Brood Reduction in White Storks Mediated through Asymmetries in Plasma Testosterone Concentrations in Chicks

Abstract

We hypothesized that increasing chick plasma testosterone concentrations, transmitted from the mothers via their eggs, enhances survival of their offspring and that the fitness of the young, depending on the maternal hormones, is influenced by parental quality. To test our hypotheses we distinguished the broods of white storks Ciconia ciconia L. where chicks died and those where all chicks survived. We analysed the plasma testosterone concentrations in the chicks, the ability of the chicks to be first to receive food and the mass of chicks before fledging in relation to their hatching order and recorded the body mass of parents and food mass delivered by them. Female storks used the asymmetries in testosterone concentrations within a brood to control brood size and adjusted the number of young hatched to match the parental ability to rear offspring. Females of poor condition altered the testosterone concentrations to produce large differences between the chicks: The first‐hatched chicks, which had high plasma testosterone levels, responded faster to the feeding parent and received more food than did their younger siblings. One or two later‐hatched chicks, which had lower testosterone levels, died in these broods. Females in good condition produced small differences in testosterone concentrations between the chicks and all chicks survived in their brood. Chicks that were raised by the females of poor condition in reduced broods were heavier than chicks that were raised by females of good condition in broods where all chicks survived. We suggest that the control of brood size by testosterone concentration, transmitted by the mother to the chicks, is a hormonal means of condition‐dependent reproductive strategy in the white stork.

The hormonal control of begging and early aggressive behavior: Experiments in black-headed gull chicks

“…In white storks (Ciconia ciconia), first hatched chicks are more aggressive, receive more food, and have higher plasma levels of testosterone than their siblings. The number of younger siblings that die is higher when the difference in testosterone levels between first and later hatched chicks is greater (Sasvári et al, 1999). Although the data are not based on experimental evidence, they suggest a role for testosterone in sibling competition and/or begging behavior.…”

Brood Reduction and Parental Infanticide — are the White StorkCiconia ciconiaand the Black StorkC. nigraexceptional?

“…The interest ing observation is that in some detailed studies on the W hite Stork breeding biology parental infanti cide was never observed. Infanticidal behaviour was not recorded during a three year study of the W hite Stork breeding at both low and high density (Sasvari et al 1999a(Sasvari et al , 1999b. Thus further detailed studies are necessary to explore the relationship betw een frequency of infanticide, parental quality and feeding conditions.…”

“…A new study of W hite Storks by Sasvari et al (1999a) has show n that one m aternal effect -the plasma testosterone concentrations in chicks, transm itted from the m others via their eggs -could have a remarkable influence on offspring survival. Sasvari et al (1999a) com pared broods of W hite Storks w here chicks died w ith those w here all chicks sur vived and analysed the plasma testosterone con centration in the chicks, their vigour and hatching order. It was found that highest testosterone levels w ere recorded in the first chick to hatch and the lowest in those chicks that hatched last.…”

Do males hatch first and dominate sex ratios in White Stork Ciconia ciconia chicks?

“…Tortosa and Redondo (1992) observed that stork parents may kill their smaller chick, which, according to our findings, might be female. Sasvari et al (1999) also noted that cases of brood reduction were related to testosterone concentrations in chick blood. The first-hatched chicks, which had high plasma testosterone levels (hence mainly males although chicks were not sexed in the original study), responded faster to the feeding parent and received more food than did their younger siblings.…”