Variation in and functional consequences of avian maternal hormone deposition in the egg

hormone deposition in the egg have recently been receiving much attention. Several studies have reported large variation in these steroids both within and among clutches (e.g. Schwabl, 1993; Gil, 1999; Whittingham and Schwabl, 2002). The majority of studies reported that enhanced yolk levels of maternal androgens appear to have beneficial effects on offspring growth and development (e.g. Schwabl, 1996; Lipar and Ketterson, 2000; Eising et al., 2001; Eising and Groothuis, in press). These beneficial effects of yolk androgens have mainly been discussed in the context of hatching asynchrony. For instance, experimental elevation of yolk androgen levels decreased the time until hatching by half a day in black-headed gull chicks [Larus ridibundus; Eising et al., 2001; but see Sockman and Schwabl, 2000 for an opposite effect in American kestrels (Falco sparverius)]. This is in accordance with the data presented by Lipar and Ketterson (2000) and Lipar (2001), who showed that increasing levels of yolk testosterone according to laying order were associated with an enlarged size of the hatching muscle in European starling (Sturnus vulgarus) and red-winged blackbird (Agelaius phoeniceus) chicks. Moreover, since body mass and tarsus length at hatching of gull chicks from androgen-treated eggs were equal to those of oil controls, maternal androgens appear to enhance overall embryonic growth. Growth after hatching can be significantly enhanced by yolk androgens (Schwabl, 1996; Eising et al., 2001). Also, long-term enhancing effects of maternal yolk androgens on competitive behaviour have been reported both in canaries (Serinus canaria; Schwabl, 1996) and in gulls (Eising and Groothuis, 2002). However, the occurrence of large withinand betweenclutch variation in the deposition of maternal androgens (e.g. Schwabl et al., 1997; Reed and Vleck, 2001; Groothuis and Schwabl, 2002) begs the question of why not all mothers invest high amounts of hormones into their eggs, unless there is a cost involved. This cost may either be incurred by the mother producing the androgens or by the chicks exposed to them. Steroid hormones can entail immunosuppressive costs (e.g. Mooradian et al., 1987; Ketterson and Nolan, 1999; Peters, 2000; but see also Ros et al., 1997; Braude et al., 1999; Hasselquist et al., 1999), which is also suggested for maternal androgens (T. G. G. Groothuis, C. M. Eising, C. Dijkstra and W. Müller, manuscript submitted; Müller et al., in press). High 3211 The Journal of Experimental Biology 206, 3211-3218 © 2003 The Company of Biologists Ltd doi:10.1242/jeb.00552