The ghosts of parasitism past: lingering frontline anti-brood parasite defenses in a former host

Abstract

Coevolutionary arms races between brood parasites and hosts provide tractable systems for understanding antagonistic coevolution in nature; however, little is known about the fate of frontline antiparasite defences when the host ‘wins’ the coevolutionary arms race. By recreating bygone species-interactions, using artificial parasitism experiments, lingering defensive behaviors that evolved in the context of parasitism can be understood and may even be used to identify the unknown agent of parasitism past. Here we present the first study of this type by evaluating lingering “frontline” nest defences that have evolved to prevent egg laying in a former brood parasite host. The Australian reed warbler Acrocephalus australis, is currently not parasitized but is known to exhibit fine-tuned egg discrimination—a defensive behavior indicative of a past brood parasite-host arms race and common in closely related parasitized species. Here, using 3 D-printed models of adult brood parasites, we examined whether the Australian reed warbler also exhibits frontline defences to adult brood parasites, and whether we could use these defences to identify the warbler’s “ghost of parasitism past”. Our findings provide evidence that the Australian reed warbler readily engages in frontline defences that are considered adaptive specifically in the context of brood parasitism. However, individuals were unable to discriminate between adults of different brood parasite species at their nest. Overall, our results demonstrate that despite a relaxation in selection, defences against brood parasitism can be maintained across multiple stages of the host’s nesting cycle, and further suggest that, in accordance with previous findings, that learning may be important for fine-tuning frontline defence.

Can social interactions rescue “lost” behavioural defences in an allopatric cuckoo host?

“…However, in contrast to our results, alarm calls produced by yellow warblers that specifically warn of brood parasitic brown-headed cowbirds seem to have persisted similarly among sites in long-term allopatry and those close to parasitised ones (Kuehn et al, 2016), although differences in plasticity were not tested for. Similarly, some frontline defences are retained by Australian reed warblers, which are not hosts of its sympatric cuckoos (Chaumont et al, 2021).…”

“…Similarly, some frontline defences are retained by Australian reed warblers, which are not hosts of its sympatric cuckoos (Chaumont et al, 2021).…”