Hormonal correlates of siblicide in Galapagos Nazca boobies

Nazca boobies (Sula granti) show unconditional obligate siblicide immediate ly after hatching, reducing the typical two-egg clutch size to one. We stud ied body mass changes and levels of testosterone (T), corticosterone (CORT) , and progesterone (P) for A-chicks (dominant, first hatched), B-chicks (su bordinate, second hatched), and singletons, during the first 7 days after h atching, when siblicide normally occurs. Mass increase with age was higher for A-chicks than for singletons and B-chicks. This exaggerated the existin g developmental advantage of A- over B-chicks that is due to hatching async hrony. In nests with two chicks, CORT titer was significantly higher in B-c hicks than in A-chicks. During ontogenetic development, CORT decreased with age for A-chicks, but did not change for singletons. P showed qualitativel y similar ontogenetic changes to CORT, remaining unchanged for A-chicks but increasing for singletons. Thus, both CORT and P levels were lower for A-c hicks than for singletons, and both hormones varied inversely with body mas s. Overall, T levels did not differ between different categories of chicks. However, one B-chick in the process of reversing the dominance relationshi p with its older, but weakened, sibling had significantly elevated T. We su ggest that CORT and P are regulated to promote exaggerated mass gain in soc ially challenged A-chicks, facilitating siblicide. Whether T induces aggres siveness during short time intervals of intense sibling rivalry needs furth er attention. (C) 2001 Academic Press.