“Parasites make up the majority of species on Earth. According to one estimate, parasites may outnumber free-living species four to one. In other words, the study of life is, for the most part, parasitology.”
– Carl Zimmer (Parasite Rex)
Given the abundance of parasites, it is surprising that not many studies have explored the relationship between hybridization and parasitism. At first glance, you might think that hybrids suffer more from parasite infections compared to “pure” species. Hybrid are often sterile and suffer from developmental problems. Their immune system might not be functioning properly, preventing them to fend off parasites. However, a study on hybrids between Red-crowned Parakeet (Cyanoramphus novaezelandiae) and Forbes’ Parakeet (C. forbesi) showed exactly the opposite: hybrids had significantly higher measures of immune function. Hybridization can increase genetic diversity, resulting in a more versatile immune system. Clearly, the relationship between hybrids and parasites is not straightforward. As with many biological questions, the answer depends on the study system.
On the Galapagos Islands, the introduced fly Philornis downsi is wreaking havoc in bird populations. Female flies lays their eggs in bird nests where the larvae feed of the blood and tissue of nestlings. Infection with Philornis often results in brood failure. This fly is considered the biggest risk factor for the extinction of two bird species: the Mangrove Finch (Camarhynchus heliobates) and Medium Tree Finch (C. pauper). The latter species hybridizes with Small Tree Finch (C. parvulus), providing the opportunity to explore how hybrids cope with parasites.
Katharina Peters and her colleagues studied the nests of Medium Tree Finches, Small Tree Finches and their hybrids on Floreana Island. Catching females turned out to be extremely difficult. They rarely descend from the nest and they don’t respond to sound recording. Therefore, the researchers focused on males and determined their genetic status. The analyses revealed that the number of Philornis parasites decreased with increasing genetic admixture. In other words, nests with hybrid males coped better with parasite infections.
This result begs the question: why do hybrids have less parasites in their nest? Perhaps the increased genetic diversity of hybrids results in a better immune system, enabling them fend off with parasite infections. Or maybe hybrids show particular behaviors that prevent parasites from colonizing the nest. A previous study showed that higher nests have more Philornis flies. Hybrids breed at intermediate heights compared to their parental species and might thus avoid the flies. Another possibility is that hybrids might show increased cleaning behavior, more efficiently removing parasites from their nestlings.
Whatever the mechanism, hybrids hold the key to combating the Philornis parasite. Paradoxically, hybridization might prevent extinction.
Kleindorfer S., Peters K.J., Hohl L., Sulloway F.J. (2016) Flight behaviour of an introduced parasite affects its Galápagos Island hosts: Philornis downsi and Darwin’s finches. In Biological invasions and animal behaviour (eds Weis J.S. & Sol D.), pp. 158-179. Cambridge, UK: Cambridge University Press.
Peters, K. J., Evans, C., Aguirre, J. D., & Kleindorfer, S. (2019). Genetic admixture predicts parasite intensity: evidence for increased hybrid performance in Darwin’s tree finches. Royal Society Open Science, 6(4):181616.
Tompkins, D.M., Mitchell, R.A., & Bryant, D.M. (2006). Hybridization increases measures of innate and cell‐mediated immunity in an endangered bird species. Journal of Animal Ecology, 75(2):559-564.
A big thank you to Katharina Peters for drawing my attention to this paper, which has been added to the Thraupidae page.