Research on brood parasites usually focuses on the evolutionary arms race between host and parasite, but findings published this week in Nature highlight the importance of a broader perspective. The paper presents seemingly paradoxical evidence that even when the parasite imposes a minimal cost, it still isn't tolerated by the host. The reason lies in an intraspecific battle being fought within the host species: the parasite is simply caught in the crossfire.

The black-headed duck Heteronetta atricapilla, a little-known South American species, is the only obligate brood parasite whose chicks require no more parental care than incubation as eggs: within a day of hatching, they leave the host nest and set off for independent life. Despite the apparently minimal costs of parasitism—limited to incubating an extra egg—the two coot species that play host to over 80% of the duck's eggs frequently reject them from their nests.

However, Bruce Lyon and John Eadie, researchers at the University of California's Davis and Santa Cruz campuses, respectively, discovered another layer of brood parasitism, operating at the intraspecific level: both coot host species also lay eggs in conspecific's nests. "Most host–parasite interactions focus solely on interspecific interactions," Lyon told The Scientist. "Our study indicates that some of these behaviors, such as the rejection of duck eggs by coots, might in fact stem from a within-species evolutionary arms race."

Arnon Lotem, at Tel Aviv University, thinks that the study has important implications for the future. "[This study shows] that there are probably many ways by which host–parasite co-evolution can develop," he said. "The conventional view is of a one-on-one (i.e., host–parasite) arms race, but Lyon and Eadie have found a more complex system, which leads to different outcomes."

Lotem, who was not involved in the study, added that the findings lend empirical support to a previously unsubstantiated conjecture. "The theoretical possibility that egg rejection might evolve primarily as a mechanism against conspecific parasitism and subsequently work against a specialist brood parasite has been considered in the past, but the discovery of a system in which this is really the case is completely novel," he said.

Graeme Ruxton, at the University of Glasgow, agrees. "There has been a historical tendency for researchers to focus on either intraspecific or interspecific brood parasitism, but this study clearly demonstrates that the two are intertwined. The findings are likely to trigger a rush among people studying interspecific parasitism to check for intraspecific brood parasitism within their host species," he said.

But the study also raises questions about the costs of parasitism, especially in cases where the parasite is precocial. "The authors convincingly demonstrate 'a lack of detectable cost of parasitism to the hosts.' If this is so, the high prevalence of intraspecific parasitism seems paradoxical," Ruxton, who was not involved in the study, pointed out. "If this is so, what is the evolutionary pressure driving one female to lay some of her eggs in her neighbor's nest? Wouldn't she do just as well, and save a bit of hassle, just to incubate them all herself?"

Lyon hopes that future work will address these questions. "One can never be sure that one has measured all possible costs. For example, we didn't attempt to measure the costs of incubation," he said, although in a closely related coot species, they appear to be minimal. Nevertheless, he said, "Failure to detect costs does not prove they don't exist."

Further studies are also needed to understand why not all duck eggs are rejected, even though they are clearly distinguishable from coot eggs. "The nice twist in Lyon and Eadie's paper is that they also show how well developed egg recognition in the host prevents the initiation of egg mimicry evolution in the parasite," said Lotem. "It is this inability or unwillingness of the host to reject all parasitic eggs that makes this parasite–host system stable."