No one could accuse a seahorse of being a hands-off father. That's because males are the ones that carry the young. Now findings published online this week by the Proceedings of the National Academy of Sciences suggest that male pregnancy not only takes the load off female seahorses, it can also drive the development of new species.

Prevailing theory holds that new species arise primarily because geographic barriers halt the flow of genes between different populations. But a number of recent theoretical studies have suggested that so-called sympatric speciation can occur, in which different populations originate in one geographical area, but do not interbreed. In the new work, Adam G. Jones of the Georgia Institute and his colleagues studied seahorses off the coast of Perth, Australia, in which the female deposits her eggs in a male's brood pouch and he fertilizes and carries the eggs until they hatch. Using genetic analyses the researchers confirmed that the creatures tend to choose mates of a similar size (a selection process known as assortative mating). This way, neither female eggs nor male pouch space is wasted. Notes Jones, "in seahorses assortative mating appears to be a consequence of male pregnancy and monogamy."

The researchers then devised a computer model to test whether this mating regime could lead to reproductive isolation and subsequent speciation. They determined that if environmental conditions favor either very small or very large body sizes as opposed to intermediate ones, new species may arise in just tens or hundreds of generations as a result of assortative mating. Male pregnancy, the authors thus conclude, "represents an unusual form of parental care with extraordinary evolutionary consequences."