Photoperiod influences reproduction in the prairie vole (Microtus ochrogaster)

Four experiments examined the role of photoperiod in the regulation of seasonal breeding in the prairie vole. Adult male voles maintained in short (8L:16D) as compared to long (16L:8D) photoperiods for 10 wk had reduced testicular and seminal vesicle weights, but fertility was not impaired. Male prairie voles reared from birth until 35 days of age in short as compared to long photoperiods also had reduced testicular and seminal vesicle weights, as well as diminished fertility. The incidence of pregnancy did not differ between long- and short-day female voles paired for 6 days with long- or short-day males (93%, 86%, 89%, and 88%, respectively). Photoperiod did not affect the incidence or the timing of postpartum pregnancies in long- or short-day females paired with long-day males through the birth of several litters. Adult male prairie voles retain only marginal sensitivity to short photoperiods, maturing males are highly responsive to short days, and adult females are insensitive to photoperiod. These data suggest that termination of the breeding season in the autumn may be due to the lack of fecund males in the population.     

The effects of mate removal on pregnancy success in prairie voles (Microtus ochrogaster) and meadow voles (Microtus pennsylvanicus).

The effects of removing the stud male have not been controlled in many studies relating pregnancy block to the presence of an unfamiliar male. We examined the effects of removing the male on pregnancy success in prairie voles and meadow voles, two species that differ in degree of paternal investment. Whereas prairie vole males provide extensive care to offspring and accelerate pup development, meadow vole males display little or no care and delay development of pups. We predicted that removal of the stud male would decrease pregnancy success in prairie voles and either have no effect or increase success in meadow voles. In experiment 1, females were in male-induced estrus, and their mates were either left with them or were removed 4 h, 1 day, 2 days, or 8 days after mating. In experiment 2, females were in postpartum estrus, and their mates were either left with them or were removed 1 day, 2 days, or 8 days after birth of their first litter. Removal of the male soon after mating in postpartum estrus decreased pregnancy success in prairie voles and increased success in meadow voles. Thus, although removal of the stud male influenced litter production, the direction of the effect varied with species.     

Elevated Fos-like immunoreactivity in the brains of postpartum female prairie voles, Microtus ochrogaster

This study investigated the expression of the protein product of the immediate early gene c-fos in the brains of female prairie voles (Microtus ochrogaster) in association with pregnancy and postparturient activities including maternal behavior, lactation and postpartum estrus. Fos expression was assessed in female voles that were late in pregnancy, nonpregnant or at one of three different times postpartum (0-8, 12-24, and 24-48 h, respectively). A significant increase in the number of cells displaying Fos immunoreactivity (Fos-ir) was observed during the 0-8 h and 12-24 h postpartum time periods in the accessory olfactory bulbs, medial preoptic area, hypothalamus (specifically, the supraoptic nucleus, ventro-medial hypothalamus, and paraventricular nucleus), lateral septum, bed nucleus of the stria terminalis, and primary somatosensory area of the brain. The number of Fos-ir cells decreased after 24 h postpartum. There were no significant changes in Fos-ir cell numbers in the primary olfactory bulbs, hippocampus, or caudate putamen. The neural activation of the medial preoptic area, accessory olfactory bulbs, hypothalamus, and bed nucleus is consistent with reports in rats of Fos induction associated with the onset of maternal behavior. In voles postpartum estrous behavior begins and ends 0-12 h after parturition. Maternal behavior, including lactation, is initiated at the same time but persists for several weeks. The highest Fos-ir cell numbers reported here coincide with the timing of postpartum estrous behavior in this species.     

A comparison of maternal behaviour in the meadow vole (Microtus pennsylvanicus), prairie vole (M. ochrogaster) and pine vole (M. pinetorum)

The maternal behaviour of three species of voles was compared using a semi-naturalistic laboratory system. Meadow vole females spent more time out of the nest, and exhibited less maternal behaviour in terms of nursing and brooding, and more non-social behaviour such as locomoting, eating, and drinking, than the females of the other two species. Across all species, maternal behaviour decreased from parturition onward, while non-social behaviour increased. Paternal care was well developed in the prairie vole and pine vole, but was never observed in the meadow vole. During the first postnatal week, male prairie voles showed a tendency to enter the nest when the female left the young unattended. This trend was not apparent in the male pine vole. The physical parameters of pup development, including eye opening and the development of fur, were similar in all three species. Behavioural development, however, was most rapid in the meadow vole, intermediate in the prairie vole, and slowest in the pine vole. These results are compared with those of previous field and laboratory studies, and are discussed with reference to the life-history strategy of each species.     

