Sex differences in the fingers of 3 to 5 month old infants do not predict concurrent salivary testosterone levels Matthew H. McIntyre a, ⁎, Gerianne M. Alexander b a Department of Anthropology, 309 Howard Phillips Hall, University of Central Florida, 4000 Central Florida Blvd., Orlando, FL 32816 USA b Department of Psychology, TAMU-2435, Texas A&M University, College Station, TX 77845 USA abstract article info Article history: Received 26 August 2010 Received in revised form 20 January 2011 Accepted 31 January 2011 Keywords: Perinatal Postnatal Prenatal Androgens 2D:4D Digit ratios Testosterone was assayed in the saliva of 32 female and 42 male 3–5 month old infants, and the lengths of their finger segments between flexion creases were measured. While expected sex differences were identified in the finger measures, these sex differences were not correlated with salivary testosterone in either sex. © 2011 Elsevier Ireland Ltd. All rights reserved. Most early mammalian secondary sex differentiation occurs under the influence of testicular testosterone produced by males in utero [1]. However, male mammals, including primates, also produce testoster- one immediately after birth [2–7]. In humans, testicular testosterone production peaks a second time between 1 and 6 months [8,9]. Salivary testosterone, the unbound, free fraction of plasma testosterone that is able to diffuse across the salivary glands, is highly correlated with the physiologically active portion of total circulating testosterone [10]. The postnatal surge is not detected in saliva [11,12] because rising levels of sex hormone binding globulin bind much of the testosterone [9] preventing its perfusion into the salivary glands, and also, presumably, into target organs. Despite the presumably low bioavailability, and although the biologically active proportion of testosterone in male neonates is well below that observed in adult men [13], the postnatal increase in testosterone levels is critical for the normal development of male genitalia and reproductive function [14,15]. For example, in a large prospective study of penile growth in male infants from birth to three years of age, higher levels of total and free testosterone at 3 months of age were associated with greater penile growth across the three years of development [16]. Thus, secondary sex differences arising prior to puberty are usually attributable to the effects of prenatal or postnatal elevation in testicular testosterone production in males relative to females, who produce less testosterone from non-testicular sources. One example of such a sex difference is relative finger length, especially the ratio of the index-to-ring-finger length (2D:4D), which in recent years has been widely used to study the effects of early sex differentiation [17,18]. The possible, if small, influence of hormones on the development of 2D:4D has been studied using a number of methods. As blood levels of testosterone are studied only in abortuses, evidence about developmental effects of prenatal hormones, such as sex differences in 2D:4D, has been somewhat indirect. Lutchmaya et al. [19] found an association between amniotic fluid testosterone: estradiol ratio and lower 2D:4D at 2 years old in sample of 18 boys and 15 girls. Other studies have considered associations with diseases affecting prenatal hormone function, especially congenital adrenal hyperplasia [20–25]. The current study investigated the possibility that established sex differences in the hands of infants are also associated with postnatal testosterone action. 1. Methods Participants include 32 girls and 42 boys from whom both a single saliva sample of adequate volume and a clear photocopy of the right hand were obtained between 3 and 5 months old. As previously described [26], saliva (b 15 ml) was collected by a sterile DeLee suction catheter from each infant at the end of a behavioral testing session. Saliva samples were immediately stored at -80 °C. Frozen samples were shipped overnight in dry ice to Salimetrics (State College, Pennsylvania), where salivary levels of testosterone and estradiol were measured in duplicate aliquots using enzyme immu- noassays (assay sensitivity b 1 pg/ml). Early Human Development 87 (2011) 349–351 ⁎ Corresponding author. Tel.: +1 407 823 4611; fax: +1 407 823 3498. E-mail address: mmcintyr@mail.ucf.edu (M.H. McIntyre). 0378-3782/$ – see front matter © 2011 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.earlhumdev.2011.01.046 Contents lists available at ScienceDirect Early Human Development journal homepage: www.elsevier.com/locate/earlhumdev