Effects of prior intrauterine position and housing on oestrous cycle length in adolescent mice

Sexual dimorphism in placental development and function: Comparative physiology with an emphasis on the pig

Across mammalian species, it has been demonstrated that sex influences birth weight, with males being heavier than females; a characteristic that can be observed from early gestation. Male piglets are more likely to be stillborn and have greater preweaning mortality than their female littermates, despite the additional maternal investment into male fetal growth. Given the conserved nature of the genome between the sexes, it is hypothesized that these developmental differences between males and females are most likely orchestrated by differential placental adaptation. This review summarizes the current understanding of fetal sex-specific differences in placental and endometrial structure and function, with an emphasis on pathways found to be differentially regulated in the pig including angiogenesis, apoptosis, and proliferation. Given the importance of piglet sex in agricultural enterprises, and the potential for skewed litter sex ratios, it is imperative to improve understanding of the relationship between fetal sex and molecular signaling in both the placenta and endometria across gestation.

Associations between maternal vitamin D status and porcine litter characteristics throughout gestation

Emerging evidence suggests an important role of vitamin D in the establishment and maintenance of pregnancy, and the regulation of foetal growth across mammalian species. However, the temporal changes in maternal vitamin D status throughout gestation in the pig and the relationship between maternal vitamin D status and litter characteristics of interest across gestation remain poorly understood and under-investigated. The abundance of 25(OH)D in maternal plasma was quantified by HPLC–MS/MS at gestational days (GD) 18, 30, 45, 60 and 90 (n = 5–11 gilts/GD). Maternal plasma 25(OH)D concentrations significantly increased between GD18 and GD30 (P < 0.05). The relationship between maternal vitamin D metabolite concentrations and litter characteristics of interest including gilt weight, ovulation rate, mean litter weight, number of live foetuses, percentage prenatal survival, and sex ratio of the litter was assessed. Maternal 25(OH)D (P = 0.059) concentrations tended to be positively associated with percentage prenatal survival on GD60. On GD90, maternal 25(OH)D (P < 0.05) concentrations were inversely associated with gilt weight. Maternal plasma 25(OH)D concentrations were inversely associated with the percentage of male foetuses in the litter on GD90 (P < 0.05). This study has provided novel insights into temporal changes in maternal vitamin D status throughout gestation and the relationship between maternal vitamin D status and the economically important litter characteristics of gilt weight, percentage prenatal survival and percentage of male foetuses in the litter. Improving the understanding of the role of vitamin D across important developmental timepoints in relation to foetal growth is essential to improve reproductive success in livestock species.

Placental miR-340 mediates vulnerability to activity based anorexia in mice

Anorexia nervosa (AN) is a devastating eating disorder characterized by self-starvation that mainly affects women. Its etiology is unknown, which impedes successful treatment options leading to a limited chance of full recovery. Here, we show that gestation is a vulnerable window that can influence the predisposition to AN. By screening placental microRNA expression of naive and prenatally stressed (PNS) fetuses and assessing vulnerability to activity-based anorexia (ABA), we identify miR-340 as a sexually dimorphic regulator involved in prenatal programming of ABA. PNS caused gene-body hypermethylation of placental miR-340, which is associated with reduced miR-340 expression and increased protein levels of several target transcripts, GR, Cry2 and H3F3b. MiR-340 is linked to the expression of several nutrient transporters both in mice and human placentas. Using placenta-specific lentiviral transgenes and embryo transfer, we demonstrate the key role miR-340 plays in the mechanism involved in early life programming of ABA.

Anorexia nervosa is characterised by self-starvation but its etiology is not completely understood. Here the authors describe how prenatal stress can induce activity-based anorexia in the offspring during early adulthood by upregulating miR-340 expression in the placenta that affects expression of nutrient transporters.

