Yesterday I had a Skype chat with Ohio State University biologist Tim Berra, who is down under in Darwin, giving talks about the town's namesake and dodging the crocs in search of elusive nurseryfish. These river-dwellers are noted for the way the males carry eggs around on a hook-like appendage sticking out of their heads. Berra has been making trips to Darwin since 2001, navigating the Adelaide River in a 15-foot aluminum boat, which he says has more than a few crocodile bite marks.

In addition to hunting exotic fish, Berra writes about evolution. Last spring he sent me a copy of his latest book, Charles Darwin: The Concise Story of an Extraordinary Man. It's just 100 pages but loaded with fascinating details about Darwin's life and science, as well as the impact of his work. It's also full of illustrations including one that explains why Darwin was so fascinated by pigeons.

Darwin the town is in the tropical North coast of Australia, where it's now sliding from the dry season into the wet. Days are over 90 degrees, he said, but he likes it hot. So far he's taken three boat trips and netted 16 male and 4 female nurseryfish.

The Australians like to take tourists out in boats and hold up big pieces of meat on sticks to rile up the 16-foot crocodiles, allowing for good photo-ops. Those boats tend to navigate the same waters where Berra is seeking nurseryfish, which can create less than ideal conditions, he said.

On this trip, he wants to start an isotope analysis to figure out whether these fish spend any time in salt water, and a DNA analysis to see where the fish fits into the evolutionary tree of life.

Why nurseryfish? Because so little is known about it, said Berra. Until he took them up in 2001, the last paper written on these fish came out in 1913. And they only exist on Northern Australia and New Guinea. The extraordinary way the males carry the eggs is not well-understood, he said. It seems likely that it's protective, helping the eggs survive in these waters, which are subject to powerful 26-foot tides.

Nobody is sure how this trait evolved, or how the eggs get on the male's head. The fish are hard to keep alive in captivity so no one has ever been able see them spawn. Berra has one hypothesis – an act that I can't describe without the running the risk that I'll get in trouble with editors.

It's possible that the egg-hooks evolved not just for egg-protection but for showing off to females. I learned from reporting my sex column/book that in many fish species, females prefer males that look like good fathers. And so males show off their parenting abilities, or fake them. This passage is based on an interview with ichthyologist Mark Sabaj of the Academy of Natural Sciences:

In some species, such as sticklebacks, males will steal eggs from other males. In one type of minnow, says Sabaj, a male will oust a rival from his nest, treating most of the other guy's eggs as caviar but leaving behind just enough to fool a female into thinking he's a nice single dad.Another popular tactic is fakery. This, Sabaj suspects, might explain one weird-looking catfish he's seen in various South American waters. Called a bristlenose pleco, the female looks normal enough but the male sprouts dozens of wormlike tentacles from his head.

Could the tentacles have evolved because females started mistaking them for fish larvae? A similar tactic almost worked for Hugh Grant's character when he borrowed a friend's child to help him score in the 2002 film, About a Boy. Other fish display fleshy yellow knobs or spots on their fins that look like eggs….So why not larvae? And the male bristlenose catfish is under pressure to look desirable since he has to be chosen by a female. Males stake out cavities in rocks and wait like eager young girls at a middle school dance as the females swim by and inspect them. The females look for males tending larvae, or so they think.

Once she makes her choice, a female enters his cavity, lays eggs all over the walls, and leaves. This may not sound like hot sex by our standards, but to them, apparently, this is as good as it gets. For all his trouble, once the male fertilizes the eggs and becomes a real father, he ends up with all the parenting and housework.

I had some trouble finding a good image of a nurseryfish with a full egg hook. I'm waiting for Dr. Berra to guide me to one. In the meantime, I found this nice picture of him (picture 2), giving a hug to a large unknown fish.