Functional significance of variation in egg-yolk androgens in the American coot

Abstract

Maternally derived hormones in cleidoic eggs have been implicated in mediating growth, behavior, and social interactions among offspring. Given these widespread and significant effects, hormonal investments have the potential to greatly influence fitness of offspring. Intraspecific variation can exist at three levels (within individual eggs, among eggs within clutches, and among eggs from different females), each of which has different implications for offspring. We characterized all three levels of variation in maternally derived androgens (testosterone and androstenedione) present in yolks of American coot eggs. We found no variation in testosterone levels within eggs which suggests that embryos are exposed to constant androgen levels during development, and that field-based yolk biopsies are an appropriate way to sample eggs for this species. Within clutches, early-laid eggs had higher androgen levels than late-laid eggs, and this pattern may exacerbate negative effects of hatching asynchrony on chicks from late-hatching eggs if androgens provide chicks with a behavioral or growth advantage over chicks from eggs with lower androgen levels. American coots lay large clutches, and unequal resource allocation among offspring may be the optimal strategy for females with access to limited resources. Most of the variation in androgen levels occurred among eggs from different females. Females nesting on ponds with two other pairs laid eggs with significantly higher androgen levels than females living on ponds with fewer pairs. This suggests that increased territory defense behaviors influence the levels of androgens allocated to eggs and may be one mechanism underlying density-dependent effects on reproduction.

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.…”

Yolk androgens do not appear to mediate sexual conflict over parental investment in the collared flycatcher Ficedula albicollis

“…Even if within-clutch variation in hormone deposition would have occurred in some individuals of our study population, the third egg would still represent the medium concentration of the whole clutch, as long as the pattern of within-clutch variation is linear (as found in many studied species with significant within-clutch variation, e.g. Reed and Vleck, 2001;Groothuis and Schwabl, 2002;Pilz et al, 2003;Tschirren et al, 2004).…”

“…In most species studied so far, including the collared flycatcher and its sister species the pied flycatcher (Ficedula hypoleuca), the between-clutch variation in yolk androgen levels is higher than the within-clutch variation (e.g. Reed and Vleck, 2001;Groothuis and Schwabl, 2002;Pilz et al, 2003;Tschirren et al, 2004;Michl et al, 2005;Müller et al, 2007b, Tobler et al, 2007T. Laaksonen, unpubl.…”

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

“…In Tree swallows Tachycineta bicolor, maternal aggressive response levels to conspecific intrusions at the nest box were reflected in the T levels in the eggs (Whittingham & Schwabl, 2002). In American coots Fulica americana (Reed & Vleck, 2001) and Black-headed gulls Larus ridibundus yolk androgen levels increased with breeding density, and in gulls these levels also decreased with increasing vegetation height, likely due to a reduced amount of social interactions between breeding individuals (Groothuis & Schwabl, 2002;.…”