F. S. vom Saal

Ethological methods to study the effects of maternal exposure to estrogenic endocrine disrupters: a study with methoxychlor.

There has been increasing interest, both at the scientific and regulatory level, in the use of ethological methods for evaluating neural effects of endocrine disrupters. We present a series of ethological studies on the effects of maternal exposure to low, environmentally relevant doses (0.02, 0.2, and 2 microg/g mother bw/day) of the estrogenic pesticide methoxychlor (MXC) on behavior. From gestation day 11 to 17, female mice spontaneously drank oil with or without MXC; their maternal behavior was examined from postpartum days 2 to 15. MXC treatment during pregnancy produced slight changes in the expression of maternal behavior: females fed the lower MXC dose spent less time nursing the pups as compared to control dams. Their maternally exposed offspring were subjected to a series of behavioral tests at different ages. Maternal exposure to MXC affected behavioral responses to novelty in both sexes at periadolescence. The onset of male intrasex aggression was delayed in males prenatally exposed to low doses of MXC, since exposed males showed low levels of aggressive interactions during early adolescence but not after they reached adulthood. When adults, MXC-exposed females, but not males showed increased exploration in an unfamiliar open-field. While a sex difference was observed in the control group, with males being significantly more active in the open field than females, prenatal treatment with some MXC doses tended to decrease the sexual dimorphism in activity levels in the novel environment. Ethology, as the evolutionary study of behavior, may provide a framework for integrating a functional perspective (i.e., evolutionary significance) to studies on proximate mechanisms that can account for behavioral alterations induced by developmental exposure to endocrine disrupters.

Intrauterine positions and testosterone levels of adult male gerbils are correlated

Those male Mongolian gerbils (Meriones unguiculatus) that developed in intrauterine positions between two male fetuses had significantly higher levels of serum testosterone, as adults, than did those adult male gerbils that developed in intrauterine positions between two female fetuses. The endogenous testosterone levels of adult male gerbils were significantly positively correlated with both the sizes of their ventral scent glands and their frequencies of scent marking. We found no evidence of pulsatile release of testosterone in adult male gerbils.

The Interaction of Circulating Oestrogens and Androgens in Regulating Mammalian Sexual Differentiation

In mammals males are the heterogametic sex, and the Y chromosome codes for differentiation of the gonads into testes. With the exception of the primary sexual characteristic, gonadal sex, all other differences between males and females are thought to be mediated hormonally via secretions from the testes. In the absence of gonads, mammalian foetuses develop into phenotypic females (Jost 1972). The ovaries of female foetuses differentiate significantly later in development than do the testes, and it appears that the ovaries of female foetuses are not steroidogenic (cf., Gibori and Sridaran 1981). In mammalian species in which the period of gestation is relatively long, sexual differentiation is usually completed by birth. In short-gestation species, such as rats and mice, however, sexual differentiation commences during the last third of pregnancy and continues during the first week to 10 days of postnatal life. Previously, it was erroneously assumed that the differentiation of sexual behaviour in mice and rats began shortly after birth (Barraclough and Leathem 1954, Young et al. 1964). There is a critical coupling of the timing of the secretion of testosterone, the primary androgen secreted by the testes of foetuses and adults, and the development of both neural and peripheral androgen-target tissues. Extrinsic factors that interfere with this normal coupling (such as maternal stress or exposure to exogenous hormones) can radically alter the course of sexual differentiation.

