European starling chicks benefit from high yolk testosterone levels during a drought year Kevin M. Pilz, a, * Martı ´n Quiroga, b Hubert Schwabl, c and Elizabeth Adkins-Regan a,d a Department of Neurobiology and Behavior, Cornell University, Ithaca, NY 14853, USA b National Institute of Limnology, 3016 Santo Tome ´, Santa Fe, Argentina c Centerof Reproductive Biology, School of Biological Sciences, Washington State University, Pullman, WA 99164, USA d Department of Psychology, Cornell University, Ithaca, NY 14853, USA Received 26 March 2003; revised 24 November 2003; accepted 9 March 2004 Available online 7 June 2004 Abstract Avian egg yolk contains androgenic hormones, such as testosterone, of maternal origin. Experimental elevation of yolk testosterone levels enhances growth of canary chicks. Success in sibling competition, due to increased begging, is presumed to underlie this growth enhancement, because canary hatchlings from testosterone-treated eggs beg longer in response to vibrational stimuli than controls. Furthermore, experimental elevation of both yolk androstenedione and testosterone increased chick growth and begging in black-headed gulls. We measured daily growth of European starling (Sturnus vulgaris) chicks hatching from testosterone-treated or vehicle-treated (control) eggs until 14 days of age, and measured begging behavior at hatching and at 5 days of age. A temporary drought caused relatively high levels of early brood reduction for this population; 2- and 3-day-old chicks were most likely to starve. We found that chicks from testosterone-treated eggs were less likely to starve than control chicks, and were heavier on the days when most brood reduction occurred. However, chicks from testosterone-treated eggs begged less than control chicks on the day of hatching, and begged similarly at 5 days of age. Thus, while yolk testosterone did increase growth during periods of (presumably) high competition, increased begging does not appear to mediate this effect. Instead, testosterone may induce more efficient energy use, for example, by decreasing ineffective begging. While our results indicate that elevated yolk testosterone enhances survival, and thus offspring and parental fitness, further evidence regarding the fitness consequences of yolk androgens are vital to understanding their role in avian life history. D 2004 Elsevier Inc. All rights reserved. Keywords: Yolk testosterone; Begging behavior; Sibling competition; Chick growth; Parental investment; Sturnus vulgaris Introduction Altricial bird chicks are unable to forage for themselves and depend on parents to provide food resources. The food supply available to chicks is thus limited, and food compe- tition becomes a central aspect of life for chicks raised with siblings. Parental optima for offspring competitiveness and growth rates are likely to depend on ecological conditions, such as food availability and the probability that all chicks in a clutch will survive to fledging, and thus may vary temporally (Bosque and Bosque, 1995; Lack, 1968; Mock and Forbes, 1994). Mechanisms by which parents could facultatively adjust offspring growth and competition should therefore be useful to parents. Direct parental manipulation of offspring phenotype could also be useful to parents for gaining an upper hand in parent – offspring conflict, since optimal levels of competition and optimal allocation of food are likely to differ from the parental and offspring perspec- tives (Parker and McNair, 1978; Trivers, 1974). While chick competition has been extensively researched by biologists interested in the evolution of signaling systems and parent – offspring interactions, the physiological mech- anisms that regulate chick competition are little understood (see Wright and Leonard, 2002, for reviews of evolutionary and mechanistic research on chick competition). Regarding hormonal mechanisms, we have limited knowledge regard- ing the role of testosterone (T) in mediating chick aggres- sion, begging, and growth. T enhances social status and the probability of aggressive behavior in chicken chicks 0018-506X/$ - see front matter D 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.yhbeh.2004.03.004 * Corresponding author. Department of Evolutionary Ecology, Museo Nacional de Ciencias Naturales, Jose Gutierrez Abascal, 2 28006 Madrid, Spain. Fax: +34-91-564-5078. E-mail address: kevin.pilz@cornell.edu (K.M. Pilz). www.elsevier.com/locate/yhbeh Hormones and Behavior 46 (2004) 179 – 192