title: Hormonal correlates of siblicide in Galápagos Nazca boobies.
abstract: Nazca boobies (Sula granti) show unconditional obligate siblicide immediately after hatching, reducing the typical two-egg clutch size to one. We studied body mass changes and levels of testosterone (T), corticosterone (CORT), and progesterone (P) for A-chicks (dominant, first hatched), B-chicks (subordinate, second hatched), and singletons, during the first 7 days after hatching, when siblicide normally occurs. Mass increase with age was higher for A-chicks than for singletons and B-chicks. This exaggerated the existing developmental advantage of A- over B-chicks that is due to hatching asynchrony. In nests with two chicks, CORT titer was significantly higher in B-chicks than in A-chicks. During ontogenetic development, CORT decreased with age for A-chicks, but did not change for singletons. P showed qualitatively similar ontogenetic changes to CORT, remaining unchanged for A-chicks but increasing for singletons. Thus, both CORT and P levels were lower for A-chicks than for singletons, and both hormones varied inversely with body mass. Overall, T levels did not differ between different categories of chicks. However, one B-chick in the process of reversing the dominance relationship with its older, but weakened, sibling had significantly elevated T. We suggest that CORT and P are regulated to promote exaggerated mass gain in socially challenged A-chicks, facilitating siblicide. Whether T induces aggressiveness during short time intervals of intense sibling rivalry needs further attention.
authors.affiliation: Department of Ecology, Ethology and Evolution, University of Illinois at Urbana-Champaign, 505 S. Goodwin Avenue, Urbana, Illinois 61820, USA.
authors.forename: E M
authors.initials: EM
authors.lastname: Tarlow
authors.forename: M
authors.initials: M
authors.lastname: Wikelski
authors.forename: D J
authors.initials: DJ
authors.lastname: Anderson
mesh_terms: D000374:Aggression / Q000502:physiology*; D000378:Agonistic Behavior / Q000502:physiology*; D000818:Animals; D001835:Body Weight / Q000502:physiology; D003345:Corticosterone / Q000097:blood*; D004291:Dominance-Subordination; D004484:Ecuador; D005260:Female; D008297:Male; D011374:Progesterone / Q000097:blood*; D012801:Sibling Relations; D013739:Testosterone / Q000097:blood*
chemical_list: D013739:Testosterone; D011374:Progesterone; D003345:Corticosterone