General and Comparative Endocrinology 151 (2007) 220–229 www.elsevier.com/locate/ygcen 0016-6480/$ - see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.ygcen.2007.01.034 EVects of artiWcial eggs on prolactin secretion, steroid levels, brood patch development, incubation onset and clutch size in the yellow-eyed penguin (Megadyptes antipodes) Melanie Massaro ¤ , Alvin N. Setiawan, Lloyd S. Davis Department of Zoology, University of Otago, P.O. Box 56, Dunedin, New Zealand Received 24 May 2006; revised 22 January 2007; accepted 25 January 2007 Available online 9 February 2007 Abstract Several studies have shown that the transition from egg laying to incubation behavior in birds is associated with changes in plasma lev- els of prolactin and steroid hormones. However, any eVect of the tactile and visual input provided by eggs at initiating these hormonal changes has not been fully investigated in wild birds. A few days before yellow-eyed penguins, Megadyptes antipodes, started egg laying, we placed an artiWcial egg into their nests or under cages next to their nest. We then investigated the eVect of the tactile and/or visual stim- ulus of such an artiWcial egg on prolactin secretion, steroid hormone levels (total androgen, estradiol and progesterone), brood patch development, incubation onset and clutch size in these penguins. Prolactin levels rose in females in response to having an artiWcial egg in the nest, while they declined considerably in males. Total androgen concentrations in males were less than 7% of those of control males and the levels prior to egg placement. Brood patch width increased in both males and females. Additionally, an egg in the nest caused yel- low-eyed penguin pairs to attend and sit prone on their nest more frequently. Females that initiated egg laying 1 or 2 days after placement of the artiWcial egg in the nest, laid a full clutch of two eggs, while most other females that were exposed to an artiWcial egg in their nest, laid only a single egg. In contrast, the visual stimulus of an artiWcial egg alone (that was placed under a cage) did not inXuence hormone levels, brood patch development, incubation behavior or clutch size. The stimulation of an egg in the nest inXuences prolactin and total androgen levels in yellow-eyed penguins, particularly in males. While brood patch development and incubation behavior were initiated and egg laying was terminated in response to an artiWcial egg in the nest, the exact endocrine mechanisms underlying these physiological and behavioral changes remain poorly understood. We encourage further studies on other bird species taking an experimental approach to investigate the direct inXuence of hormones in initiating brood patch development and incubation behavior. Moreover, such experi- mental studies will widen our understanding of the endocrine mechanisms that regulate clutch size.  2007 Elsevier Inc. All rights reserved. Keywords: Brood patch development; Clutch size; Egg laying; Hormones; Incubation onset; Megadyptes antipodes; Parental behavior; Yellow-eyed penguin 1. Introduction There is considerable evidence that incubation behavior in birds is aVected by plasma levels of prolactin (e.g., Gold- smith, 1991; Buntin, 1996; Lormée et al., 1999). Prolactin secretion is known to increase steadily in incubating birds during egg laying and incubation, when it usually reaches its highest levels, before declining immediately after hatch- ing in precocial species or at the end of the brooding period in altricial species (Goldsmith, 1991; Buntin, 1996; Vleck, 2002). In contrast, plasma levels of gonadal steroids are generally at their highest during the courtship and egg lay- ing period and decline as incubation is commenced (e.g., Groscolas et al., 1986; Hector et al., 1986; Ball, 1991). Of particular interest is the endocrine control of the transition from courtship to incubation behavior as the exact timing of incubation onset in relation to egg laying may inXuence * Corresponding author. Present address: School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand. Fax: +64 3 364 2590. E-mail address: melanie.massaro@canterbury.ac.nz (M. Massaro).