Grug's motto ("never not be afraid") seems like paranoia at first, considering "never not" is a double-negative synonymous with "always". However, if you read it literally, it starts to make sense: he's essentially telling his family to never be devoid of fear, facing new or unknown things with an appropriate amount of caution instead of rushing blindly into something you're not prepared for. It's only his own misplaced paranoia that makes his motto seem like paranoia in turn, which is what he realizes by the end of the movie. So when Grug tells Eep the logical antithesis, "never be afraid", he's basically telling her to never be paranoid, and face new things with an open mind while not forgetting to be safe.
Light and/or illumination is a major theme of this series, as it represents the family being able to see beyond their own borders and limits and be able to take steps forward.
The logo jokes for each film are themselves lit up by various light sources (a spark for the first film, and Belt using a torch for the second).
The first film has the Croods encounter fire, and a star-filled night.
Both films involve the Croods and sunlight at the finale: the first film has the Croods running towards the light, and the second has Eep and Guy enjoying a sunrise.
Eep gets the Thunder Sister title of Fire Heart in the second film, as she is a Fiery Redhead who is the most rebellious and romantic of the Crood family.
Eep and Guy use fire to bring down the Spiny Mandrilla in the second film.
As a foil to the above, we have tar: liquid darkness that keeps you from moving forward. No wonder Guy considers it his enemy.
Remember how Guy told Eep that he hates the dark? He later revealed to Grug that his family died in a tar pit, which can be described as having a dark/darkness-like appearance. Guy's hatred of darkness came from the most tragic day of his life.
Even more so, if you consider that Cro-Magnon parents smart enough to produce Guy aren't likely to stumble into a tar pit, unless they can't see the ground in front of them. They may well have died because they were walking at night and didn't see it in time.
Ugga's hairstyle changed after she, Gran, and Sandy escape the man eating flowers. In other words, she blossomed.
You really can tell time by putting your fingers between the sun and the horizon. One finger's width is equal to about 15 minutes. So the "Two-Kunckle Warning" is just the Croods' way of saying that there's only a half hour left till sunset.
In the first movie, Grug and Ugga express their love by touching foreheads. In the second movie, however, Eep and Guy repeatedly go in for an outright kiss. Naturally, as they're the ones who are most open to trying something new.
From what anthropogeny has told us, it's clear that kisses and hugs existed well before modern humans started developing. Many primates (and even non-primates like cats) have been known to wrap their paws around each other or make lip contact for several reasons. The question is not whether or not they existed at all, it's whether or not they existed as simple gestures of love or affection. So while Eep and Guy might not have revolutionized the concept, they still introduced the element of romance to it.
In this universe, the word "Dog" could easily have derived from "Douglass."
Every time Grug comes face to face with Chunky the Death Cat, he smacks or steps on Chunky's paw and Chunky swats back at him. Some Real Life cats and their owners enjoy play-swiping at one another's hands/paws as a game, foreshadowing how the two will eventually bond.
For all their aggressiveness in the initial egg-hunting sequence, none of the Croods even consider actually killing the trip gerbils or other animals that are competing with them. Early confirmation that, for all their brutishness, they're actually quite gentle people.
Grug's casual remark about how he "ate last week" sounds like exaggeration, either on Grug's part as to how long he's gone without food or on the writers' part about how food-deprived the Croods are. But later we see just how much food the cave family can scarf down in a single sitting, when they devour a huge bird's entire carcass (minus the normal human amount Guy eats) in a matter of minutes. So he literally could have gone that long without food, if the family's last substantial meal was large enough to let them gorge that way.
The art style that the opening is drawn in has a very rough texture... in other words, it's very crude.
Technically, the Croods were right when they referred to the stars as suns. The sun is a star, after all.
When Guy spots Grug crossing the ravine on his improvised flying contraption, he says that Grug is "riding the sun", even though it was actually just a stick on fire. But earlier in the movie, Grug's wife called fire 'a baby sun'. So in a way, Grug was riding the sun.
Additionally, the place the Croods are trying to reach (the territory beyond the Crood's earthquake-ridden home) is consistently referred to as "Tomorrow". Despite being the most resistant to Guy's fanciful ideas, Grug is the one who really did end up riding the sun to Tomorrow.
Gran is the eldest and the only one who wears lizard skin. Similarly, reptiles roamed the Earth way earlier than mammals. Her lizard skin symbolizes that she's been around for a really, really long time.