Hormones and endocrine-disrupting chemicals: low-dose effects and nonmonotonic dose responses

For decades, studies of endocrine-disrupting chemicals (EDCs) have challenged traditional concepts in toxicology, in particular the dogma of "the dose makes the poison," because EDCs can have effects at low doses that are not predicted by effects at higher doses. Here, we review two major concepts in EDC studies: low dose and nonmonotonicity. Low-dose effects were defined by the National Toxicology Program as those that occur in the range of human exposures or effects observed at doses below those used for traditional toxicological studies. We review the mechanistic data for low-dose effects and use a weight-of-evidence approach to analyze five examples from the EDC literature. Additionally, we explore nonmonotonic dose-response curves, defined as a nonlinear relationship between dose and effect where the slope of the curve changes sign somewhere within the range of doses examined. We provide a detailed discussion of the mechanisms responsible for generating these phenomena, plus hundreds of examples from the cell culture, animal, and epidemiology literature. We illustrate that nonmonotonic responses and low-dose effects are remarkably common in studies of natural hormones and EDCs. Whether low doses of EDCs influence certain human disorders is no longer conjecture, because epidemiological studies show that environmental exposures to EDCs are associated with human diseases and disabilities. We conclude that when nonmonotonic dose-response curves occur, the effects of low doses cannot be predicted by the effects observed at high doses. Thus, fundamental changes in chemical testing and safety determination are needed to protect human health.

Mouse females devoid of exposure to males during fetal development exhibit increased maternal behavior

Many sex differences can be found in the expression of aggression and parental nurturing behaviors. It is important to determine if these are modulated by prenatal conditions. Here, using assisted reproduction technologies, we generated females that were (mixed-sex) or were not (same-sex) exposed to males during fetal development, raised them by cross fostering among fosters' own female only pups to control for effects of postnatal environment, and compared their reproductive abilities and behavior. There were no differences between females from the two prenatal conditions in estrus cycle length and length of time spent at individual estrus cycle stages. Both types of females had similar ovulation efficiency and bred equally well yielding comparable litter size and progeny sex ratio. Females from the two prenatal conditions were also indistinguishable in social behavior and exhibited normal social responses towards unfamiliar females in the three-chamber social approach and social proximity tests. When urine was collected from both types of females and used as a point source in a scent-marking paradigm, exposed males showed a similar distribution and extent of urinary scent marking in response to urine from each type of female but tended to engage in higher durations of sniffing the urine from same-sex females. When females were tested in a resident-intruder paradigm 3 days after giving birth, same-sex females exhibited enhancement of pup grooming and an overall decrease of non-pup activity prior to male intruder introduction, and after introduction were more defensive as evidenced by higher rates of burying, open-mouth threat/lunges, and attacks towards the male, and decreased latencies to display these defensive behaviors. Our results suggest that females devoid of male exposure during fetal development have reproductive abilities similar to those of females from mixed-sex pregnancies, and have normal social interactions with other females. However, they exhibit hyper-maternal behavior both in terms of the care and defense of pups in front of a male intruder, and potentially produce a pheromonal milieu that renders them more attractive to males during olfactory investigations.

Bisphenol-A and the great divide: a review of controversies in the field of endocrine disruption

In 1991, a group of 21 scientists gathered at the Wingspread Conference Center to discuss evidence of developmental alterations observed in wildlife populations after chemical exposures. There, the term "endocrine disruptor" was agreed upon to describe a class of chemicals including those that act as agonists and antagonists of the estrogen receptors (ERs), androgen receptor, thyroid hormone receptor, and others. This definition has since evolved, and the field has grown to encompass hundreds of chemicals. Despite significant advances in the study of endocrine disruptors, several controversies have sprung up and continue, including the debate over the existence of nonmonotonic dose response curves, the mechanisms of low-dose effects, and the importance of considering critical periods of exposure in experimental design. One chemical found ubiquitously in our environment, bisphenol-A (BPA), has received a tremendous amount of attention from research scientists, government panels, and the popular press. In this review, we have covered the above-mentioned controversies plus six additional issues that have divided scientists in the field of BPA research, namely: 1) mechanisms of BPA action; 2) levels of human exposure; 3) routes of human exposure; 4) pharmacokinetic models of BPA metabolism; 5) effects of BPA on exposed animals; and 6) links between BPA and cancer. Understanding these topics is essential for educating the public and medical professionals about potential risks associated with developmental exposure to BPA and other endocrine disruptors, the design of rigorously researched programs using both epidemiological and animal studies, and ultimately the development of a sound public health policy.

