Yolk androgens reduce offspring survival

Abstract

Females may favour some offspring over others by differential deposition of yolk hormones. In American kestrels (Falco sparverius), we found that yolks of eggs laid late in the sequence of a clutch had more testosterone (T) and androstenedione (A4) than yolks of first-laid eggs. To investigate the effects of these yolk androgens on nestling 'fitness', we injected both T and A4 into the yolks of first-laid eggs and compared their hatching time, nestling growth and nestling survival with those of first-laid eggs in which we injected vehicle as a control. Compared to controls, injection of T and A4 at a dose intended to increase their levels to those of later-laid eggs delayed hatching and reduced nestling growth and survival rates. Yolk androgen treatment of egg 1 had no effect on survival of siblings hatching from subsequently laid eggs. The adverse actions of yolk androgen treatment in the kestrel are in contrast to the favourable actions of yolk T treatment found previously in canaries (Serinus canaria). Additional studies are necessary in order to determine whether the deposition of yolk androgens is an adaptive form of parental favouritism or an adverse by-product of endocrine processes during egg formation. Despite its adaptive significance, such 'transgenerational' effects of steroid hormones may have helped to evolutionarily shape the hormonal mechanisms regulating reproduction.

Hormone-mediated maternal effects in birds:mechanismsmatter but what do we know of them?

“…For example, nestling corticosterone concentrations were higher in American kestrel chicks hatched from androgen-treated than from control eggs (Sockman & Schwabl 2000); similar results were obtained for Japanese quail (Daisley et al 2005). In American kestrels, androgen-exposed chicks also showed reduced growth that may have indirectly led to their higher corticosterone levels or vice versa.…”

“…Circulating progesterone is associated with onset of broodiness (e.g. Cheng 1979;Sockman & Schwabl 2000). These local intra-ovarian and systemic signalling actions of ovarian steroids in the female have to be considered when proposing adaptive functions for their presence in the egg.…”

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

“…In some bird species, androgens deposited in the egg yolk have been found to increase competitive ability of chicks (correlative data in Schwabl, 1993; experimental data in Eising et al, 2001), chick begging (experimental data : Schwabl, 1996;Schwabl and Lipar, 2002;Eising and Groothuis, 2003;Boncoraglio et al, 2006;von Engelhardt et al, 2006) and growth (experimental data: Schwabl, 1996;Eising et al, 2001;Pilz et al, 2004;Navara et al, 2005Navara et al, , 2006Tschirren et al, 2005). In other studies, however, no positive effect of elevated yolk androgens on begging or growth was found (experimental data: Sockman and Schwabl, 2000;Andersson et al, 2004;Pilz et al, 2004;Uller et al, 2005;Saino et al, 2006), or the effect depended on offspring sex (e.g. Saino et al, 2006;Müller et al, 2008;Sockman et al, 2008).…”

Maternal androgens in egg yolks: relation with sex, incubation time and embryonic growth

“…Sex steroids of maternal origin can be found in the egg yolks of many oviparous species (Japanese quail, Adkins-Regan et al, 1995;Zebrafinch, Canary, Cattle egret, American kestrel, Schwabl, 1993, 1996a,b, Schwabl et al, 1997, Sockman and Schwabl, 2000, Whittingham and Schwabl, 2002turtles, Janzen et al, 1998;Zebrafinch, Gil et al, 1999; Dark-eyed junco, Red-winged blackbird, Lipar et al, 1999;damsel fish, McCormick, 1999;Painted turtle, Bowden et al, 2000;Black-headed gull, Eising et al, 2001; Lesser blackbacked gull, Royle et al, 2001;Tree swallow, Whittingham and Schwabl, 2002). It has been suggested that yolk hormone levels correlate with circulating plasma hormone levels in the female (Canary, Schwabl, 1996a) and may vary adaptively with the speciesÕ reproductive strategy and social requirements (Cattle egret, Schwabl et al, 1997, Black-headed gulls: Eising et al, 2001Groothuis and Schwabl, 2002).…”