In between. A male hybrid flycatcher.

Beating Hybridization's Bad Rap

Mating with a member of another species is generally a bad idea. Cross a horse and a donkey, for example, and the offspring is sterile. But now, a study shows that some female birds can benefit by pairing with males of another species--as long as they sneak in some same-species matings on the sly.

The findings are based on 2 decades of research into Europe's pied flycatchers and collared flycatchers, two closely related species. Today, they interbreed rarely in two areas where their ranges overlap, the Swedish island of Gotland and the Czech Republic. Ben Sheldon of Oxford University, his student Thor Veen of Uppsala University, and researchers studying the birds in the Czech Republic decided to tease apart the costs and benefits of hybridization for female flycatchers.

When female collared flycatchers paired with male pied flycatchers, their offspring were somewhat less able to reproduce than the descendents of same-species pairs, the group found. But the females could compensate. First, breeding with a pied flycatcher occurred at a later time in the year, when chances of breeding success go up, possibly because food is more abundant. Through an unknown mechanism, the mixed pairs produced more male than female offspring, and males are more likely to survive. And the females secretly mated with males of their own species, such that half their young were pure collared flycatchers, cared for by the cuckolded pied flycatcher male.

All told, collared females can pass on more of their own genes at later dates in the breeding season by opting for pied males, the authors report in the 3 May issue of Nature--so long as they cheat on those males. The study sheds new light on the evolution of adultery in birds, Sheldon says. While previous studies have shown only very slight advantages to illicit matings, now "we have a situation where the advantages of this behavior can be huge."

The study boasts "huge sample sizes on something people usually dream of having a few cases of," says evolutionary biologist Dennis Hasselquist of Sweden's Lund University. And the outcomes "may alter the view of hybridization in animals as some form of mistake," comments Loeske Kruuk of the University of Edinburgh, "because it is now clear that we cannot take the consequences for fitness at face value."

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