The impact of unabated stimulation by human chorionic gonadotropin on the steroid hormone environment of pregnant rats and the spontaneous expression of ovarian cysts in female progeny

Unabated stimulation by low doses of human chorionic gonadotropin (hCG) induces ovarian cysts in pregnant rats. In order to determine the impact of these in vivo treatments on the hormonal milieu of pregnancy, and the potential impact of an aberrant cystic-ovary state during pregnancy on the resulting female offspring, pregnant rats were treated with either 0 (control), 1, or 3 IU hCG twice daily for at least 9 days, beginning on day 13 of pregnancy. Serum was harvested from control and hCG treated animals on days 15, 17, 19, and 22 of pregnancy. Control pregnant rats and animals treated with 1 IU hCG shared similar serum profiles for progesterone (P4), androstenedione (A4), 5alpha-androstane-3alpha,17beta-diol (3alpha-diol), androsterone (A5), and estrone (E1) between days 15 and 22 of pregnancy. Testosterone serum concentrations were similar for control and 1 IU hGG-treated pregnant rats between days 15 and 19 of pregnancy; whereas, on day 22, 1 IU hGG-treated pregnant rats displayed lower serum testosterone than control pregnant rats (P < or = 0.05). In contrast, serum estradiol (E2) concentrations for 1 IU hCG- treated pregnant rats were greater than E2 values observed for control rats on days 15-19 of pregnancy (P < or = 0.05). Serum testosterone and 3alpha-diol values for 3 IU hCG-treated pregnant rats differed from those of control pregnant rats only on day 19 when these values were transiently greater for these hCG-treated animals compared with serum values for control pregnant rats (P < or = 0.05). Serum A4 values for 3 IU hCG-treated pregnant rats were elevated compared to values for control pregnant rats only on days 15 and 17 (P < or = 0.05). In contrast, serum E1, A5, and E2 were elevated on days 19-22, 17-22, and 15-22, respectively, in 3 IU hCG-treated pregnant rats compared to control pregnant rats (P < or = 0.05). No pups from control pregnant rats displayed ovarian cysts during the time they were observed postnatally. In contrast, 6 of 25 pups from 3 IU hCG-treated pregnant rats displayed cystic ovaries, without corpora lutea, on day 55 of age. Serum steroid concentrations for these cyst-bearing progeny were similar to those of female progeny from control pregnant rats, whereas female progeny without ovarian cysts from 3 IU hCG-treated pregnant rats displayed differences in serum steroid values from those of progeny from control pregnant rats (P < or = 0.05). The data support the concept that an aberrant, yet physiologic hormonal environment associated with the induction of ovarian cysts during pregnancy in rats, can lead to the spontaneous establishment of an ovarian cystic state in at least a subset of the female progeny. Further, the date suggest that tonically increased ovarian estrogen production during pregnancy, reflected by tonically elevated peripheral serum estrogen concentrations, may play a pivotal role in the etiology of an ovarian cystic state in this subset of daughters from hCG-induced, cyst-bearing pregnant rats.

The onset of puberty in female mice as reflected in urinary steroids and uterine/ovarian mass: interactions of exposure to males, phyto-oestrogen content of diet, and ano-genital distance

Development of puberty in female mice was examined in relationship with the ano-genital distance index (AGDI), phyto-oestrogen content of diet and exposure to males post weaning. Throughout gestation and post-natal development, females were exposed to a regular diet or a nutritionally similar diet deficient in phyto-oestrogens. After segregation at weaning on the basis of short or long AGDI, an indirect measure ofin uteroandrogen exposure, females were housed alone or underneath two outbred adult males for 2 weeks. Female urinary samples were collected non-invasively throughout this exposure, then assayed for oestradiol, progesterone and creatinine. Females were then killed and uterine and ovarian mass was determined. Urinary oestradiol was substantially reduced in females raised on the phyto-oestrogen-free diet. Oestradiol levels were more dynamic over days in urine of male-exposed females, especially among those on the regular diet. Urinary progesterone was not strongly influenced by diet. Progesterone was more dynamic in urine of male-exposed females, and was generally elevated compared with levels in isolated females, the size of this effect dependent on AGDI, diet and whether the measure was adjusted for creatinine. Urinary creatinine was elevated by the phyto-oestrogen-free diet and reduced by male exposure, tending to decline over days in females exposed to males. Male exposure increased uterine and ovarian mass and was influenced by AGDI in interaction with diet and male exposure.

