Egg yolk androgen levels increase with breeding density in the European Starling,Sturnus vulgaris

Abstract

Summary1. High breeding density can cause elevated plasma androgen levels in adult birds. Since maternal androgens are deposited into egg yolk, high breeding density may result in elevated yolk androgen levels as well. 2. The relationship between breeding density and yolk androgen levels was examined in the European Starling, Sturnus vulgaris . The concentration and total content of yolk androstenedione and yolk testosterone were measured in eggs from 24 clutches distributed across nine different colonies of nestboxes. 3. Yolk androstenedione and testosterone levels were significantly higher in colonies where a greater proportion of nestboxes had active nests. 4. Yolk testosterone levels were significantly higher, and yolk androstenedione levels were marginally higher, in colonies with a greater absolute number of active nests. 5. Yolk androgen levels were not related to the number of active nests in adjacent nestboxes. 6. We conclude that female starlings nesting in colonies with higher breeding densities transfer more androgen to their eggs. 7. This relationship may be mediated by increased interfemale aggression, particularly towards floater females searching for mates or nests to brood parasitize, in high-density colonies. Such a relationship between maternal environment and maternal yolk androgens may represent adaptive maternal modification of offspring phenotype or a non-adaptive physiological constraint which females cannot avoid.

Heritable Variation in Maternal Yolk Hormone Transfer in a Wild Bird Population

“…Indeed, we found significant year effects on yolk A4 concentrations as well as a significant negative association between female body condition and yolk A4. No such effects were observed for yolk T. Furthermore, experimental manipulation of environmental conditions (e.g., parasitism [Tschirren et al 2004], food abundance [Verboven et al 2003], and immune challenge of the mother [Gil et al 2006a]) had stronger effects on the yolk A4 than on the yolk T content of eggs (but see Reed and Vleck 2001;Groothuis and Schwabl 2002;Pilz and Smith 2004). The finding that repeatability of A4 was substantially higher when correcting for variation among years indicates that although females deposit variable amounts of A4 into the eggs depending on environmental conditions, they deposit similar amounts relative to each other, showing an interesting interaction between plasticity on the one hand and individual consistency on the other.…”

“…These opposing costs and benefits indicate that the optimal allocation of maternal yolk androgens into the eggs will depend on current or future environmental or social conditions. Indeed, it is well documented that factors such as breeding density (Schwabl 1997;Reed and Vleck 2001;Groothuis and Schwabl 2002;Mazuc et al 2003;Pilz and Smith 2004), timing of breeding (Schwabl 1996;Bowden et al 2000;Pilz et al 2003;Müller et al 2004), food abundance (Verboven et al 2003;Gasparini et al 2007;Warner et al 2007), partner attractiveness (Gil et al 1999(Gil et al , 2004Tanvez et al 2004;Loyau et al 2007;Kingma et al 2008), or parasitism (Tschirren et al 2004) influence maternal yolk androgen deposition.…”

Trans-generational effects of prenatal stress in quail

“…These prenatal stress effects could be mediated by modifications of steroid hormone levels of maternal origin present in eggs [15]. Indeed, yolk testosterone levels increase in eggs of females living under stressful conditions during laying [13,14,16,17]. The effects of prenatal exposure to elevated yolk testosterone levels have been studied mainly by applying yolk hormonal injections.…”

Maternal yolk androgens in European starlings: affected by social environment or individual traits of the mother?

“…Also in our own study species, the European starling, Sturnus vulgaris, Pilz & Smith (2004) have shown that yolk androgen levels increase with increasing breeding density. This relationship may be mediated by higher levels of inter-female aggression, in particular towards floater females searching for mates or opportunities to parasitize broods in large colonies.…”

“…Based on the data presented by Pilz & Smith (2004), our experimental design allowed us to study allocation patterns of maternal yolk androgens in captive European starlings breeding in outdoor aviaries in relation to breeding densities and nest site availability in two subsequent years. We hypothesize that yolk androgen levels increase with increasing breeding density and with increasing competition for nest boxes of the same polygynous male.…”