Reproduction And Immune Homeostasis In A Long-Lived Seabird, The Nazca Booby (Sula granti)
The evolution of longevity requires that the marginal investment in selfmaintenance at the expense of reproductive effort is favored by realizing a longer reproductive lifespan. This can occur when extrinsic mortality factors (weather, predators, etc.) are less important than intrinsic mortality factors, such as the physiological cost of reproduction. Long-lived pelagic seabirds have low annual reproductive output and prolonged offspring growth periods that are thought to have evolved to accommodate marine resource variability. The life-history theory of senescence predicts that these same taxa should minimize per diem reproductive costs and shift effects of resource variability to the offspring. To address this prediction, we measured parental effort, offspring growth, and one aspect of self-maintenance (serum immunoglobulin G concentration [IgG]) in a long-lived pelagic seabird, the Nazca Booby (Sula granti). We collected data on 38 families in the 2002-2003 breeding season on Isla Española, Galapagos Islands, Ecuador. Offspring body-mass growth showed variable trajectories, but a variable nestling period allowed similar (sex-specific) fledging mass to be attained. Growth of two structural traits was most variable when the traits were growing most rapidly, but again attained sex-specific targets at fledging. Offspring [IgG] showed marked inter-individual variation, but the ontogeny of [IgG] was unrelated to morphological growth. Mothers spent more time at sea than fathers, and both parents spent more time at sea for offspring of the larger (female) sex at the time of peak body mass. Foraging effort did not show consistent inter-individual variation but was correlated between pair members. Sex-specific body mass of the parents showed consistent inter-individual variation as it declined across the nestling period, with a greater decline in parents raising daughters. In parents, [IgG] was stable across the nestling period and was correlated among family members.
The plasticity of offspring growth and the consistency of self-maintenance of the parents accord with the predictions of the life-history theory of senescence. To our knowledge, this is the first study to use a longitudinal analysis to assess intra- and inter-individual variation in parental effort, offspring growth, and a measure of immune-mediated self-maintenance in a wild vertebrate population. Received 7 December 2006, accepted 6 July 2007.