Male copulatory behavior and the induction of ovulation in female voles: a quest for species specificity.

Two experiments were conducted to investigate species specificity in the neuroendocrine responsiveness of female prairie voles to the copulatory patterns of males. In Experiment 1, prairie vole males mated for one ejaculatory series were not significantly more effective in inducing ovulation in prairie vole females than montane voles mated with prairie vole females for one series, two series, or to satiety. Mating with conspecific males did result in significantly more implanted embryos than did heterospecific matings. In Experiment 2, it was found that, when the amount of vaginal stimulation was both low and equated across groups, prairie vole males were significantly more effective in triggering ovulation in female prairie voles than were either meadow voles or montane voles. Although there appears to be some species specificity to the “vaginal codes” of these congeneric species, its biological significance is unclear.     

Physiology of a Microgravity Environment Selected Contribution: Effects of spaceflight during pregnancy on labor and birth at 1 G

The events of parturition (labor,delivery, maternal care, placentophagia, and onset of nursing) wereanalyzed in female Norway rats (Rattus norvegicus) flown oneither 11- or 9-day-long spaceflights beginning at the approximatemidpoint of their pregnancies. Each space shuttle flight landed on the20th day of the rats' pregnancies, just 48-72 h beforeparturition. After spaceflight, dams were continuously monitored andrecorded by time-lapse videography throughout the completion ofparturition and onset of nursing (days 22 and23). Analyses of parturition revealed that, compared with groundcontrols, flight dams displayed twice the number of lordosiscontractions, the predominant labor contraction type in rats.The number of vertical contractions (those that immediately precedeexpulsion of a pup from the womb), the duration of labor, fetalwastage, number of neonates born, neonatal birth weights, placentophagia, and maternal care during parturition, including theonset of nursing, were comparable in flight and ground control dams.Our findings indicate that, with the exception of labor contractions,mammalian pregnancy and parturition remain qualitatively andquantitatively intact after spaceflight during pregnancy.

     

Influence of male gonadal hormones and familiarity on pregnancy interruption in prairie voles

Pregnancy interruption (PI) was examined in female prairie voles, Microtus ochrogaster, exposed to stimuli from males 7 to 12 days after pairing. Urine from unfamiliar males interrupted pregnancy when placed directly on the external nares of newly mated females, but urine from familiar stud males was without effect. Castration of males did not reduce the efficacy of unfamiliar male urine in interrupting pregnancy. The neuroendocrine system of female prairie voles responded selectively to male urine as a function of its familiarity; the efficacy of male stimuli leading to PI was not dependent on gonadal hormones.     

Lever pressing for pups: Evidence for hormonal influence upon maternal behavior of mice

Five experiments were performed to determine the extent to which hormones influence maternal behavior of the female mouse. The number of pup-reinforced lever presses served as the measure of maternal behavior. In Experiment 1 it was found that postpartum mice performed significantly more pup-reinforced lever presses than intact or ovariectomized virgins. The second experiment showed that the augmentation of responding was unique to the postpartum period since responding was not facilitated during the last three days of pregnancy. Experiment 3 established that the postpartum augmentation of responding was mediated by the termination of pregnancy and not by parturition per se. It also was observed that the earlier during gestation pregnancy was terminated the greater the decline in response rate. Experiment 4 proved that ovarian secretions play a critical role in the pregnancy-terminated facilitation of pup-reinforced lever pressing. The final experiment demonstrated that the termination-of-pregnancy effect on responding in all likelihood is mediated by the synergistic actions of estrogen stimulation and progesterone withdrawal. The results show that the maternal behavior of the mouse, like that of the rat, is responsive to hormonal stimulation.     

