Watch out for those sneaky males!

A courting midshipman fish with his mate and a small sneaker male
Picture from the website of Dr. Andrew Bass at Cornell University

As I stated in my last post many animal species are socially monogamous but genetic analyses show that some level "cheating" occurs resulting some percentage of the offspring having a different genetic father.  The vast majority of fish species have external fertilization so if a male can get close enough he can release sperm and fertilize the eggs. Some, usually young or sub-dominant, males called sneaker males use such opportunistic fertilization opportunities as their primary mating strategy!  These males try to look as unobtrusive and non-threatening as possible and will often take on female coloration so that the dominant male partner won't notice him and try to chase him away.  Dr. Andrew Bass at Cornell University has done extensive research on the midshipman fish Porichthys notatus.  In this species males develop into one of two types: one type of male builds a nice cozy nest and lures a female in with his lovely (to female midshipman fish ears) loud humming vocalization.  This guy is really works hard to woo the girl!  The sneaker male doesn't bother with such niceties, he just rushes into the nest of a pair and attempts to fertilize the eggs.  However, this strategy does come with some costs, sneaker males must release more sperm to get a smaller percentage of eggs fertilized successfully and thus have much larger testes than courting males.  In this case a subset of males have found that forgoing courtship and "stealing" fertilizations is a viable alternative mating strategy.