Sexual Inequality – an Evolutionary View

All of our wrangling on child care policy, sexism and morning after pills would turn upside down if we humans had evolved the mating strategy of the nurseryfish. No one knows exactly what sex between nurseryfish entails, but when they're finished, the male leaves with fertilized eggs stuck on an appendage that grows out of the top of his head.

He protects the eggs while the female swims off for a life of self-actualization, adventure, and further trysts. Nurseryfish are among a few species – including seahorses – where the familiar sexual equation seems reversed.

But why do we humans see the females as the usual caretakers of offspring – shouldering most of the burden of reproduction? The answer goes back to the very definition of male and female. In everything from rhinos to rhododendrons, the sexes are defined by the relative sizes of the sex cells they bring to the reproductive table. The female is defined as the supplier of an egg, which is by definition bigger than sperm and full of all the cellular equipment needed to start a new life. The male brings the sperm, which is small and spare and carries just his genetic material and little else.

It's not at all obvious why it has to be that way, since sex is all about merging and exchange of genes, and can be done perfectly well with two equal and opposite "mating types" rather than two sexes who share unequally in the burden of reproduction.

A more egalitarian situation probably predated the evolution of male and female sexes, said biologist James Umen of Donald Danforth Plant Science Center in St. Louis. He is looking into the issue by studying two closely related species of algae, one which reproduces using sperm and egg, while the other combines two equal-sized cells – a practice known as isogamy.
Organisms that practice isogamy are neither male nor female but divide into "mating types", labeled plus and minus,or something similar.

This was probably our ancestral state, said Umen, but things changed when organisms became more complex, going from single cells to many cooperating cells. When multi-celled organisms reproduce, they need to get their offspring off to a proper start, which means one parent or the other has to contribute more than just one cells-worth of material.

It would be nice if both parents could go 50/50, but that situation tends to fall apart thanks to something appropriately called disruptive selection. That begins when some troublemakers start gaming the system by producing increasing numbers of small sex cells (proto-sperm) in order to up their chances of successful reproduction.  Others respond to these proto-sperm not by competing with them for numbers, but by taking up the slack, producing larger, more amply-supplied sex cells (proto-eggs).

Anyone who produces middle-sized sex cells loses out, Umen said, because they can't compete with sperm for abundance and speed, and they're too small to make a successful progeny if they merge with them. They can't beat the males and they can't join them. And so once disruptive selection starts, the sperm get smaller and the eggs get bigger and all the inequality and injustice that goes with having two sexes follows.

If it's any consolation, the species of algae with the more egalitarian isogamous systems are stuck in a state of single-cell simplicity, while the more complex, multicellular species use sperm and eggs. So it's possible that the male/female divide is the price we pay for being more complex than pond scum.

That doesn't mean that males always get to mate and then leave when the fun is over. In some species the males contribute as much parental care as females. In some penguins, it's the male who incubates the eggs over the frigid Antarctic winter.

Among fish, most species just leave their fertilized eggs to fend for themselves. In others, the females may provide care and in others that job goes to the male. Some male fish carry their fertilized eggs in their mouths. Others build nests in which the females deposit eggs, after which the males stay and tend to them. And then there's the nurseryfish, in which the male carries eggs on a hook-like appendage that grows from the top of his head.

"Forehead brooding is just one part of this incredibly interesting spectrum of parental care among fishes," said Ohio State University evolutionary biologist Tim Berra, who has been studying nurseryfish for the last decade. "It's the only fish we know that has this system." 
Profound questions remain about nurseryfish, he said, such as how the forehead brooding system evolved, and how the eggs get on the hook during nurseryfish sex. Nurseryfish have never been seen  mating in captivity.

And in the wild, they live along the North Coast of Australia in waters that are subject to ferocious tides and also happen to be teeming with large crocodiles. Last fall, Berra was stalked by a crocodile that was longer than his 15-foot aluminum boat. The croc eventually got caught in the gill net that Berra was using to capture nurseryfish. He and the crocodile escaped alive (crocodiles can drown if caught in a fish net.) The net, however, was destroyed. The incident made the front page of the local paper with the headline, "5M[5 meter] Croc Shock for Professor".

Berra said that DNA tests reveal the closest relatives to the nurseryfish are the cardinalfishes – a species  in which the male carries the fertilized eggs around in his mouth. It's not an easy system for the males, who have to forego food while brooding, lest they swallow their genetic legacy. So it would be like being pregnant and on a diet at the same time. But then, evolution doesn't care what's fair – it only favors what works.

Contact Faye Flam at 215-854-4977, fflam@phillynews.com, or @fayeflam on Twitter. Read her blog at philly.com/evolution.