Experimentally elevated testosterone levels enhance courtship behaviour and territoriality but depress acquired immune response in Red Bishops Euplectes orix RALF EDLER, 1 WOLFGANG GOYMANN, 2 INGRID SCHWABL 2 & THOMAS W. P. FRIEDL 1 * 1 Animal Physiology and Behaviour Group, Institute for Biology and Environmental Sciences, Carl von Ossietzky University Oldenburg, PO Box 2503, D-26111 Oldenburg, Germany 2 Max-Planck-Institut fu¨rOrnithologie, Abteilung fu¨rVerhaltensneurobiologie, Eberhard-Gwinner Straße, D – 82319 Seewiesen, Germany The immunocompetence handicap hypothesis (ICHH) posits that the hormone testos- terone mediates a trade-off between investment in reproduction and immunological con- dition. In this study, we tested the ICHH in the Red Bishop Euplectes orix, a polygynous weaverbird. Males of this strongly sexually dimorphic species show an elaborate court- ship display to attract females and compete aggressively with other males for nesting sites in breeding colonies. We experimentally elevated testosterone levels in breeding male Red Bishops kept in an aviary with a subcutaneous implantation of testosterone-releasing pellets. We then compared behaviour, development of territory size and immunological condition (as assessed through a white blood cell count) of the experimental group with a control group treated with placebos. In addition, we measured the primary and second- ary response to phytohaemagglutinin (PHA) to investigate the effect of testosterone on both innate and acquired immunity. Males with elevated levels of testosterone enlarged their territories and conducted more courtship behaviour, while showing a decrease in health, expressed by an increased heterophil ⁄ lymphocyte ratio compared with the con- trol group. Males of the control group showed an increase of the secondary response to PHA, as expected under the assumption that repeated exposure to an antigen enhanced the immune response due to acquired immunity. However, males with experimentally increased testosterone levels did not show such an enhanced immune response in the sec- ondary PHA test (although sample size and power of the statistical tests were low), indi- cating that testosterone treatment might directly or indirectly suppress some component of the acquired immune response. Keywords: acquired immune response, heterophil ⁄ lymphocyte ratio, immunocompetence handicap hypothesis, polygynous weaverbird, repeated PHA-test, territorial behaviour. The immunocompetence handicap hypothesis (ICHH) proposed by Folstad and Karter (1992) suggests that the hormone testosterone acts as a double-edged sword. Although testosterone has been shown to increase reproductive behaviour such as courtship and territorial defence as well as the probability of extra-pair copulations (Wingfield et al. 1990, Collins & Borgia 1992, Johnsen 1998, Vleck & Brown 1999, de Ridder et al. 2000, McDonald et al. 2001), it is also believed to be costly for the males. Proposed costs of enhanced testosterone levels may take the form of an increase in male–male competition and therefore decreased survivorship (Dufty 1989), an increased metabolic rate with higher energy requirements (Røskaft et al. 1986), a suppression of spermato- genesis (Turek et al. 1976) or a decrease in male *Corresponding author. Email: Thomas.Friedl@uni-oldenburg.de ª 2010 The Authors Journal compilation ª 2010 British Ornithologists’ Union Ibis (2011), 153, 46–58