Prolactin secretion in a brood parasite, the brown‐headed cowbird, Molothrus ater

Abstract

Blood samples were collected from a free‐living population of male and female brown‐headed cowbirds, Molothrus ater, during the breeding season and plasma levels of prolactin were measured by radioimmunoassay. Prolactin concentrations in both sexes increased significantly in mid to late May; peak levels were reached by June and were maintained throughout June and into July. Prolactin levels in juveniles were significantly lower than in adults but were still elevated. Prolactin has been implicated in parental care in several avian species; however, cowbirds are brood parasites with complete absence of parental behaviour, suggesting that prolactin must function in another capacity in cowbirds, or that cowbirds have developed target tissue insensitivity to this hormone.

Does prolactin mediate parental and life-history decisions in response to environmental conditions in birds? A review

“…This could explain why expression of parental behavior is not correlated with prolactin under all circumstances (e.g. Dufty et al, 1987). Supporting this idea, the effect of prolactin injection on parental behavior varies greatly between experienced and nonexperienced individuals in ring doves (Wang and Buntin, 1999).…”

“…and circulating prolactin levels can also change in respect to multiple factors that are not linked to parental behavior, such as day length, time of the day or metabolic needs (Meier et al, 1969;Hall et al, 1986;Dawson, 2006;Holberton et al, 2008). Therefore, observed changes in prolactin levels may not always be related to changes in the expression of parental cares, as suggested by Dufty et al (1987). Second, the influence of prolactin on parental behavior may not only depend on circulating prolactin levels but also on the location and density of prolactin receptors.…”

“…For instance, incubation starts in doves and pigeons before prolactin levels begin to rise (Silver, 1984) and prolactin levels are not correlated with parental provisioning or brood defense behavior in several studies (Ketterson et al, 1990;Silverin and Goldsmith, 1984;Kosztolanyi et al, 2012;Wojczulanis-Jakubas et al, 2013). Another striking example highlighted by Williams (2012) is the prolactin cycle of the brood parasitic brown-headed cowbird: Despite the absence of incubation behavior and parental provisioning, prolactin levels increase in this species in the same way as many other species that provide intense parental care (Dufty et al, 1987). Do all these examples mean that prolactin is not as tightly linked to parental behavior as previously thought?…”

Endocrine and neuroendocrine regulation of fathering behavior in birds

“…This second explanation is borne out by findings that in some species, seasonal rises in prolactin secretion appear to be photoperiodically driven (e.g., European starlings, Dawson and Goldsmith, 1983; White-crowned sparrow, Zonotrichia leucophrys, Hiatt et al, 1987; song sparrow, Melospiza melodia, Wingfield and Goldsmith, 1990), and can occur even in species that show no paternal care whatsoever (e.g., Brown-headed cowbird, Molothrus ater, Dufty et al, 1987). This seasonal pattern of prolactin secretion may be associated not with reproductive development or parental care, but rather with the onset of photorefractoriness (Dawson and Goldsmith, 1983;Dufty et al, 1987;Goldsmith and Nicholls, 1984). Changes in central nervous system sensitivity to hormones, rather than changes in the profile of circulating hormones themselves, may also explain why strictly non-paternal birds such as cowbirds still exhibit high prolactin titers during breeding.…”

“…The preoptic area (POA) of the hypothalamus is known to be the central site mediating the onset of parental care in mammals and birds (e.g., Buntin, 1996;Buntin et al, 2006;Slawski and Buntin, 1995). Differences in sensitivity of the POA to prolactin have been hypothesized to underlie the lack of association with prolactin and paternal behavior in cowbirds (Dufty et al, 1987), with cowbirds showing quantitatively lower levels of prolactin binding than parental species at this site (Ball, 1991). Whether such differences in tissue sensitivity exist across species showing a diversity of paternal care strategies, however, is poorly understood.…”