Anogenital distance measured at weaning is correlated with measures of blood chemistry and behaviors in 450-day-old female mice

In female mice, anogenital distance (AGD), measured at weaning, provides an estimate of uterine exposure to testosterone from flanking male mouse littermates. A variant of the anogenital distance index (AGDI) that uses the residual value of AGD after accounting for the effect of weight by regression (AGDWTRES) was measured at weaning in F(2) female mice from a C57BL/6J x DBA2/J cross. AGDWTRES was used to examine the relationship between intrauterine environment and blood chemistry variables and activity-related behaviors when the females were 450 days old. Longer AGDWTRES values correlated with lower levels of calcium, cholesterol, phosphorus, iron, and protein, which is opposite to the expected direction, based on underlying sex differences for blood chemistry. A positive correlation was found between AGDWTRES and two activity-related measures (the number of rears in a test of exploration, and the number of sectors of a rod that are entered by the mouse). These findings suggest that in utero proximity to males, as indexed by AGDWTRES, may have effects on fundamental aspects of blood chemistry and behavior that extend well into mouse middle age, and could play an important role in health.

Risk-taking behavior in adolescent mice: psychobiological determinants and early epigenetic influence

Epidemiological research has emphasized that adolescence is associated with some temperamental and behavioral traits that are typical of this age and that might substantially contribute to both psychological and psychobiological vulnerability. The contribution of the important developmental rearrangements in neurobiological and neuroendocrinological processes has received surprisingly little investigation. The present review summarizes recent work in animal models, indicating that adolescent rodents exhibit marked peculiarities in their spontaneous behavioral repertoire. When compared to adults, adolescents show an unbalanced and 'extremes-oriented' behavior, consisting of an increased novelty seeking, together with decreased novelty-induced stress and anxiety, an increased risk-taking behavior in the plus-maze, as well as elevated levels of impulsivity and restlessness. Age-related discontinuities in the function of monoaminergic systems, which are a main target of abused drugs, can perhaps account for such a profile. In particular, a peculiar function within reward-related dopaminergic brain pathways actually seems to underlie the search for novel and rewarding sensations, as well as changes in the magnitude of psychostimulant effects. The role played by early epigenetic factors in the shaping of novelty-seeking behavior of adolescent and adult rodents are also reviewed. Two examples are considered, namely, subtle variations in the hormonal milieu as a function of intrauterine position and precocious or delayed maturation of nutritional independence as a function of changes in time of weaning. As for spontaneous drug consumption, a prominent vulnerability to the oral intake of nicotine during early adolescence is reported. In conclusion, adolescence in rodents may represent a suitable animal model with enough face- and construct-validity. Actually, this model is able to show behavioral features that resemble those found in human adolescents, including vulnerability to the consumption of psychoactive drugs.

Intrauterine position effects

A review of the literature suggests that individual variability in sex-related traits may be influenced by variations in hormonal exposure during fetal development. In litter-bearing mammals, fetuses develop in utero and may be subjected to differing hormonal environments based upon the sex of neighboring fetuses. Female fetuses developing between two males tend to show masculinized anatomical, physiological and behavioral traits as adults. Female fetuses developing without adjacent males, on the other hand, tend to show more feminized traits as adults. These traits include permanently altered hormone levels, reproductive organs, aggressive behaviors, secondary sex ratios and susceptibility to endocrine disruption. This intrauterine effect is due to the transfer of testosterone from male fetuses to adjacent fetuses. While these effects have been most clearly demonstrated in mice, other rodents and swine also show intrauterine position (IUP) effects. Some of these effects are similar to the influence of prenatal stress on adult phenotypes. A few reports on human twins suggest that variability in some masculine and feminine traits may be due to intrauterine hormonal signals. IUP effects may impact a number of scientific fields of research such as endocrine disruption, toxicology, population biology, animal production and health.

Novelty seeking in periadolescent mice: sex differences and influence of intrauterine position

In rodents, beside basic sex differences, a certain degree of within-gender phenotypic variation can also be provided in utero by hormones from adjacent fetuses. We investigated novelty-seeking behavior in two groups of male and female mice from know intrauterine position: 2M (between males) and 0M (between females). Subjects were assessed during periadolescence (postnatal days 33--43), an ontogenetic phase, which is characterized by an elevated expression of this novelty-seeking behavior. Periadolescent mice underwent a familiarization session for 3 consecutive training days with one side of a two-chamber apparatus. On testing day 4, the opening of a partition, which allowed mice to freely move from the familiar compartment to a novel one, produced an increased behavioral arousal in all animals. Marked sex differences were found, with females being in general more active than males, whereas the latter showed significantly higher levels of novelty seeking than females. Uterine position failed to affect the profile of novelty preference in females, whereas within the male group 2M subjects expressed a marked profile of novelty seeking. The differential titers of sex hormones reported to characterize the 0M and 2M condition early in fetal development are suggested to account for the individual variability in the seeking for novelty within the male group during puberty.

Sensation seeking in opposite-sex twins: an effect of prenatal hormones?

Intrauterine hormones and position with respect to male and female littermates influence sexually dimorphic adult behavior in litter-bearing animals. Opposite-sex dizygotic twins offer the opportunity to examine analogous effects on sex-related human behaviors. To illustrate this approach, Sensation Seeking Scale (SSS) scores from 422 British twin pairs, including 51 opposite-sex pairs (Zuckerman, M., et al., J. Consult. Clin. Psychol. 46:139-149, 1978), were reanalyzed. Zuckerman et al. (1978) have shown that some aspects of sensation seeking are consistently increased in males relative to females. In comparing age-adjusted data for opposite and same-sex twins, our reanalysis demonstrated the predicted increase in sensation seeking in female members of opposite-sex pairs. Results were significant for measures of disinhibition, experience seeking, and overall sensation seeking. In contrast, male opposite-sex twins were not significantly different from male same-sex twins. Although psychosocial explanations of the increased sensation seeking in opposite-sex female twins cannot be excluded, these finding are consistent with hypothesized in utero hormonal influences on later behavioral development.

Pheromonal regulation of the mouse estrous cycle by a heterogenotypic male

The role of male pheromones in estrous cyclicity was studied in mice selected for different reproductive traits. When females are exposed to males of their own strain, estrous cycles are highly regular in females selected for increased embryo survival (line E). In contrast, cycle regularity is reduced by exposure of line E females to males from a strain characterized by irregular estrous cycles (line CN-). To investigate the inhibition of estrous cyclicity and the role of androgen in this phenomenon, line E females were housed in the olfactory presence of E males and later rehoused with one of the following: intact or castrated males of line E (homogenotypic condition) or line CN-heterogenotypic condition) or castrated CN- males provided with testosterone replacement. A final exposure to homogenotypic (line E) males was provided. Estrous cyclicity was decreased when line E females were rehoused with intact or castrated CN- males. Metestrus was prolonged by intact CN- males, whereas diestrus was prolonged in the presence of castrated CN- males. Androgen treatment did not enable castrated CN- males to prolong metestrus. These results demonstrate that: 1) heterogenotypic pheromones inhibit estrous cyclicity in line E; and 2) the inhibitory influence of line CN- males on line E estrous cyclicity is mediated by factors in addition to or other than testosterone.

In utero contiguity to males does not influence morphology, behavioral sensitivity to testosterone, or hypothalamic androgen binding in CF-1 female mice

Physiological and behavioral systems presumably influenced by prenatal exposure to testosterone (T) were compared in CF-1 female mice from known uterine positions. Anogenital distance did not differ among females that developed in utero between two females (0M), adjacent to one male (1M), or between two males (2M) at birth, at weaning on Day 21, or on Day 60 postpartum. The age of vaginal opening and mean estrous cycle length also were similar among the groups. When ovariectomized and implanted with a T-containing silastic capsule, the mean number of days of treatment required to activate male-like aggressive behavior also did not differ among the three positional classifications. Finally, androgen binding in combined hypothalamic-preoptic-septal cytosol was assessed after 8 days of T treatment, and no systematic variation in [3H]DHT binding related to uterine position was found. These results indicate that contiguity to male fetuses did not induce variation among CF-1 females in morphological, behavioral, or biochemical systems thought to be influenced by prenatal exposure to T.

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.

Effects of social stress and intrauterine position on sexual phenotype in wild-type house mice (Mus musculus)

Wild-type house mice were used to test the effect of intrauterine position on anogenital distance (AGD) and to verify whether crowding stress would masculinize female pups, developing at all intrauterine positions, as has been demonstrated in CF-1 mice stressed by restraint, heat, and light. Stress of crowding was documented by comparing aggressive behavior, litter birth weights, and plasma corticosterone levels among females in different densities. AGDs were recorded from pups born to females housed from day 12 to 19 of gestation either individually with their mate (nonstressed) or in one of two group-housed densities. Female pups from nonstressed dams positioned between two males in utero (2M females) had longer AGDs than females positioned between two females (0M females). AGDs of males from nonstressed dams did not differ on the basis of intrauterine position. Group-housed pregnant females in the higher of two densities had female pups with longer AGDs than female pups of other dams. However, variance in female pup AGD was no different among dams in different densities. These results extend to the wild house mouse previous findings in albino mice that intrauterine position influences sexual phenotype. In addition, social stress can induce masculinization of female pups in wild mice as physical stress has been shown to do in albinos. This suggests that intrauterine position effects and their modification by crowding stress can potentially influence the dynamics of wild house mouse populations.

Fetal effects on sexual behavior and aggression in young and old female mice treated with estrogen and testosterone

During fetal life female mice (Mus musculus) that develop between two male fetuses (2M females) have higher blood concentrations of testosterone than do females that do not develop next to a male fetus (0M females). In the first experiment reported here, sexual receptivity and sexual attractiveness to males were examined in young (5 month old) and old (17 month old) ovariectomized, estrogen- and progesterone-treated 0M and 2M female mice that were placed in like-age pairs with a male. Most males inseminated the 0M female prior to inseminating the 2M female regardless of age. In addition, 0M females were more likely to exhibit lordosis when mounted than were 2M females. When the same young females were 9 months of age and the old females were 21 months of age, they were treated with testosterone and again placed together in pairs along with a sexually receptive female. Young 2M females exhibited more aggression toward the testosterone-treated female partner, and also exhibited more mounting of the receptive female, than did young 0M females. But, both old 0M and old 2M females were highly aggressive and exhibited mounting. An increase in sensitivity to the effects of testosterone on behavior thus occurs during aging in 0M females, which are relatively insensitive to testosterone in young adulthood. In contrast, when treated with estrogen and progesterone, 0M females were more attractive to males and were more sexually receptive than 2M females regardless of age.

Testosterone induced mounting behavior in adult female rats born in litters of different female to male ratios

Female rats can show mounting behavior towards isosexual or heterosexual conspecifics. The present experiments were designed to study whether the prenatal presence of male fetuses would affect display of this mounting behavior in adulthood. Therefore mounting behavior, after gonadectomy and during continuous treatment with testosterone propionate (TP) was studied in female rats which were born in litters without male siblings ("all female" litters) and in litters with a variable number of male siblings. Litter composition at birth did not affect display of adult mounting behavior during protracted tests (a total of 6 tests during 8 weeks of treatment with TP). The data indicate that TP induced mounting behavior in adulthood occurs independent of the prenatal presence of male fetuses. If mounting behavior in adulthood has to be "organized" by prenatally present androgen (the current way of thinking), then the present data indicate that female rat fetuses provide themselves with these hormones. It would then seem inappropriate to judge adult mounting behavior as a sign of "masculinization" of the rat brain.

The regulation of infanticide and parental behavior: implications for reproductive success in male mice

Infanticide has been proposed to be a pathological response to overcrowding or other forms of environmental stress and thus a maladaptive behavior. However, in male house mice this behavior is predictable and is modulated by learning. Committing infanticide can increase a male's reproductive success and in some situations may therefore be an adaptive behavior.