The Interaction between Organizational and Activational Effects of Testosterone in the Control of Early Aggression in Birds: A Comment on Sasvári, Hegyi & Péczely

Corresponding author: Albert F. H. Ros, Unidade de InvestigacE aA o em EcoEtologo a, Instituto Superior de Psicologia Aplicada, Rua Jardim do Tabaco, 34, 1149±041 Lisboa, Portugal. E-mail: aros@ispa.pt Recently, Sasva ri et al. (1999) published a paper in this journal on the relationship between brood reduction and testosterone levels in the asynchronously hatching chicks of the white stork (Ciconia ciconia) in which they showed that levels of testosterone in plasma, drawn at 5 and 15 d after hatching, decreased with laying order. They compared nests in which one or more chicks died at between the age of 5 and 15 d after hatching with nests in which no chick died in this period. In correlation with the mortality, parents of the ®rst group were in lower physical condition and delivered less food to the chicks than parents of the second group. Interestingly, in the ®rst group relative to the second group, levels of testosterone were increased nearly twofold in chicks that hatched ®rst and decreased about twofold in chicks that hatched last. Furthermore, chicks that hatched ®rst were twice as likely to be the ®rst to be fed in the ®rst group as compared with the second group. They did not observe overt aggression between siblings between the age of 5 and 15 d after hatching. The authors argued that the mortality di€erence in nests was a€ected by testosterone-dependent competition between the chicks, which in turn was a result of maternal transmission of hormones to the egg yolk. However, they did not take measurements of androgen concentrations in yolk samples to substantiate this rather complicated mechanism. Since testosterone levels were still elevated at day 5 after hatching and increased from day 5 to day 15 after hatching, a maternal Ethology 107, 851Ð853 (2001) O 2001 Blackwell Wissenschafts-Verlag, Berlin ISSN 0179±1613