ELSEVIER Behavioural Brain Research 62 (1994) 119-125 BEHAVIOURAL BRAIN RESEARCH Research Report Males located caudally in the uterus affect sexual behavior of male rats in adulthood E.J. Houtsmuller a'*, J. Juranek b, C.E. Gebauer b, A.K. Slob a, D.L. Rowland b a Department o! Endocrinology and Reproduction, Facultyof Medicine and Health Sciences, Erasmus Universi O" Rotterdam, PO Box 1738, 3000 DR Rotterdam, The Netherlands b Department of Psycholog3'. Valparaiso Universio'. Valparaiso, IN 46383. USA Received 26 February 1993: revised 19 November 1993; accepted 5 January 1994 Abstract It has been suggested that the sexual differentiation of female rats is affected by androgens from male fetuses in the uterine horn (intra-uterine position phenomenon). Effects of adjacent males, as well as of males located caudally in the uterus have been reported. The present study investigated whether male rats, like females, are affected by the presence of either caudal or adjacent male litter- mates. When tested in adulthood for sexual behavior, males that had male fetuses located caudally in the uterine horn showed shorter latencies to the first mount or intromission and shorter latencies to ejaculation, and exhibited more mounts and intromissions per minute than males that lacked caudal male siblings in the uterus. The presence of adjacent males did not significantly affect the parameters studied in this experiment. Key word~v Intra-uterine position; Sexual behavior; Ejaculation latency; Sexual differentiation; Caudal; Prenatal environment; Male rat 1. Introduction As a result of exposure to higher levels of testosterone perinatally, male rats show more masculine sexual behav- ior (mounts, intromissions and ejaculations) in adulthood than females [2]. Testosterone secretion in male rat fe- tuses starts around day 14 of gestation, and peaks on days 18 and 19 [3,22,28]. This testosterone not only affects males" own sexual differentiation, but supposedly affects female fetuses in the same uterine horn as well [9,19,26]. The manner in which androgens from males reach fe- male fetuses remains uncertain. It has been reported that females thai develop in utero between two males are more masculinized behaviorally (i.e., show more mounting be- havior) and somatically (i.e., have larger anogenital dis- tances at birth) than females located between two females. This finding prompted the hypothesis that androgens from males reach females by diffusion through the amniotic membrane {"adjacent male hypothesis") [9,26]. Support for this hypothesis has most often been provided in studies * Corresponding author. Present address: Dept. of Biological Sciences. University of Notre Dame, Notre Dame, 1N 46556, USA. Fax: (1)(219) 631-7413. E-mail: Elisabcth J. Houtsmuller @nd.edu 0166-4328,94 $7.110 © 1994 Elsevier Science B.V. All rights reserved SSDI 01 66-4328(94)00025- B using mice and gerbils (e.g. [8,25,29,30]), although non- supportive findings in mice and rats have been reported as well [ 16,17,19]. In view of their disparate findings, Meisel and Ward [19] proposed the alternative hypothesis that androgens from males reach females through the blood supply, which supposedly flows from the cervix in a ros- tral direction [ 12]. This hypothesis was derived from the observation that female rats that have males located cau- dally (i.e. females that are located "upstream" from males with regard to bloodflow) in the uterus are somatically and behaviorally more masculinized than females without such males. Several, though not all (e.g [3,4]), studies have provided subsequent support for this hypothesis in rats [17,20], hamsters [24] and ferrets [18]. In studies addressing this ~'intrauterine position phenomenon", effects of male fetuses on females have been described extensively in a variety of species. Only recently it was reported that males, like females, may also be affected by hormones secreted by littermates in the same uterine horn. Specifically, male mice located between two females had higher levels of estradiol prenatally and showed, surprisingly, more mounting behavior and less aggression in adulthood than other males [27]. Male ger- bils that developed in utero between two males were found