Effects of maternal stress on puberty, fertility and aggressive behavior of female mice from different intrauterine positions

We examined the effects of maternal stress (bright light and heat) during the last third of pregnancy on subsequent reproductive and behavioral characteristics of female mice from different intrauterine positions. Female mice that develop in utero between two male fetuses (2M females) differ from females that develop between two female fetuses (0M females) in their serum concentrations of both testosterone and estradiol during the fetal period of sexual differentiation. After birth, 0M and 2M females differ in a wide range of reproductive characteristics. We examined the effects of maternal stress on the response to social cues regulating the timing of first vaginal estrus and the length of the first postpubertal estrous cycle when 4 0M or 4 2M females were housed together next to an adult male. Maternal stress decreased the inhibitory effect of being housed with other females in terms of the length of the first postpubertal estrous cycle, but this only occurred in 0M females. We found no effect of maternal stress or intrauterine position on the capacity to mate and remain pregnant, regardless of whether 0M or 2M females were stressed or not stressed during early pregnancy prior to implantation. While there was no effect of prior intrauterine position on interfemale aggression or behavior toward young, maternal stress did tend to reduce the likelihood that females (in diestrus) would exhibit aggression toward other females.

Responsiveness to testosterone of male gerbils from known intrauterine positions

Following either a) castration or b) both castration and implantation with capsules releasing a constant, physiological dose of testosterone, adult male Mongolian gerbils that had matured in intrauterine positions between two male fetuses still scent marked with greater frequency than did male gerbils that had matured in intrauterine positions between two female fetuses. We also found significant positive correlations between the relative frequency of scent marking exhibited by individual male gerbils when intact, after castration and after both castration and implantation with capsules releasing testosterone. Each of these findings is consistent with the view that differential exposure to testosterone, as a consequence of fetal intrauterine position, has lasting effects on the organization of scent-marking by male gerbils.

Intrauterine position effects in female swine: Subsequent reproductive performance, and social and sexual behavior

Uterine position, relative to the sex of adjacent fetuses, is known to affect the subsequent behavior and reproductive performance of rodents. This experiment was designed to determine if similar effects are present in female swine. Donor sows were slaughtered at Day 112 of gestation, the uteri removed and piglets delivered manually. Uterine position was recorded for each pig as between 2 females (0M, n=16), between a male and a female (1M, n=25) or between 2 male fetuses (2M, n=17). Females were reared in heterosexual littermate groups until 120 days and thereafter in unisexual groups of six. Females were checked twice daily for estrus using a mature boar beginning on Day 175. 2M females tended (P<0.15) to reach puberty sooner (187 days) than 0M (192 days) and 1M (194 days). The length of the estrous cycle and ovulation rates did not differ among groups over the first three estrous cycles. Duration of receptivity was similar among groups until the third estrus when the duration of standing heat was shorter for 2M females, both when measured in the presence (2.8, 2.7 and 2.1 days for 0M, 1M and 2M, respectively; P<0.10) and the absence of the male (2.1, 1.3 and 0.9 days for 0M, 1M and 2M, respectively; P<0.01). When in their home pen, 0M females received the fewest mounts from pen mates (P<0.01), but mounted most often. There were no differences in the frequency of anogenital sniffing or rooting the sides of pen mates. The frequency of agonistic activity was similar among groups, however 0M females participated in fewer fights (χ2=5.2, P<0.10) and won fewer than expected by chance (05; χ2=6.0, P<0.05). Twenty gilts (n=8, 5 and 7 for 0M, 1M and 2M gilts, respectively) were ovariectomized and treated with estradiol benzoate. When tested in the boars absence, the proportion of females showing a positive response to back pressure and the duration of this response was greater for 2M females (P<0.005). Relative attractivity was tested by allowing the boar simultaneous fenceline access to 0M–2M gilt dyads. When matched for stage of receptivity, 0M and 2M gilts were equally attractive. The results indicate that prenatal intrauterine position has little effect on adult agonistic behavior, length of the estrous cycle and ovulation rate, but may affect the subsequent sexual behavior of female swine.

