turtles stack in order to receive sunlight and warmth. If there is no space, then the turtle will climb onto the back of an already basking turtle.
Turtles need UV light and heat and will do whatever it takes to receive this. However, there are other reasons for their odd behavior.
Turtles Stack To Deter Potential Predators
Other turtle breeders and enthusiasts believe that turtles stack to appear less vulnerable to predators. In the wild, size matters. Prey that is much larger deters predators.
A group of turtles stacked on top of one another may appear more intimidating and in theory would scare off potential predators
Turtles Stack To Display Dominance
Many turtle keepers have noticed that usually, the more dominant turtles are usually closer to the top of these turtle piles.
In this case, it can be seen that the more dominant turtle expresses this dominance by trying to receive more light and warmth than the others.
bamiller3 on September 23rd, 2021 at 12:29 UTC »
It’s turtles all the way down.
mylopolis on September 23rd, 2021 at 12:42 UTC »
Yurtle will rise again!
Spartan2470 on September 23rd, 2021 at 13:26 UTC »
Here are higher quality and less cropped versions of these images.
As for why they did this, according to here: