The transfer of non-genetic resources from mother to the offspring often has considerable consequences for offspring performance. In birds, maternally derived hormones are known to influence a variety of morphological, physiological and behavioural traits in the chick. So far, the range of these hormonal effects involves benefits in terms of enhanced growth and competitive ability as well as costs in terms of immunosuppression. However, since yolk hormones can enhance growth and begging activity, high levels of these hormones may also involve energetic costs. Here, we show experimentally that elevated levels of prenatal testosterone increase resting metabolic rate in nestling zebra finches (Taeniopygia guttata). Surprisingly, however, elevation of prenatal testosterone did not result in higher growth rates and, thus, differences in resting metabolism do not seem to be linked to nestling growth. We conclude that apart from immunosuppressive effects, high levels of egg steroids may also entail costs in terms of increased energy expenditure.

Maternal hormones can have a considerable impact on offspring phenotype in many vertebrate species (e.g. Clark & Galef 1995; McCormick 1999; Uller & Olsson 2003; Groothuis et al. 2005a). In oviparous species, hormones transferred from the mother to the egg vary remarkably within as well as among clutches (e.g. Conley et al. 1997; Janzen et al. 1998; McCormick 1999; Groothuis et al. 2005a). Recent studies on birds suggest that this variation may be maintained owing to a trade-off between beneficial and costly effects of egg hormones on offspring performance. Egg hormones promote pre- and postnatal growth (e.g. Groothuis et al. 2005a; Gil et al. 2006; but see Sockman & Schwabl 2000) as well as competitive ability of the chicks (e.g. Groothuis et al. 2005a; von Engelhardt et al. 2006), but they also negatively affect immune function of the offspring (Andersson et al. 2004; Navara et al. 2005; Groothuis et al. 2005b; Müller et al. 2005; but see Tschirren et al. 2005; Navara et al. 2006). However, since egg hormones increase growth and activity levels, they may not only entail costs in terms of immunosuppression, but also in terms of increased energy expenditure. It has previously been suggested that such costs would be manifested in higher metabolic rates in chicks hatching from eggs with high hormone levels (Eising et al. 2003; Gil 2003). The only study so far addressing this cost experimentally has not found any effects of egg hormones on chick metabolism (Eising et al. 2003). Neither resting metabolic rate nor daily energy expenditure was increased in precocial black-headed gull (Larus ridibundus) chicks that were exposed to elevated prenatal androgen levels. In the present study, we report data from an altricial passerine, the zebra finch (Taeniopygia guttata), showing a positive relationship between elevated prenatal testosterone levels and nestling resting metabolic rate.

Offspring hatched from eggs with elevated T-levels had significantly higher RMR at 14–15 days of age than offspring hatched from control eggs (figure 1; table 1). The effect of egg treatment was independent of the sex or the exact age of the nestling. RMR was strongly influenced by individual body mass, but the effect of egg treatment on RMR appears not to be due to treatment-specific differences in growth rate. Growth rate from hatching to the age of 20 days did not differ between the two treatment groups (change in body mass over time by treatment interaction, effect of egg treatment, both p>0.28). No effect of sex on growth rate was found (change in body mass over time by sex interaction, effect of sex, both p>0.35). Moreover, body mass during RMR measurements (day 14 or 15) was not affected by egg treatment (F_1,38.8=0.46, p=0.50) or sex (F_1,27.5=0.22, p=0.64), and there was no significant interaction between sex and treatment (F_1,39.4=0.91, p=0.35).