Sex differences and developmental effects of oxytocin on aggression and social behavior in prairie voles (Microtus ochrogaster)

Various hormones, including sex steroids and neuropeptides, have been implicated in aggression. In this study we examined (1) sex differences in intrasexual aggression in na?ve prairie voles; (2) the effects of developmental manipulations of oxytocin on intrasexual aggression; and (3) changes in patterns of intrasexual aggression after brief exposure to an animal of the opposite sex. Within 24 h of birth, infants were randomly assigned to receive either an injection of oxytocin (OT) or oxytocin antagonist (OTA) or to one of two control (CTL) groups receiving either isotonic saline or handling without injection. As adults, animals were tested twice in a neutral arena; before (Test 1) and 24 h after (Test 2) a 4-h exposure to an animal of the opposite sex. In Test 1, CTL males were more likely to show aggressive and less likely to show social behavior than CTL females. No significant treatment differences were observed within either sex in Test 1. In Test 2, after brief exposure to a male, females treated with OT became more aggressive and less social than OTA or CTL females. Male aggressive behavior did not change after exposure to a female. An increase in aggression and decline in social behavior toward other females, seen here in OT-treated females, is typically observed only following several days of female-male cohabitation. These findings demonstrate a sex difference in intrasexual aggression and suggest that neonatal exposure to OT may facilitate the onset of the mate-guarding component of pair bonding in female prairie voles.     

Sex differences and effects of neonatal aromatase inhibition on masculine and feminine copulatory potentials in prairie voles

Copulatory behaviors in most rodents are highly sexually dimorphic, even when circulating hormones are equated between the sexes. Prairie voles (Microtus ochrogaster) are monomorphic in their display of some social behaviors, including partner preferences and parenting, but differences between the sexes in their masculine and feminine copulatory behavior potentials have not been studied in detail. Furthermore, the role of neonatal aromatization of testosterone to estradiol on the development of prairie vole sexual behavior potentials or their brain is unknown. To address these issues, prairie vole pups were injected daily for the first week after birth with 0.5 mg of the aromatase inhibitor 1,4,6-androstatriene-3,17-dione (ATD) or oil. Masculine and feminine copulatory behaviors in response to testosterone or estradiol were later examined in both sexes. Males and females showed high mounting and thrusting in response to testosterone, but only males reliably showed ejaculatory behavior. Conversely, males never showed feminine copulatory behaviors in response to estradiol. Sex differences in these behaviors were not affected by neonatal ATD, but ATD-treated females received fewer mounts and thrusts than controls, possibly indicating reduced attractiveness to males. In other groups of subjects, neonatal ATD demasculinized males' tyrosine hydroxylase expression in the anteroventral periventricular preoptic area, and estrogen receptor alpha expression in the medial preoptic area. Thus, although sexual behavior in both sexes of prairie voles is highly masculinized, aromatase during neonatal life is necessary only for females' femininity. Furthermore, copulatory behavior potentials and at least some aspects of brain development in male prairie voles are dissociable by their requirement for neonatal aromatase.     

Trichostatin A (TSA) facilitates formation of partner preference in male prairie voles (Microtus ochrogaster)

In the socially monogamous prairie voles (Microtus ochrogaster), the development of a social bonding is indicated by the formation of partner preference, which involves a variety of environmental and neurochemical factors and brain structures. In a most recent study in female prairie voles, we found that treatment with the histone deacetylase inhibitor trichostatin A (TSA) facilitates the formation of partner preference through up-regulation of oxytocin receptor (OTR) and vasopressin V1a receptor (V1aR) genes expression in the nucleus accumbens (NAcc). In the present study, we tested the hypothesis that TSA treatment also facilitates partner preference formation and alters OTR and V1aR genes expression in the NAcc in male prairie voles. We thus observed that central injection of TSA dose-dependently promoted the formation of partner preference in the absence of mating in male prairie voles. Interestingly, TSA treatment up-regulated OTR, but not V1aR, gene expression in the NAcc similarly as they were affected by mating — an essential process for naturally occurring partner preference. These data, together with others, not only indicate the involvement of epigenetic events but also the potential role of NAcc oxytocin in the regulation of partner preference in both male and female prairie voles.     

