to the ‘threat simulation theory’ of dreaming. This hypothesis suggests that dreams serve a distinct purpose; they are a biological defense mechanism providing an evolutionary advantage, simulating potential threats and allowing us to learn from them (1). This suggests that dreaming, as we currently know it, evolved in our ancestral environment, millions of years ago, and helped us ‘practise’ scenarios in which we might be threatened (11). Thus, individuals who had more frequent or more vivid dreams were more likely to survive such scenarios when they really occurred, passing on their dreaming habits to their offspring (11). Other theories around the evolutionary function of dreams involve their operation as important mechanisms in memory consolidation and emotional processing (6). Although it is still somewhat a mystery as to the exact role that dreams have played in the evolutionary success of our species, it seems that they are integral in the comprehension of our surrounding world, and help us navigate through complex, and possibly dangerous, situations
So, if androids dream of electric sheep, do octopuses dream of Costasiella kuroshimae? We may never know, but mounting evidence across the animal kingdom suggests that myriad traits we thought were unique to humans are shared across many lineages (9). Octopuses are extremely intelligent, having to navigate through complex environments, avoiding threats both from predators, and competitors of their own species. It may be that octopuses use dreams as 'practice' for such situations, with vivid dreamers more likely to survive nasty altercations. So, next time you have a bad dream, remember that the evolutionary pathways which have led to it are truly extraordinary, and there may be an octopus out there experiencing something surprisingly similar.
References
1. Linden S van der. The Science Behind Dreaming. Scientific American. July 26, 2011. https://www.scientificamerican.com/article/the-science-behind-dreaming/
2. National Geographic. Animals dream too—here’s what we know. National Geographic. 2022. https://www.nationalgeographic.com/animals/article/animals-dream-too-heres-what-we-know
3. National Geographic. Do octopuses dream? National Geographic. 2023. https://www.nationalgeographic.com/animals/article/octopuses-dreaming-nightmares-sleep
4. Meyer F. How Octopuses and Squids Change Color. Smithsonian Ocean. October, 2013. https://ocean.si.edu/ocean-life/invertebrates/how-octopuses-and-squids-change-color
5. Nahmad-Rohen L, Qureshi YH, Vorobyev M. The Colours of Octopus: Using Spectral Data to Measure Octopus Camouflage. Vision (Basel). 2022;6(4):59. doi:10.3390/vision6040059
6. Scarpelli S, Bartolacci C, D’Atri A, Gorgoni M, Gennaro LD. The Functional Role of Dreaming in Emotional Processes. Frontiers in Psychology. 2019;10:459. doi:10.3389/fpsyg.2019.00459
7. Thompson B, Bundell S. Do octopuses dream? Neural activity resembles human sleep stages. Nature. 2023. doi:10.1038/d41586-023-02169-5
8. Marshall M. Timeline of the evolution of life on Earth. New Scientist. July 14, 2009. https://www.newscientist.com/article/1937806-timeline-the-evolution-of-life/
9. Medeiros SL de S, Paiva MMM de, Lopes PH, et al. Cyclic alternation of quiet and active sleep states in the octopus. iScience. 2021;24(4). doi:10.1016/j.isci.2021.
9. How smart is an octopus? University of Queensland. November 22, 2021. https://qbi.uq.edu.au/article/2021/11/how-smart-octopus
10. Valli K, Revonsuo A, Pälkäs O, Ismail KH, Ali KJ, Punamäki RL. The threat simulation theory of the evolutionary function of dreaming: Evidence from dreams of traumatized children. Consciousness and Cognition. 2005;Neurobiology of Animal Consciousness14(1):188–218. doi:10.1016/S1053-8100(03)00019-9
11. Delgado M, Sulloway F. Attributes of Conscientiousness Throughout the Animal Kingdom: An Empirical and Evolutionary Overview. Psychological Bulletin. 2017;143(8):823-867. doi:10.1037/bul0000107
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