Glucocorticoids mediate egg rejection in a brood parasite host

Abstract

10Avian brood parasites and their hosts are engaged in a coevolutionary battle that can result in the 11 evolution of sophisticated trickery by parasites and novel defence behaviours in hosts. Despite the clear 12 evolutionary and ecological significance of host behaviour, however, we know very little about the 13 mechanisms that regulate host defences, which limits our understanding of both inter-and intraspecific 14 variation in host responses to parasitism. Here we tested whether corticosterone, a hormone known to be 15 upregulated in hosts exposed to parasitism, also mediates one of the most frequent host defencesthe 16 rejection of foreign eggs. We experimentally reduced corticosterone levels in free-living brood parasite 17 hosts, American robins Turdus migratorius, using mitotane and found that the likelihood of model egg 18rejection was significantly lower in the mitotane-treated birds relative to the sham-treated birds. These 19 results demonstrate a causal link between glucocorticoids and egg rejection in hosts of avian brood 20 parasites, but the physiological and sensory-cognitive pathways that regulate this effect remain unknown. 21 22

Inter-Individual Variation in Anti-Parasitic Egg Rejection Behavior: A Test of the Maternal Investment Hypothesis

“…In a recent observational study, where female robins were captured, aged, weighed, and assessed for circulating glucocorticoid levels, birds with higher body mass and higher corticosterone plasma concentrations, but not with greater age, were less likely to reject deep blue model eggs ( Abolins-Abols and Hauber 2020 ). Nonetheless, the causal role of these factors still remains to be examined in robins and in most other rejecter host species of avian brood parasites, for example, by manipulating the circulating hormone levels of the incubating females experimentally ( Abolins-Abols and Hauber 2019 ).…”