Multi-male mating, probability of conception, and litter size in the prairie vole (Microtus ochrogaster)

We conducted a mating experiment in the laboratory using prairie voles, Microtus ochrogaster, to document that multi-male mating (MMM) can occur in this supposedly monogamous species and to test two hypotheses for the advantages of MMM in female mammals. The two hypotheses are that MMM (1) increases the probability of pregnancy and (2) increases litter size. We also tested the hypothesis that multiple copulations, rather than multiple partners, increases litter size and/or probability of pregnancy. Females were given a choice of mating with any of three males, each of which was tethered in a separate compartment. The mate choice bouts were recorded on videotape. We recorded the number of copulations and number of males with which females mated over a 24 h period. Litter size and probability of pregnancy were not significantly different for females that mated with one, two or three males. Increasing numbers of copulations, independent from the number of males, also did not increase litter size but did significantly increase the probability of pregnancy. MMM, at least in prairie voles, must serve some function other than increasing litter size and probability of conception.     

Adult female prairie voles and meadow voles do not suppress reproduction in their daughters

Reproductive suppression of young females by conspecific females has been reported from laboratory studies on several species of rodents, including the prairie vole, Microtus ochrogaster, but not meadow voles, M. pennsylvanicus. We exposed female prairie voles and meadow voles to two treatments: a mother and one 23-26-day-old daughter paired with a strange male and a 23-26-day-old daughter paired with a different strange male. We found no differences in the proportion of daughters breeding or the time to sexual maturation for daughters raised in the two treatments for either species. Thus, we have no indication that mothers had any adverse effect on reproductive efforts of their daughters. These results differ from previous studies that concluded young female prairie voles were reproductively suppressed by female relatives. The difference between our and previous studies on reproductive suppression is that we examined breeding in young females rather than proximate measures of growth and reproductive development. We question the evolutionary significance of reproductive suppression among related female microtine rodents, especially in that it has not been documented from field populations.     

Relation between circulating sex steroid concentrations and sexual behaviour during pregnancy and post partum in the domestic rabbit

Sexual behaviour of 16 female and 12 male rabbits was studied during pregnancy and early post partum. The main behavioural events of the male (nuzzling and mounting) did not differ in the presence of receptive or non-receptive females. When introduced into the cage of the male, receptive and non-receptive females flattened to the floor or circled around. Sexual receptivity to males decreased in early pregnancy and increased to a maximum a few days around parturition; on Days 1 and 6 post partum, all experimental rabbits submitted to mating. Two groups of females were distinguished: one group submitting to mating whatever the stage of pregnancy, the second being receptive only during the few days before parturition or post partum. During pregnancy and early post partum there was no relation between the colour of the vulva and the female sexual behaviour. Does remained sexually receptive even when progesterone concentrations were high. Nevertheless, the number of receptive females was higher when progesterone concentrations decreased around parturition and the mean daily progesterone values were consistently higher in non-receptive than in receptive females. Oestrogen concentrations during pregnancy were very low and were not related to receptive behaviour.     

Increasing oxytocin receptor expression in the nucleus accumbens of pre-pubertal female prairie voles enhances alloparental responsiveness and partner preference formation as adults

Oxytocin receptors (OXTR) in the nucleus accumbens (NAcc) promote alloparental behavior and partner preference formation in female prairie voles. Within the NAcc there is significant individual variation in OXTR binding and virgin juvenile and adult females with a high density of OXTR in the NAcc display an elevated propensity to engage in alloparental behavior toward novel pups. Over-expression of OXTR in the NAcc of adult female prairie voles using viral vector gene transfer facilitates partner preference formation, but has no effect on alloparental behavior, even though OXTR antagonists infused into the NAcc blocks both behaviors. We therefore hypothesized that long-term increases in OXTR signaling during development may underlie the relationship between adult OXTR density in the NAcc and alloparental behavior. To test this hypothesis, we used viral vector gene transfer to increase OXTR density in the NAcc of prepubertal, 21 day old female prairie voles and tested for both alloparental behavior and partner preference formation as adults. Consistent with a developmental impact of OXTR signaling, adults over-expressing OXTR from weaning display both increased alloparental behavior and partner preference formation. Thus, the relatively acute impact of elevated OXTR signaling in the NAcc on partner preference formation previously reported appears to be dissociable from the effects of longer term, developmentally relevant OXTR signaling necessary for modulating alloparental behavior. These results are consistent with the hypothesis that oxytocin can have both long-term “organizational” effects as well as acute “activational” effects on affiliative behaviors.     

