The evolution of host specialisation in avian brood parasites

Abstract

Traditional ecological theory predicts that specialisation can promote speciation; hence, recently derived species are specialists. However, an alternative view is that new species have broad niches, which become narrower and specialised over time. Here, we test these hypotheses using avian brood parasites and three different measures of host specialisation. Brood parasites provide an ideal system in which to investigate the evolution of specialisation, because some exploit more than 40 host species and others specialise on only one. We find that young brood parasite species are smaller and specialise on a narrower range of host sizes, as expected, if specialisation is linked with the generation of new species. Moreover, we show that highly virulent parasites are more specialised, supporting findings in other host-parasite systems. Finally, we demonstrate that different measures of specialisation can lead to different conclusions, and specialisation indices should be designed taking into account the biology of each system.

Avoidance of host resistance in the oviposition-site preferences of rose bitterling

“…Most host-parasite relationships incur significant costs to both partners and result in a rapid coevolutionary arms race when an adaptation in one partner is followed by reciprocal counter-adaptations in the other (Dawkins and Krebs 1979). This process is dynamic and at a particular evolutionary point, hosts may be better adapted to defend themselves from the parasite or, alternatively, the parasite can be better adapted to exploit its host (Rothstein and Robinson 1998;Sorensen et al 2004;Medina and Langmore 2016). The parasite specialization scenario considers that the present coevolutionary state is primarily resolved by traits evolved by the parasite, with parasite adaptations currently overcoming the defences evolved by the host.…”

“…This situation produces variable host-parasite associations across geographic regions, leading to local differences in parasite specialization. An example is the European cuckoo (Cuculus canorus L.) where different populations specialize on different locally accessible host species (Medina and Langmore 2016).…”

“…In other host-parasite systems, however, parasites may coevolve with a single host species across their entire range. Parasite specialization at the species level requires generalised adaptations to exploit a host species and may be favoured when either parasite or host species disperse over large distances (Medina and Langmore 2016). In African indigobirds (Vidua spp.…”

Bayesian inference supports the host selection hypothesis in explaining adaptive host specificity by European bitterling

“…Mussels have evolved defences against bitterling parasitism, primarily by ejecting their eggs and developing embryos (Reichard et al 2006 , 2009 , 2012 , 2015 ), but also by rapidly closing their siphons to prevent bitterling oviposition and by diverting the bitterling ovipositor into their mantle cavity rather than their gills (Reichard et al 2010 ). Mussel body size appears not to affect bitterling oviposition preference or quality as a host (Smith et al 2004 ), though in avian brood parasites this trait is a strong predictor of host specificity (Medina and Langmore 2016 ). There is good evidence for coevolution between bitterling and host mussels across their respective distributions.…”

Diet specialization and brood parasitism in cuckoo species

“…Many cuckoo species are characterized by a reproductive strategy known as brood parasitism (Davies, 2000). Brood parasitic species lay their eggs in the nests of other birds, sparing themselves the expense of rearing their own young (Erritzøe et al, 2012; Medina, Langmore, & Norris, 2016). Parasitism strongly influences the evolution of life histories of species (Krüger, 2007; Møller, 1997).…”