Plasma concentrations of prolactin during the breeding cycle in the Cape gannet ( Sula capensis): A foot incubator

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Plasma prolactin was measured in the Cape gannet ( Sula capensis) which differs from most other avian species in that although both sexes share equally in incubation duties, neither sex possesses an incubation patch but rather incubates a single egg with its foot webbing. Five main categories of birds were studied: (1) nonbreeders, (2) unmated birds which were nevertheless defending a nest site, (3) recently mated birds, (4) incubators, and (5) brooders with nestlings. Prolactin titers in the plasma were 8–9 ng/ml in nonbreeding gannets, increasing to 17.7 ± 1.6 and 16.7 ± 1.2 ng/ml in incubating females and males, respectively. Plasma concentrations of prolactin progressively increased in nonbreeding gannets from 5.5 ± 0.4 ng/ml in 1-year-old birds to 8.7 ± 1.6 ng/ml in birds defending a nest site but without an egg. Blood samples taken from both incubators and nonincubators every 3 hr over 27 hr did not reveal any significant daily cycling in prolactin titers. Plasma osmolarity did not vary significantly between nonincubators and incubators, being 300.1 ± 3.2 and 298.2 ± 3.0 mOsm/kg, respectively. No positive correlation was found between possible osmotic stress during incubating and plasma concentrations of prolactin. Plasma concentrations of luteinizing hormone and testosterone increased while prolactin decreased, significantly, 24 hr after clutch loss. The putative inhibition of the pituitary-gonadal axis by prolactin is discussed together with other possible roles of prolactin during the breeding season in gannets.