Humans mostly dream during the RAPID EYE MOVEMENT (REM) phase of sleep that happens about 90 minutes into falling asleep. During these phase, the eyes move rapidly the heartbeat becomes faster and blood pressure is almost as if one was awake. This phase of sleep is also found in many animals, but to understand what animals dream of we need to get into their heads. Cat's brains have an area that prevents them from moving while sleeping.
In an experiment conducted back in the 1960s, this part of the brain was removed. As a result, cats began moving more during their sleep. As they entered into the REM phase, they started moving their heads as if they were responding to a call. They even walked around and became predatory like they were chasing mice. Cats sleep for around 15 hours a day. During their sleep, they may start twitching, making odd sounds, or moving their paws as if running, which indicates that they might be dreaming. Some birds also turned out to sing in their sleep.
Zebra finches, for example, silently replay the songs they learned during the day while sleeping. When they sing during the day, a special group of neurons gets activated. When zebra finches playback their songs in their sleep, the same group of neurons fire again. Zebra finches are not born knowing these songs and notes but have to learn them later in life. It could be that zebra finches are rechearsing their melodies in their sleep to remember them better.
What's interesting is that, when these birds sleep for long periods, they would not only replay their song but also try different variations like changing or tempo. Which means, sleep may not only allow birds to dream but also offers them a chance to be more creative other animals like cuttlefish behave even more interestingly in their sleep.
During their sleep, cuttlefish demonstrate REM like behavior lasting between two and three minutes. When they enter deep sleep, their arms twitch and they even change their body color. No one knows whether or not this change in color is in response to their dreams. Cuttlefish are renowned for their sophisticated camouflage as well as displaying mesmerizing colors to attract their mates and prey.
So it's possible that they're dreaming of chasing a prey going after a mate, or escaping from a predator. So, why are animals being selective at what to remember during their dreams? How do they choose how do they choose what is worth remembering?