On the Need for a National (U.S.) Research Program to Elucidate the Potential Risks to Human Health and the Environment Posed by Contaminants of Emerging Concern

Department of Civil Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States Leetown Science Center, U.S. Geological Survey, Kearneysville, West Virginia 25430, United States School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, Arizona 85287, United States Great Lakes WATER Institute, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53204, United States Iowa Water Science Center, U.S. Geological Survey, Iowa City, Iowa 52244, United States Lowell Center for Sustainable Production, University of Massachusetts-Lowell, Lowell, Massachusetts 01854, United States Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States Department of Biology, University of St. Thomas, Saint Paul, Minnesota 55105, United States Department of Biology, North Carolina State University, Raleigh, North Carolina 27695, United States Department of Chemical and Environmental Engineering, University of Arizona, Tucson, Arizona 85721, United States Division of Biological Sciences, University of Missouri-Columbia, Columbia, Missouri 65211, United States Product Safety & Regulatory Affairs, Procter & Gamble, Cincinnati, Ohio 45201, United States Humphrey Institute of Public Affairs, University of Minnesota, Minneapolis, Minnesota 55455, United States

Choice of animal feed can alter fetal steroid levels and mask developmental effects of endocrine disrupting chemicals

Exposure of fetuses to endocrine disrupting chemicals (EDCs), such as the estrogenic drug diethylstilbestrol (DES), disrupts development of the reproductive system and affects other aspects of adult phenotype including diseases, consistent with the developmental origins of health and disease hypothesis. To determine whether diet could influence the effects of DES, we compared mice fed a commonly used combination of soy-based Purina 5008 (breeding and lactation) and 5001 (post-weaning) with mice fed soy-based Purina 5002 throughout life. We exposed fetal CD-1 mice (F1) in utero on different feeds to a 0 (controls), low (0.1 mg/kg/day) or high (50 mg/kg/day) dose of DES via feeding the dam (F0) on gestation days 11–17. Compared to 5008, 5002 feed significantly increased serum estradiol in control fetuses. On 5008 (but not 5002) feed, DES significantly increased fetal serum estradiol at a low dose and reduced it at a high dose. Diet influenced the effects of in utero DES on F1 female onset of puberty and the uterine response to estradiol (an inverted-U dose–response relationship seen for DES on uterine weight with 5008/5001 feed was not observed with 5002). Both low- and high-dose DES reduced daily sperm production (DSP) in adult F1 males on 5008/5001 feed, whereas males fed 5002 showed no DES-induced reduction in DSP. Thus, we observed a number of low-dose effects of in utero DES exposure on Purina 5008/5001 feed that were not observed using Purina 5002, a feed commonly used in industry-funded toxicological studies conducted for regulatory purposes. Received 24 August 2010; Revised 26 October 2010; Accepted 2 December 2010; First published online 28 January 2011

Social inhibition of infanticide in male house mice

Although a variety of behavioural mechanisms are known to inhibit infanticide in male house mice, the potential for social subordination to inhibit infanticide has not been widely investigated. Sixty male CF-1 stock house mice (Mus domesticus) were grouped five per cage from weaning until 5 months of age. Each male was then isolated and tested for infanticide at 1-day and 25-days after being singly housed. Only 13% were infanticidal after 1-day of isolation, but after 25-days of isolation, 44% were infanticidal (P < 0.0001). Thus, being group-housed with other males inhibits infanticide in most CF-1 male mice. From an ecological perspective, the cage-by-cage data suggest that subordination is a behavioural counterstrategy by which a dominant breeding male can inhibit his rivals from killing pups, thus maintaining social stability and reproductive success in a typical house mouse microdeme.

THE EMERGING SCIENCE OF ENDOCRINE DISRUPTION

Environmental Health Sciences, White Hall VA 22987-0125 USA Department of Zoology, University of Florida, Gainesville, FL, 32611-7300 USA Dipartimento di Biologia Evolutiva e Funzionale, Parma University, 43100 Parma, Italy Department of Family and Community Medicine, University of Missouri-Columbia School of Medicine, Columbia, Missouri, 65211 USA 5 Division of Biological Sciences, University of Missouri-Columbia, Columbia, Missouri, 65211 USA