Goose egg yolk hormones 221 Goose egg yolk hormones doi: 10.3184/175815513X13740595171803 Hormone levels in the outer layer of domestic goose (Anser anser domesticus) eggs change over the laying season John P. Kent a* , Kenneth J. Murphy a,b,d , Attila Salamon a,b , Thomas J. Hayden b and Erich Möstl c a Ballyrichard House, Arklow, Co. Wicklow, Ireland b School of Biology and Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland c Department of Biomedical Sciences/Biochemistry, University of Veterinary Medicine, Veterinärplatz 1, A-1210 Vienna, Austria d Present address: Kings College London, Department of Forensic and Neurodevelopmental Science, Institute of Psychiatry, De Crespigny Park, London, SE5 8AF, UK *E-mail: john.kent@ucd.ie ABSTRACT In general, avian species produce clutches of more than one egg. Differential investment in egg weight and hormone levels is possible and their effects on development and behaviour can be measured. Here we investigated changes in three hormones (testosterone, T; androstenedione, A 4 and progesterone, P 4 ) in the outer layer of yolks of infertile domestic goose eggs over the course of a laying season. There was a significant change in the concentration of all three hormones: T and P 4 increased in a linear manner across the season whilst A 4 increased until mid season and then steadily decreased. The correlation between T and P 4 (r=0.613, P<0.001), and between T and A 4 (r=0.746, P<0.001) were both positive, but the relationship between A 4 and P 4 , while positive was not strong (r=0.379, P<0.001), although still significant. The increase in P 4 over the laying season is consistent with the transition from a laying to a broody state. The increase in T is consistent with a trade-off for the seasonal decline in egg weight. Keywords: androstenedione; eggs; geese; maternal investment; progesterone; testosterone www.avianbiologyresearch.co.uk AVIAN BIOLOGY RESEARCH 6 (3), 2013 221–226 1. INTRODUCTION The maternal interuterine environment in mammals influences offspring development. In house mice (Mus musculus), females occupying an interuterine environment between two males had first oestrus at a later age and had longer oestrus cycles, a shorter reproductive life, and were more aggressive, than females positioned in utero between two other female embryos (vom Saal 1984; for review see Clark and Galef, 1998). From these findings a role for pre-natal hormones on post-natal development started to become apparent. The finding that egg yolk was a source of maternal testosterone (T) and that yolk T influenced postnatal development in birds (Schwabl, 1993; 1996a) further stimulated interest in what are known as maternal effects (Kirkpatrick and Lande, 1989; Groothuis and Schwabl, 2008). The eggs of oviparous species are a perfect model for this since their development largely takes place outside the female’s body and therefore facilitates experimental manipulation in a controlled manner. In the egg yolk of black-headed gulls (Larus ridibundus; Eising et al., 2001; Groothuis and Schwabl, 2002; Eising and Groothuis, 2003; Müller et al., 2004), lesser black- backed gulls (Larus fuscus; Royle et al., 2001; Verboven et al., 2003) and common terns (Sterna hirundo; French Jr et al., 2001) T levels increase with laying order. This increase in T in later laid eggs is thought to accelerate embryonic development and therefore plays a compensatory role in the context of hatching asynchrony since the later laid eggs may receive less incubation time (Müller et al., 2004); incubation starts during laying in Canada geese (Branta canadensis; Cooper, 1978) and barnacle geese (Branta leucopsis; Hübner et al., 2002). Hegyi and Schwabl (2010) found that T reduced early structural growth and mass gain post-natally in Japanese quail (Coturnix japonica), while Hegyi et al. (2011) showed that collared flycatcher (Ficedula albicollis) eggs with relatively higher yolk T levels were less likely to hatch. However, Boonstra et al. (2009) found that T decreased with laying order in Canada geese, but T was measured at the centre of the egg yolk (see Discussion). There was no relationship between T and laying order in Japanese quail (for review see Groothuis et al., 2005 and Gil, 2008). Japanese quail are noted for their synchronised hatching that is facilitated by pre-hatching social facilitation mechanisms (see Freeman and Vince, 1974) and while T has no apparent role in hatching synchrony it was found to influence early behavioural development (Daisley et al. 2005; Okuliarová et al. 2007).