Host defences against avian brood parasitism: an endocrine perspective

Abstract

Host defences against avian brood parasites are the outcome of well-documented coevolutionary arms races, yet important questions about variation in hosts' antiparasitic response traits remain poorly understood. Why are certain defences employed by some species or individuals and not by others? Here, we propose that understanding variability in and the evolution of host defences can be facilitated by the study of the underlying physiological mechanisms. Specifically, because antiparasitic strategies involve behaviours that have been shown to be hormonally regulated in other contexts, we hypothesize that host responses to brood parasites are likely to be mediated by related endocrine mechanisms. We outline the hallmarks of the endocrine bases of parasite defence-related avian behaviours, review the current understanding of antiparasitic host tactics and propose testable hypotheses about the hormonal mechanisms that may mediate host defences. We consider these mechanisms in a life-history framework and discuss how endocrine factors may shape variation in host defences. By providing a hypothesis-driven mechanistic framework for defences against parasitism, this perspective should stimulate the study of their endocrine bases to enhance our understanding of the intricate arms races in avian host-parasite systems.

Glucocorticoids mediate egg rejection in a brood parasite host

“…Host adults show elevated baseline and stress-induced corticosterone levels in response to brood 135 parasitism [9,10]. Until now, the consequences of this endocrine response on host antiparasitic behaviours 136 have been unknown [8]. Here we experimentally investigated if corticosterone plays a role in mediating 137 host defences against brood parasites.…”

“…For example, a rise in glucocorticoid levels in response to a parasitic egg (or an adult brood 156 parasite) may activate an action pattern of discrimination, recognition, and rejection [6]. Another 157 possibility is that the effect of glucocorticoids on egg rejection is mediated by a general suppression of 158 affiliative maternal behaviour [8]. Corticosterone has been shown to have a suppressive effect on the 159 parental behaviour across different bird species [16,38; but see 39].…”

“…A handful of recent studies suggest that host responses to avian brood parasitism are, in part, 33 mediated by steroid hormones [8]. For example, host parents caring for a parasitic chick show higher 34 stress-induced corticosterone levels compared to parents caring for only their own young [9].…”

Shared transcriptional responses to con- and heterospecific behavioral antagonists in a wild songbird

“…Overall, the behavioral and gene expression data indicate that similar physiological pathways may be involved in the cognitive recognition and motor responses to distinct antagonistic threats, in this case conspecific and heterospecific intruders. Many host species of brood parasites have evolved aggressive anti-parasitic behaviors 7 , yet the cognitive and physiological mechanisms involved in this behavioral evolution remain undiscovered 10 . Our study suggests that some of proximate responses to conspecific intruders were co-opted in the evolution of anti-parasitic aggression towards female cowbirds.…”

“…What constitutes the physiological basis of anti-parasite responses remains largely unknown in avian host-parasite systems 10 . For example, recent work in wild-caught juvenile male red-winged blackbirds (Agelaius phoeniceus; hereafter: redwings) found no differences in immediate early gene (IEG) expression levels within the auditory forebrain in response to the calls of adult female brood parasites (brown-headed cowbirds Molothrus ater; hereafter: cowbirds) versus a harmless control species (mourning dove Zenaida macroura; hereafter: doves), whereas responses were stronger to conspecific adult female calls 11 .…”