PI: Douglas W. Moc, CO-PI: Hubert G. Schwabl IBN-9604370 In recent years, the general assumption that parents value all of their offspring equally has come into question. In many animals and plant groups, parents produce greater numbers of concurrent young than can ordinarily be supported fully, forcing a subsequent adjustment downward in family size. The underlying reasons for parental overproduction stem from multiple sources of environmental unpredictability (including resource unpredictability, accidental losses of progeny, partial-brood predation, cannibalism by family members or others, ect.), but the fact of it simultaneously generates sibling rivalry, which creates incentives for parents to produce a hierarchical sibship to minimize the costs of such rivalry. Thus, parents confer special advantages on some offspring and/or handicaps on other, so called marginal brood members. The best-studied manifestation of biological parental favoritism is probably asynchronous hatching in many predatory birds species. There, early onset of incubation gives embryos in the first- laid eggs a developmental head-start that facilitates their success in any ensuing sibling competitions for limited food. Hubert Schwabl s recent discovery that female laboratory canaries deposit increasing amounts of testosterone into yolks as the laying process proceeds suggested a new mechanism for parental manipulation of offspring vigar and inspired the collection of fresh eggs from cattle egrets (Bubulcus ibis), a species known to practice fatal sibling aggression (or siblicide ). The marginal third egg of this species contained much less yolk testosterone (plus two similar steroids) than those of its two nestmates, which normally hatch 1-3 days earlier. The fact that the egret pattern (decreasing steroid dose) is exactly opposite that found in canaries, but congruent with the effects of hatching asynchrony, suggests that hormone depositions into yolks could be a widespread and flexible parental adaptation for rigging intra-brood competitive dynamics and survival. The proposed study aims to explore this matter through two field studies. First, the cattle egret system will be experimentally manipulated by injecting various levels of a 3-steroid cocktail into last-laid eggs, then measuring the subsequent competitive behaviors (begging and fighting) of the hatchlings under field conditions. If steroid-manipulation is part of an evolved parental strategy for managing sibling competition, broods with boosted last- eggs should show escalated fighting and begging, ultimately to the detriment of parental interests. Second, to test whether the mother s cutback on steroids is a general phenomenon and associated with families in which sib- competition is especially keen, an exploratory survey will be made of the other siblicidal species, each paired with a closely-related but nonaggressive sister species. As with cattle egret pilot work, clutches of eggs will be collected as soon as each is laid, their yolks frozen for later laboratory assays of steroid content. Through this work, we expect to increase understanding of the reproductive strategies of parents under natural conditions and to determine whether maternal steroid dosing is a general mechanism used to enhance parental fitness.

Agency
National Science Foundation (NSF)
Institute
Division of Integrative Organismal Systems (IOS)
Type
Standard Grant (Standard)
Application #
9604370
Program Officer
Jerry O. Wolff
Project Start
Project End
Budget Start
1997-04-01
Budget End
2002-09-30
Support Year
Fiscal Year
1996
Total Cost
$102,691
Indirect Cost
Name
University of Oklahoma
Department
Type
DUNS #
City
Norman
State
OK
Country
United States
Zip Code
73019