At the beginning of the second movie, Eep mentions that Grug didn't like Guy at first, but that eventually Guy won him over. While saying this, it shows Grug being mesmerized by Guy's fire, which obviously isn't how Grug actually accepted Guy. However, this is Eep's retelling, and she didn't see Grug and Guy's conversation in the tar pit. Instead, she shows Grug being won over the same way she was—by seeing Guy make fire.
The bird/barracuda hybrid that Guy and Eep "hunt" is evidently a male, as it takes an interest in the "egg"-laying female puppet and has different plumage from this (phony) prospective mate, as birds often do. However, it's also seen caring for an egg when encountered. In both real-world ratites (ostriches, emus, and other birds with a similar flightless build to the movie's), and real-world bony fishes which engage in nest-guarding behavior, it tends to be partially or solely the male that protects the eggs.
The scene where Eep gets annoyed at Grug for telling her the water was dangerous only to unquestioningly listen to Guy when he says the same thing is clearly Played for Laughs, but Eep would have a valid reason to think that way. She knows for a fact that Grug is extremely paranoid and has been treating every new thing as bad or dangerous, even when it clearly isn't. Guy, on the other hand, has been exploring and living in the outside world all his life, and thus knows what is actually dangerous and what isn't.
It makes sense in a way why Grug is so eager for his mother-in-law to kick the bucket (despite the fact that they honestly care for each other deep down). Grug is considered by both himself and the family as the family's main provider, and even with them scrabbling for every bite they could get and him often sacrificing his own food so his family can eat, they'd still have to go days without food sometimes (as evidenced by Grug saying he "ate last week"). With his elderly mother-in-law gone, he could potentially feed his wife and children even just the smallest bit more, maybe with something left over for himself.
The Croods live relatively close to the lush jungle they are eventually driven into by the great disaster. Such a place surely has tons of food, yet the Croods live in a barren desert a short distance away, where they were perpetually hungry. Why? Because the jungle also has a much higher concentration of predators, and Grug would rather keep his family out of danger in the desert.
In addition, if Eep had climbed all the way up the cliff and walked just a short distance, she would have easily found the jungle. Because of the Croods' insistence on avoiding danger and anything "new," they almost missed out on an oasis right next to them, one they'll never be able to reach again due to the great disaster.
Tar is among the stickiest natural substances in the world. Even managing to get out of it, how would Grug and Guy be able to keep their cloths? Guy in particular, as his pants are apparently loose enough to need a belt.
When in doubt, go with the Doylist explanation: To avoid having two cast members naked for the rest of the film and avoid breaking tension for them to procure new clothes from whatever animal was in killing range.
If Belt could find suitable materials for a she-Chunky Cat puppet and functional orchestra in their immediate vicinity in a matter of minutes, he could probably scrounge up some predator-stripped discarded animal skins without too much trouble.
Guy watched his family die a slow, horrific death. He saw each of them breathe their last breath as they sank in the tar pit, helpless to save them. And he was only a child at the time. Imagine the trauma and survivor's guilt he must suffer every day because he wasn't able to save them.
The second movie eases this by showing that Guy's parents told him to run and not look back before they were completely submerged. While it's still horrifying to think of a small child having to turn his back on his own parents, at least it's not as bad as watching them drown in tar....
How many animals died during "The End"? To say nothing of all those beautiful plants in the forests the family passed through, some of which were implied to be sentient. At least some of the animals could flee, but any plants that were endemic to the devastated areas effectively went extinct.
The Croods lost their neighbors one by one. It's Played for Laughs, but think about the friendships they formed and then lost in a blink of an eye.
When Guy first meet's Eep's family, he tells Eep to stay back because they're animals, even going so far as to ask her to close her eyes because he's going to have to kill them. This means Guy has encountered other cavemen before and things didn't go too well...
Of course, it's just as likely that Guy had heard stories about cave people from his parents, possibly as a warning not to engage them. Remember, he initially mistook Eep as a fellow Cro-Magman despite her animalistic movements, suggesting he'd never actually seen a cave person. The second movie seems to back this up, with the Bettermans knowing what cave people are but saying they thought the species was "extinct".
, you can easily see that there was no way to reach the jungle until their cave was destroyed. The very thing that kept them safe also prevented them from having an easier life.