I think that the reason is intuitive.

You can equate a bluegills big eartab and bright colors to a whitetail deer's rack. It's a demonstration of genetic fitness. It tells the female "Hey, look at me, I've got resources to spare. My genes are so good, and I'm so good at foraging that I can commit some of my extra energy into a pretty display. Mate with me and you'll have babies that have the same abundance of resources, and your genes will be carried on forever".

A smaller male bluegill doesn't have what it takes yet to make a nice display, or be a good fighter for nest space. Even if he finds a nice nest, the female probably won't choose him anyway because of the way he looks. Consequently he's going to take a chance, by not reproducing this year, and spend his time foraging heavily so he can have a better chance to pass on his genes next year.


Holding a redear sunfish is like running with scissors.