Differential body condition regulation by males and females in response to experimental manipulations of brood size and parental effort in the blue-footed booby

1. In long-lived species, such as seabirds, the allocation of resources between self-maintenance and reproduction is of particular interest because only a small reduction in adult survival may have a large negative effect on lifetime reproductive success. There is much debate about whether seabirds have a fixed or flexible level of investment in their current reproduction, and it has been proposed that parents can regulate the risk of an increase in mortality under the control of a mass threshold. 2. Body mass change as response to experimental manipulations of parental effort was examined in blue-footed boobies (Sula nebouxii Mine-Edwards), a sexually size-dimorphic seabird, with females approximately 31% heavier than males. 3. First, paternal effort was manipulated by trimming the flight feathers and thereby handicapping males during the chick-rearing period. Mass remained stable in handicapped males, while there was a reduction of female body mass as response to the handicapped partner. The handicapping of males had a significant effect on chick mass. 4. Secondly, an experiment was made by enlarging and decreasing broods of two chicks to three and one during the first week after hatching. Body mass of females, but not males, was lower in enlarged broods, and body mass of males, but not females, was higher in the reduced broods when the chicks were 45 days old. Chick body masses were lower among enlarged broods than among reduced and control broods. 5. Overall, these and other results revealed that blue-footed boobies have a sex-specific body mass regulation. Males had a fixed body mass at the end of experiments; they were working at some physiological maximum or were unwilling to pay the cost in terms of future survival, when the effort demanded was increased. Females performed a flexible effort and were working with a buffer of nutritional reserves that they used when necessary. Below a critical level, females preferentially allocated resources to the maintenance of their body condition at the expense of investment in current reproduction.