Species and individual differences in juvenile female alloparental care are associated with oxytocin receptor density in the striatum and the lateral septum

The neuropeptide oxytocin has been implicated in the regulation of affiliative behavior and maternal responsiveness in several mammalian species. Rodent species vary considerably in the expression of juvenile alloparental behavior. For example, alloparental behavior is spontaneous in juvenile female prairie voles (approximately 20 days of age), takes 1-3 days of pup exposure to develop in juvenile rats, and is nearly absent in juvenile mice and meadow voles. Here, we tested the hypothesis that species differences in pup responsiveness in juvenile rodents are associated with oxytocin receptor (OTR) density in specific brain regions. We found that OTR density in the nucleus accumbens (NA) is highest in juvenile prairie voles, intermediate in juvenile rats, and lowest in juvenile mice and meadow voles. In the caudate putamen (CP), OTR binding was highest in prairie voles, intermediate in rats and meadow voles, and lowest in mice. In contrast, the lateral septum (LS) shows an opposite pattern, with OTR binding being high in mice and meadow voles and low in prairie voles and rats. Thus, alloparental responsiveness in juvenile rodents is positively correlated with OTR density in the NA and CP and negatively correlated with OTR density in the LS. We then investigated whether a similar receptor-behavior relationship exists among juvenile female prairie voles by correlating individual variation in alloparental behavior with variation in OTR density. The time spent adopting crouching postures, the most distinctive component of alloparental behavior in juveniles, was positively correlated with OTR density in the NA (r = 0.47) and CP (r = 0.45) and negatively correlated with OTR density in the lateral septum (r = -0.53). Thus, variation in OTR density in the NA, CP, and LS may underlie both species and individual differences in alloparental care in rodents.     

Increasing oxytocin receptor expression in the nucleus accumbens of pre-pubertal female prairie voles enhances alloparental responsiveness and partner preference formation as adults

Oxytocin receptors (OXTR) in the nucleus accumbens (NAcc) promote alloparental behavior and partner preference formation in female prairie voles. Within the NAcc there is significant individual variation in OXTR binding and virgin juvenile and adult females with a high density of OXTR in the NAcc display an elevated propensity to engage in alloparental behavior toward novel pups. Over-expression of OXTR in the NAcc of adult female prairie voles using viral vector gene transfer facilitates partner preference formation, but has no effect on alloparental behavior, even though OXTR antagonists infused into the NAcc blocks both behaviors. We therefore hypothesized that long-term increases in OXTR signaling during development may underlie the relationship between adult OXTR density in the NAcc and alloparental behavior. To test this hypothesis, we used viral vector gene transfer to increase OXTR density in the NAcc of prepubertal, 21 day old female prairie voles and tested for both alloparental behavior and partner preference formation as adults. Consistent with a developmental impact of OXTR signaling, adults over-expressing OXTR from weaning display both increased alloparental behavior and partner preference formation. Thus, the relatively acute impact of elevated OXTR signaling in the NAcc on partner preference formation previously reported appears to be dissociable from the effects of longer term, developmentally relevant OXTR signaling necessary for modulating alloparental behavior. These results are consistent with the hypothesis that oxytocin can have both long-term “organizational” effects as well as acute “activational” effects on affiliative behaviors.     

