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Endocrine regulation of egg rejection in an avian brood parasite host.

ABSTRACT: Parasite-host coevolution can lead to novel behavioural adaptations in hosts to resist parasitism. In avian obligate brood parasite and host systems, many host species have evolved diverse cognitive and behavioural traits to recognize and reject parasitic eggs. Our understanding of the evolution and ecology of these defences hinges on identifying the mechanisms that regulate them. We hypothesized that corticosterone, a hormone linked to stress response, vigilance and the suppression of parental behaviour, stimulates the rejection of foreign eggs by brood parasite hosts. We experimentally reduced circulating glucocorticoid levels with mitotane injections in American robins Turdus migratorius and found that the mitotane-treated birds rejected foreign eggs at a lower frequency compared to the sham-treated subjects. This is the first study to causally identify a potential mechanism of a widespread defence behaviour, and it is consistent with egg rejection being mediated by stress physiology.

SUBMITTER: Abolins-Abols M

PROVIDER: S-EPMC7336846 | biostudies-literature | 2020 Jun

REPOSITORIES: biostudies-literature

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Endocrine regulation of egg rejection in an avian brood parasite host.

Abolins-Abols Mikus M Hauber Mark E ME

Biology letters 20200624 6

Parasite-host coevolution can lead to novel behavioural adaptations in hosts to resist parasitism. In avian obligate brood parasite and host systems, many host species have evolved diverse cognitive and behavioural traits to recognize and reject parasitic eggs. Our understanding of the evolution and ecology of these defences hinges on identifying the mechanisms that regulate them. We hypothesized that corticosterone, a hormone linked to stress response, vigilance and the suppression of parental ...[more]

PMID: 32574535

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Project description:BackgroundAntagonistic species often interact via matching of phenotypes, and interactions between brood parasitic common cuckoos (Cuculus canorus) and their hosts constitute classic examples. The outcome of a parasitic event is often determined by the match between host and cuckoo eggs, giving rise to potentially strong associations between fitness and egg phenotype. Yet, empirical efforts aiming to document and understand the resulting evolutionary outcomes are in short supply.Methods/principal findingsWe used avian color space models to analyze patterns of egg color variation within and between the cuckoo and two closely related hosts, the nomadic brambling (Fringilla montifringilla) and the site fidelic chaffinch (F. coelebs). We found that there is pronounced opportunity for disruptive selection on brambling egg coloration. The corresponding cuckoo host race has evolved egg colors that maximize fitness in both sympatric and allopatric brambling populations. By contrast, the chaffinch has a more bimodal egg color distribution consistent with the evolutionary direction predicted for the brambling. Whereas the brambling and its cuckoo host race show little geographical variation in their egg color distributions, the chaffinch's distribution becomes increasingly dissimilar to the brambling's distribution towards the core area of the brambling cuckoo host race.ConclusionHigh rates of brambling gene flow is likely to cool down coevolutionary hot spots by cancelling out the selection imposed by a patchily distributed cuckoo host race, thereby promoting a matching equilibrium. By contrast, the site fidelic chaffinch is more likely to respond to selection from adapting cuckoos, resulting in a markedly more bimodal egg color distribution. The geographic variation in the chaffinch's egg color distribution could reflect a historical gradient in parasitism pressure. Finally, marked cuckoo egg polymorphisms are unlikely to evolve in these systems unless the hosts evolve even more exquisite egg recognition capabilities than currently possessed.

Project description:Abstract One of the most effective defenses of avian hosts against obligate brood parasites is the ejection of parasitic eggs from the nests. Despite the clear fitness benefits of this behavior, individuals within so‐called “egg‐rejecter” host species still show substantial variation in their propensity to eliminate foreign eggs from the nest. We argue that this variation can be further understood by studying the physiological mechanisms of host responses to brood parasitic egg stimuli: independent lines of research increasingly support the hypothesis that stress‐related physiological response to parasitic eggs may trigger egg rejection. The “stress‐mediated egg rejection” hypothesis requires that hosts activate the stress‐response when responding to parasitic egg stimuli. We tested this prediction by asking whether hosts showed differential stress response when exposed to host‐like (mimetic) or parasite‐like (non‐mimetic) eggs. We experimentally parasitized incubating American robins Turdus migratorius, a robust egg‐rejecter host to obligate brood parasitic brown‐headed cowbirds Molothrus ater, with mimetic or non‐mimetic model eggs. To assess the stress response, we measured the heart rate in incubating females immediately after experimental parasitism. We also measured plasma corticosterone and, in a subset of birds, used RNA‐sequencing to analyze the expression of proopiomelanocortin (POMC), a precursor of adrenocorticotropic hormone, 2 h after experimental parasitism. We found that egg type had no effect on heart rate. Two hours following experimental parasitism, plasma corticosterone did not differ between the differently‐colored model egg treatments or between rejecter and accepter females within the non‐mimetic treatment. However, females exposed to non‐mimetic eggs showed an upregulation of POMC gene expression (before FDR correction) in the pituitary compared with females treated with mimetic eggs. Our findings suggest that in an egg‐rejecter host species, non‐mimetic parasitic eggs may increase the activity of the stress‐related hypothalamic–pituitary–adrenal axis compared with mimetic eggs, although the temporal dynamics of this response are not yet understood. Stress‐related signaling has been shown to mediate anti‐parasitic behaviors in avian brood parasite hosts. Here we tested if brood parasite‐like model eggs activated the hypothalamic–pituitary–adrenal (HPA) axis and autonomic nervous system compared with host‐like model eggs. We show that, compared with experimental parasitism with mimetic eggs, parasitism with nonmimetic eggs does not affect blood corticosterone or heart rate, but that exposure to parasitic eggs may increase the expression of POMC gene, a precursor to ACTH, a central hormone in the HPA axis.

Dataset Information

Endocrine regulation of egg rejection in an avian brood parasite host.

Publications

Endocrine regulation of egg rejection in an avian brood parasite host.

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