Socially regulated estrogen in an eavesdropping brood parasite

Abstract Social regulation of reproductive hormones is a means by which conspecific males and females orchestrate successful reproductive efforts. There is variation, however, in the range of social cues that will initiate a hormone response in the receiver of social signals. We investigate whether social cues modify activity within the hypothalamic-pituitary-gonadal (HPG) axis and the specificity of this response in a social parasite that is known to eavesdrop on the communication signals of other species: the brown-headed cowbird ( Molothrus ater ). Brown-headed cowbirds are obligate brood parasites that do not build nests or care for their own young. Instead, obligate brood parasites always leave their eggs in the nest of a host species thereby receiving the benefits of parental care toward offspring without paying any of the costs. Thus, social parasites must coordinate their breeding attempts with conspecifics as well as potential heterospecific hosts and therefore, social parasites such as cowbirds rely on the communication signals of host species to help locate nests to parasitize during the breeding season. Here, we explore whether the vocal signals of potential host species can also be used as a social cue that modifies the HPG axis of female brown-headed cowbirds. Results reveal that both conspecific and heterospecific song-exposed females exhibit significantly greater circulating estradiol concentrations as compared to silence-exposed females. While conspecific song induces the greatest elevation in circulating estradiol, there is no significant difference in circulating estradiol levels in females exposed to either conspecific or heterospecific songs. This pattern suggests both song types are effective at evoking a reproductive physiological response. On the other hand, circulating progesterone concentrations did not differ among the song- and silence-exposed groups nor did the size of the female’s ovarian follicles. These results indicate that heterospecific vocal communication signals can effectively be used as a social cue that simultaneously provides necessary information regarding breeding status of hosts and modifies breeding condition of the eavesdropper. Highlights Brood parasites may coordinate reproductive attempts with potential hosts. Therefore, parasites eavesdrop on other species, which helps them find nests. Vocal signals from conspecific and heterospecifics elevate estrogen in parasites. Hearing potential host songs may enhance reproductive hormones in brood parasites.