COMMENTARIES Phenotypic correlates of digit ratio in a wild bird: implications for the study of maternal effects LA ´ SZLO ´ Z. GARAMSZEGI * , GERGELY HEGYI †, ESZTER SZO ¨ LLO ˜ SI †, BALA ´ ZS ROSIVALL †, JA ´ NOS TO ¨ RO ¨ K †, MARCEL EENS * & ANDERS P. MØLLER ‡ *Department of Biology, University of Antwerp yDepartment of Systematic Zoology and Ecology, Eo ¨tvo ¨s Lora ´nd University, Budapest Hungary zLaboratoire de Parasitologie Evolutive, Universite ´ Pierre et Marie Curie, Paris (Received 16 August 2006; initial acceptance 6 October 2006; final acceptance 6 November 2006; published online 23 August 2007; MS. number: SC-1301R) Keywords: age; collared flycatcher; digit ratio; effect size; Ficedula albicollis; maternal hormones; song; testosterone Digit ratio, the relative length of the second digit (2D) to the fourth digit (4D), as measured in childhood and adulthood, is generally assumed to mirror exposure to prenatal steroids in humans (Manning 2002). Specifically, high prenatal androgens and/or low prenatal oestrogens may result in longer 4D relatively to 2D, as expressed by 2D:4D. Although much recent research focused on the phenotypic correlates of digit ratio, providing promising results, including a correlation between digit ratio and the ratio of fetal testosterone and fetal oestradiol in hu- mans (Lutchmaya et al. 2004), findings are often contra- dictory and the role of prenatal testosterone in the development of digits is far from proven (Putz et al. 2004; Cohen-Bendahan et al. 2005; McIntyre et al. 2006). In birds, prenatal hormone regimes may involve compo- nents of maternal effects due to androgens (Schwabl 1993). Such maternal effects via maternal hormones are of central interest in evolutionary biology, because yolk testosterone can have short-term or long-term organizational effects on offspring, and thus mothers can actively generate phe- notypic plasticity in an adaptive way (Gil 2003; Groothuis et al. 2005). Maternal hormones are thus subject to different selection pressures both at the maternal and offspring sides, which often involve evolutionary conflicts. Such trade-off provides a proximate basis for a key concept in evolutionary ecology (Gil 2003; Groothuis et al. 2005). Given its plausible mechanistic link with prenatal steroids, digit ratio has been suggested to be used for studying the effects of early maternal hormones (Burley & Foster 2004). The measurement of digit ratio in the field would provide an easy way to get insights into how an in- dividual’s behaviour is shaped by its maternal environ- ment. For example, variance in digit ratio is associated with variance in reproductive success probably due to dif- ferences in attractiveness and mating success (Manning et al. 2000; Putz et al. 2004; Forstmeier 2005; Saino et al. 2006), which may have obvious evolutionary implications for maternal effects. The use of digit ratio offers practical benefits as, in wild populations, it is often impractical to measure or manipulate testosterone levels in an egg, and then to follow the offspring hatched from it to study re- productive performance in adulthood. An experimenter may have little chance to recapture the individuals of in- terest when adult, because of dispersal and frequently high juvenile mortality rate (in fact, if an egg is used for testosterone measurement, it cannot produce offspring). Because these difficulties do not apply to digit ratio, it may be expected that subsequent field studies in birds will prefer using digit ratio that can be measured in adults instead of directly assessing yolk testosterone levels in the egg. However, it is dangerous to adopt this preference without justifying the link between digit ratio and mater- nal testosterone level, because correlations between be- haviour and digit ratio may be falsely interpreted in the context of maternal effects (Forstmeier 2005). In fact, hitherto studies have provided conflicting evidence for the underlying assumption linking digit ratio to maternal effects in birds. Originally, Burley & Foster (2004) showed in a laboratory population of the zebra finch, Taeniopygia guttata, that digit ratio varies systemati- cally between sexes, with laying order and with the Correspondence: L. Z. Garamszegi, Department of Biology, University of Antwerp, Campus Drie Eiken, Universiteitsplein 1, B-2610 Antwerp, Belgium (email: laszlo.garamszegi@ua.ac.be). 641 0003e 3472/07/$30.00/0 Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2007, 74, 641e647 doi:10.1016/j.anbehav.2006.11.023