The effects of peptides on partner preference formation are predicted by habitat in prairie voles

This study tested the hypothesis that intraspecific variations in mating systems are correlated with differences in the capacity of peripheral arginine vasopressin (AVP) to facilitate partner preferences. It has been hypothesized that differences in environmental conditions, Kansas being more xeric than Illinois, are responsible for some of the intraspecific differences in the mating systems between Kansas (KN) and Illinois (IL) prairie voles. We predicted that prairie voles from KN would be more behaviorally sensitive to peripheral AVP than prairie voles from IL. To test this hypothesis 60- to 120-day-old male and female, lab-reared, prairie voles originating from KN and IL received three subcutaneous injections of AVP or isotonic saline. Animals were then placed with an adult member of the opposite sex, designated a "partner," for a 1-hour period of cohabitation and subsequently tested for preference for the familiar partner versus a comparable stranger. Only KN males treated with AVP displayed a significant preference for the partner. Using the same experimental paradigm we also examined the ability of peripheral oxytocin (OT) to facilitate partner preference in KN prairie voles. OT facilitated partner preference in females, but not males. This finding was consistent with previous results describing the effects of peripheral OT in IL prairie voles. We also examined the hypothesis that the differential response of KN and IL males would be associated with differences in the distribution of AVP (V1a) receptors. However, there was no apparent difference in the distribution of V(1a) receptors between KN and IL males. The results of this study indicate that there is both intraspecific and intersexual variation in the regulation of social behavior in prairie voles. In addition, these findings suggest that the proximate causes of intraspecific variation may be predicted by knowledge of the habitat of origin.     

Is Mating Alone Enough to Inhibit Infanticide in Male Bank Voles?

Infanticide, the killing of conspecific young, is commonly recognized as an adaptive behavioural strategy enhancing the fitness of the perpetrator. Infanticide is supposed to be inhibited in several male rodent species after mating with a time lag to the time when perpetrators own offspring would be born. This is because males with no parental care do not recognize their own offspring. It has been suggested that copulation alone is enough to inhibit infanticidal behaviour in male rodents. Infanticidal behaviour occurs in more than 50% of male bank voles (Myodes glareolus), and offspring loss because of infanticide may have a great effect on breeding success and population recruitment. In a laboratory experiment, we studied whether infanticidal male bank voles after successful mating stop the killing of pups. Infanticidal males were paired with a female until successful copulation. After the young were born, the males’ infanticidal behaviour was studied from the time of expected birth of own pups until their post‐weaning age. We predicted that mated infanticidal males are inhibited from committing infanticide especially during the time period when pups are less than 10 d old. Against our prediction, 67% of the infanticidal males continued the killing of pups in the age of 3 d. Infanticidal behaviour remained stable, and half of the males were infanticidal still at the age of weaning of pups. Our results are contradictory to previous studies, as we observed no inhibition of infanticide during early life of pups nor increase in infanticide again when their own offspring would have reached the ‘safe’ age and size after weaning. We suggest that mating alone is not sufficient to inhibit infanticide. Thus, we suggest that other cues of the female with whom the male mated with or on her territory are needed for inhibition to occur.     

Comparison of the "nursing" and other parental behaviors of nulliparous and lactating female rats

Virgin female rats display maternal behaviors after continuous exposure to pups (sensitization) that are in some respects similar to those of postpartum females. We herein provide a detailed comparison of the "nursing" and other parental behaviors of maternally sensitized virgin females and postpartum lactating dams. Ovariectomized and intact virgin females were exposed to pups until displaying maternal behavior. On the females' fourth day of maternal responsiveness, the pups were removed for 3 h and then returned, and subject-litter interactions were observed for 45 min. Behavior of maternal virgins was compared with that of lactating dams observed on day 4 postpartum interacting with either suckling pups or pups unable to suckle due to perioral anesthesia. Ovariectomy had no effect on behavior of virgins. Retrieval and licking of pups were deficient in virgins compared with lactating dams. Suckled dams showed prolonged kyphosis (upright crouched nursing), whereas nonsuckled dams displayed little kyphosis but rather were often in a hunched position over pups. Some aspects of quiescent "nursing" behaviors of virgins were surprisingly similar to those of suckled dams, including the latency to and duration of quiescence. Nonsuckling pup stimulation elicited more kyphosis in virgins than in lactating dams, which was still much less than in suckled dams. Virgins also "nursed" pups in hunched and prone postures. Differences between sensitized and postpartum females in their maternal behaviors likely reflect differences in motivation as well as sensory inputs they receive from pups. In particular, sensory regulation of "nursing" behaviors is influenced by reproductive state because nonsuckling pups elicit different postural responses in sensitized and lactating mothers.