Experimental shift in hosts' acceptance threshold of inaccurate-mimic brood parasite eggs

Abstract

Hosts are expected to evolve resistance strategies that efficiently detect and resist exposure to virulent parasites and pathogens. When recognition is not error-proof, the acceptance threshold used by hosts to recognize parasites should be context dependent and become more restrictive with increasing predictability of parasitism. Here, we demonstrate that decisions of great reed warblers Acrocephalus arundinaceus to reject parasitism by the common cuckoo Cuculus canorus vary adaptively within a single egg-laying bout. Hosts typically accept one of their own eggs with experimentally added spots and the background colour left visible. In contrast, hosts reject such spotted eggs when individuals had been previously exposed to and rejected one of their own eggs whose background colour had been entirely masked. These results support patterns of adaptive modulation of antiparasitic strategies through shifts in the acceptance threshold of hosts and suggest a critical role for experience in the discrimination decisions between inaccurate-mimic parasite eggs and hosts' own eggs.

Interspecific egg rejection as ecological collateral damage from selection driven by conspecific brood parasitism

“…More specifically, why aren't all duck eggs rejected? Intermediate levels of egg rejection are not unique to our study, and they remain a puzzling feature of brood parasitehost coevolution in general, including interspecific brood parasitism (de la Colina et al, 2012;Hauber et al, 2006;Rothstein, 1990;Takasu, 1998). We now review several hypotheses for intermediate rejection and discuss their relevance to coots.…”

“…That all birds appear able to recognize eggs, but do not always reject them, indicates that the explanation for intermediate rejection rates lies in the factors that trigger egg rejection. The growing literature on context-dependent egg rejection, a characteristic that enables hosts to modulate the costs and benefits of egg rejection, indicates that the stimuli that trigger egg rejection can be numerous and complex (Davies et al, 1996;Hauber et al, 2006;Hoover and Robinson, 2007;Moskat and Hauber, 2007).…”

“…One possibility is that coots have a 'life boat' response during floods and waves, whereby they ensure that their own eggs (and any eggs they cannot distinguish from their own eggs) are actively maintained inside the nest bowl while they rapidly build up their nest to avoid inundation of their clutch, while any eggs that fall outside the host's egg recognition template are treated as inert objects and the nest is incidentally built over them. Alternatively, coots may only discriminate against duck eggs when the ecological benefits of correct rejection outweigh the costs of false acceptance (Davies et al, 1996;Hauber et al, 2006;Sherman et al, 1997). Because the benefits of rejecting the relatively benign eggs from blackheaded ducks are low (Lyon & Eadie, 2004), rejection may only pay under extreme conditions, such as when birds need to rapidly build up their nests to save their own eggs.…”

Experimental shifts in egg–nest contrasts do not alter egg rejection responses in an avian host–brood parasite system

“…b The relationship between egg-nest contrast, plotted as the mean JNDs between unmanipulated host nest lining However, egg rejection behaviours of hosts are also influenced by the physical and biotic properties of the nest environment. Some of these factors include the intensity (Langmore et al 2005 ) and composition (Honza et al 2011 ) of the light illuminating the nest (Honza et al 2014 ), the number and colour of the other eggs in the nest (Lang et al 2014 ;Moskát et al 2014a ;Yang et al 2014 ), variation in the arrangement of the whole clutch (Polaciková et al 2013 ; but see Hanley et al 2015 ), and the prior presence of parasitic eggs in the clutch (Hauber et al 2006 ;Moskát and Hauber 2007 ;Moskát et al 2014b ). Similarly, important factors include the date of clutch initiation by the host (de Mársico et al 2013 ), and the presence of brood parasitic adults near the nest or in the breeding habitat (Davies and Brooke 1988 ;Mosknes and Røskaft 1989;Bártolet al 2002 ).…”

Acceptance threshold theory can explain occurrence of homosexual behaviour

“…However, although the acceptance threshold theory has been tested in different contexts (e.g. [5,6]), there is currently no study that has investigated whether the occurrence of SSB depends on the costs of making mistakes and benefits of mating with a female. Indeed, in their recent review about SSB, Scharf & Martin [1] explicitly stated that the next research step should be predicting under which conditions males are expected to show SSB more or less often and to test these predictions experimentally.…”