Hormonal influences on booby sibling competition

Behavioral ecologists are becoming increasingly interested in the hormonal mediation of social behaviors, because by discovering and comparing levels of key hormones in individuals we can better understand the proximate mechanisms behind these behaviors. Studies in parental care, migration, and resource defense, among other fields, have benefited from endocrinological research. Sibling aggression serves as a relatively new application for theories surrounding the hormonal basis for resource defense and aggression.  We are collaborating with Dr. Martin Wikelski's lab at Princeton University  in studies of hormones that might be involved in sibling aggression.

We are specifically interested in understanding what hormonal differences might account for obligate siblicide in Nazca boobies and facultative siblicide in blue-footed boobies. Hatching rate and food acquisition explain the differences at the ultimate level, but the proximate level has not yet been explored. Elise Donnelly is focusing on the endocrinological basis for siblicide in boobies for her Masters thesis. After taking and analyzing blood samples from Nazca and blue-footed booby chicks we can compare the amount of key hormones between the two species and between chicks of different status, i.e. those of different hatching order as well as those with and without a sibling. NZBOsibcide31.jpg (31081 bytes)
Three hormones are being studied: testosterone, progesterone, and corticosterone. Testosterone is associated with aggressive behavior in many organisms. The roles of progesterone are not as clear, yet it is believed to affect levels of aggression, have organizational roles (toward higher aggression) in young organisms, and alter growth patterns. Finally, cortiscosterone is related to stress; individuals under high stress usually show high levels of this hormone, which could increase muscle metabolism and encourage submissive behavior.  Our data show that all Nazca booby chicks normally have very low levels of testosterone. The figure below gives testosterone concentrations (ng/mL) for chicks of different status at early stages (1-4 days) on the left side and later stages (5-8 days) on the right side (Tarlow et al. 2001).

On the left we see elder (A) chicks before the second egg had hatched, chicks from a one-egg clutch, and B chicks 1-2 days after hatching. On the right we include A chicks once the B chick has hatched, older chicks from a one-egg clutch, and A chicks after they have evicted the B chick. There was one uncommon case of a B chick attempting to evict its sibling. Evidently during aggression testosterone levels are elevated, as shown by the single point on the figure. Except for this sample, the other data incorporated into this figure were not from samples taken during an aggressive interaction. Therefore, with testosterone we expect to find high levels only when an individual is in an aggressive interaction. There are costs to having high testosterone, such as increased metabolic needs and possibly a weakened immune system, so that we would not expect elevated levels to be maintained.

Tarlow, E. M., M. Wikelski, and D. J. Anderson. 2001. Hormonal correlates of siblicide in Nazca boobies. Hormones and Behavior 40:14-20.

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