Extra-pair copulations might be a suitable strategy to circumvent the fitness costs of interbreeding.
Hybridization can be costly. When a bird pairs up with a partner from another species, their offspring can suffer from numerous complications. In extreme cases, the young hybrid birds might be sterile or show genetic defects. Or the hybrids might look or behave so differently that they are not able to attract a partner. From an evolutionary point of view, these hybrids are a dead-end and their parents get charged a significant fitness cost.
But there might be a way to avoid these fitness costs: when your heterospecific (i.e. from another species) partner is out foraging, why not pay a visit to your conspecific (i.e. from the same species) neighbor? These so-called extra-pair copulations can be a strategy to produce “pure” offspring despite being paired with a partner from another species. A recent study in the Journal of Evolutionary Biology tested this idea in a well-studied crow hybrid zone.
The hypothesis that birds avoid the fitness costs of hybridization through extra-pair copulations leads to a clear prediction: there should be more extra-pair copulations by birds from hybrid nests compared to purebred nests. Ulrich Knief and his colleagues explored this prediction in the hybrid zone between carrion crows (Corvus corone) and hooded crows (C. cornix). Previous work on this hybrid zone revealed that mate choice is based on plumage coloration and that hybrids (with aberrant plumage patterns) suffer some reduction in fitness. This provides the ideal setting to test a possible role of extra-pair copulations.
Moreover, the genetic basis of these plumage patterns have been unraveled (see this blog post). A region on chromosome 18 and a locus on chromosome 1 explain the variation in plumage color in these crows. The researchers used this information – specifically the region on chromosome 18 – to assign individuals to different hybrid classes (purebred individuals, F1-hybrids, F2-hybrids, backcrosses, etc.). Another set of molecular markers was applied to calculate the kinship between juveniles from the same nest. Do they have the same parents or not?
The results revealed a very low incidence of extra-pair copulations. Only three nests (out of 43) contained extra-pair young, which amounts to an extra-pair nest rate of just 7 percent. In total, there were only four extra-pair young (ca. 3 percent). Statistical tests did not uncover significant differences in extra-pair copulations between pure and hybrid nests.
It could be that the costs for hybridization are so low that there is no need to avoid it. Indeed, the crow hybrids are fertile and often manage to find a partner. Similar findings have been reports in other avian systems with low fitness costs for hybrids, such as chickadees and warblers. There is, however, one case where birds do engage in extra-pair copulations to circumvent hybridization, namely Ficedula flycatchers. These small passerines have to cope with high fitness costs when interbreeding: hybrid males suffer a fitness reduction in mate choice and hybrid females are sterile. Here, scientists found patterns in support of the extra-pair copulation hypothesis: 14.5 percent of the birds from pure nests engaged extra-pair adventures while 59 percent of the birds from hybrid nests had paid a visit to their neighbors.
Crows do show extra-pair copulations, but the fitness costs for hybrids are currently too low to culminate in strong selection for this strategy. Once these fitness costs start increasing, this strategy might become more common. If so, you will read it here!
Knief, U., Bossu, C. M., & Wolf, J. B. (2020). Extra‐pair paternity as a strategy to reduce the costs of heterospecific reproduction? Insights from the crow hybrid zone. Journal of Evolutionary Biology, 33(5), 727-733.
This paper has been added to the Corvidae page.