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Come spring, bluegills set up spawning

Come spring, bluegills set up spawning beds – where the eggs are laid and protected. Central to spawning is what biologists call the parental male: a big bluegill that not only competes to reproduce with females, but also protects the eggs laid on the spawning bed.

Parental males clear the spawning beds of debris and guard them.

The bigger and badder the male, the better the location of the spawning bed. At the center of the colony, in the best position to fend off predators and other males, is the biggest and baddest bluegill of all.

“These males put all their energy into defending the nest and competing with the other male bluegills for the best females,” says Andrew Rypel, a research biologist with the Wisconsin Department of Natural Resources and bluegill expert. “All their energy goes into body growth. Being a parental male is a tough business, but his first goal is to attract a fantastic female mate. The female leaves after laying eggs, and the male has to guard the nest.” So far, this does not sound too outlandish: big males battle it out for the best females. The most fit and aggressive individuals are most attractive. The strongest pass on their genes. It’s a pretty standard wildlife narrative, right?

But here’s the wild card: Not all males participate in the festivities. As the jocks of the bluegill realm wage their battles, puny little specimens are watching on the sidelines. Biding their time.

The Little Fish with Very Big Gonads

“Bluegill males can have two completely different life histories,” says Rypel. “Some males opt out completely from growing big and guarding spawning beds. They become what we call sneaker males.”

Sneaker males are small. They do not look like they could win any battle. To an untrained eye, they may look like a different species.

Whereas the parental male puts all his energy into a big, strong body, the sneaker male puts all the energy into giant gonads.

“The sneaker male has stunted growth, but his testes are blown up like balloons,” says Rypel. “Whatever energy he acquires goes right into the testes.”

While the parental males are battling and protecting the spawning bed, the sneaker malesare roving around. And when the parental males are courting females, or fighting, the sneaker males, true to their name, sneak right onto the spawning bed.

“Because they have these huge testes, they have a lot of sperm,” says Rypel.

And so the sneaker male passes on his genes, too. It’s like that film where the quiet, wimpy guy wins out over the preening jock.

-Cool Green Science