What looks like the ultimate bad choice in romance–a mate from a different species–in some conditions may not be so dumb after all, according to a new analysis of flycatchers.
Two European species do face some poor consequences when they mix, acknowledges Ben C. Sheldon of the University of Oxford in England. Yet mismatched birds show quirks that reduce the damage, such as a good sex ratio of nestlings and an extra willingness to sneak off for a quick visit to a member of the right species.
For birds starting late in mating season, such patterns can make up for the lowered fertility of the mixed-species nest, Sheldon and his team report in the May 3 Nature. "It was quite amazing when you actually did the sums," he says. "An apparently highly maladaptive behavior turns out to be an adaptation."
He predicts that the flycatcher study will affect ideas about such evolutionary puzzles as how species split apart and why birds cuckold their mates.
"On the face of it, breeding with the wrong species is costly and extremely stupid," Sheldon says. Misalliances often spawn weaklings with low, if any, fertility.
Scientists rarely find mismatches in the wild. However, on the Swedish island of Gotland and in a swath across the Czech Republic, a few pied flycatchers, Ficedula hypoleuca, mingle with abundant collared flycatchers, Ficedula albicollis. In the two areas, where observers have studied flycatchers for 20 years, researchers noted some 200 two-species families. The team used DNA analysis and field records to analyze these birds and their descendants.
These mixed pairs bear offspring that seem healthy until it's time for them to breed. "Females are a complete waste, and males aren't very good, but they're better than nothing," Sheldon reports. The hybrid males can mate with either species to provide their parents with at least some grandchildren.
However, the mixed-species parents had some protection against a paucity of grandchildren. For example, their nestlings included fewer of the infertile daughters than the sexually functioning sons.
The researchers also found that collared females nesting with the other species made many trysts with males of their own kind. Nestlings resulting from those outings provided the main counterbalance to fertility loss, says Sheldon.
The researchers used grand-offspring, including step-grand-offspring, as the index of success. Overall, these countermeasures boosted the success of mismatched partners to roughly 80 percent of that of the same-species pairings.
Moreover, the gap essentially disappeared late in the season, says Sheldon. For same-species pairs, a 3-day delay in laying eggs results in 20 percent fewer fledglings. Yet fertility of mixed-species pairs peaks late in the season. With these conflicting trends, a female flycatcher arriving late could do as well, if not better, with a supposedly disastrous mate choice.
This study fits into what evolutionary biologist Michael L. Arnold of the University of Georgia in Athens calls a renaissance of interest in natural hybridization. Over the past half century, "it's just been sort of a blind spot for us," he says. However, now "we're finding that hybridization occurs just about everywhere we've really looked," Arnold says. And it's not always bad. The possibilities are "wondrously variable," he says.
As Sheldon puts it, "If we go back to look at other animals supposedly misbehaving, we may find more adaptations."
Ben C. Sheldon
Department of Zoology
University of Oxford
Oxford OX1 3PS
Hasselquist, D. 2001. Hybrid costs avoided. Nature 411(May 3):34.