Why are sea turtle eggs round?

Blog Post By: Karoline, Sea Turtle Intern

           Hello! My name is Karoline, and I am one of the sea turtle interns this summer. Before coming to Edisto, I had only ever seen one sea turtle, and didn’t know much about them. I’ve only been here a little over a week, but I have already learned so much about sea turtles. With this new information comes even more in-depth questions that I need answers to! One question I will always have when I learn a new sea turtle fact is: why? This week, something I have been wondering about is turtle eggs. As anyone who has seen turtle eggs before knows, they look like ping pong balls. So, my egg question for this week is: why are they shaped like ping pong balls?
         Basically: The reasoning is up for debate…but don’t worry, there are some fascinating hypotheses out there!
              The main ideas on turtle shape come from bird research. Birds with round eggs tend to be weak fliers, and strong fliers have longer eggs (making the eggs have more volume without needing to be wider). Even though turtles don’t fly (that we know of…), it’s interesting to note that weak flyers have round eggs and sea turtles do as well. Another piece of data that has been found about the shape of bird eggs is what some people call the “rolling factor”. It is exactly what you would think: the rounder the egg, the more intense the “rolling factor”. So, birds who nest on the face of a cliff would need a less round egg shape so that the eggs don’t roll off the cliff! Turtles lay eggs in egg chambers (the eggs are under the sand), so the “rolling factor” doesn’t apply, meaning that the eggs can be perfectly round without the fear of them rolling away affecting the evolution of their shape. Some relevant hypotheses I found on the egg shape of birds that seem like great ideas to apply to turtles are: conservation of warmth, space, and calcium. If you have ever done or seen a sea turtle egg relocation, you know that we have to be careful because the eggs are tightly packed together. A sphere seems to be the optimum shape for tight packing (versus a more conical shape). I would assume that tight egg packing inside the turtle is extremely important, as the turtle needs to have high reproduction rates since chances of survival are so low. Not only would tight packing be good for conserving space in the turtle, but maybe it would be optimal for the eggs to share warmth as they are close together. And, having round eggs would mean less shell surface area, which conserves the calcium used to make the eggs.
Overall, there are some great hypotheses out there, but we definitely need some more turtle specific research on egg shape. I volunteer!